[1/9] drm: execution context for GEM buffers v3

Commit Message

Christian König Feb. 28, 2023, 8:33 a.m. UTC

This adds the infrastructure for an execution context for GEM buffers
which is similar to the existinc TTMs execbuf util and intended to replace
it in the long term.

The basic functionality is that we abstracts the necessary loop to lock
many different GEM buffers with automated deadlock and duplicate handling.

v2: drop xarray and use dynamic resized array instead, the locking
    overhead is unecessary and measureable.
v3: drop duplicate tracking, radeon is really the only one needing that.

Signed-off-by: Christian König <[e-mail]>
---
 Documentation/gpu/drm-mm.rst |  12 ++
 drivers/gpu/drm/Kconfig      |   6 +
 drivers/gpu/drm/Makefile     |   2 +
 drivers/gpu/drm/drm_exec.c   | 249 +++++++++++++++++++++++++++++++++++
 include/drm/drm_exec.h       | 115 ++++++++++++++++
 5 files changed, 384 insertions(+)
 create mode 100644 drivers/gpu/drm/drm_exec.c
 create mode 100644 include/drm/drm_exec.h

Comments

Danilo Krummrich Feb. 28, 2023, 7:13 p.m. UTC | #1

On 2/28/23 09:33, Christian König wrote:
> This adds the infrastructure for an execution context for GEM buffers
> which is similar to the existinc TTMs execbuf util and intended to replace

"existing"

> it in the long term.
> 
> The basic functionality is that we abstracts the necessary loop to lock
> many different GEM buffers with automated deadlock and duplicate handling.
> 
> v2: drop xarray and use dynamic resized array instead, the locking
>      overhead is unecessary and measureable.

"unecessary", "measurable"

> v3: drop duplicate tracking, radeon is really the only one needing that.
> 
> Signed-off-by: Christian König <[e-mail]>
> ---
>   Documentation/gpu/drm-mm.rst |  12 ++
>   drivers/gpu/drm/Kconfig      |   6 +
>   drivers/gpu/drm/Makefile     |   2 +
>   drivers/gpu/drm/drm_exec.c   | 249 +++++++++++++++++++++++++++++++++++
>   include/drm/drm_exec.h       | 115 ++++++++++++++++
>   5 files changed, 384 insertions(+)
>   create mode 100644 drivers/gpu/drm/drm_exec.c
>   create mode 100644 include/drm/drm_exec.h
> 
> diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
> index a79fd3549ff8..a52e6f4117d6 100644
> --- a/Documentation/gpu/drm-mm.rst
> +++ b/Documentation/gpu/drm-mm.rst
> @@ -493,6 +493,18 @@ DRM Sync Objects
>   .. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
>      :export:
>   
> +DRM Execution context
> +=====================
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_exec.c
> +   :doc: Overview
> +
> +.. kernel-doc:: include/drm/drm_exec.h
> +   :internal:
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_exec.c
> +   :export:
> +
>   GPU Scheduler
>   =============
>   
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index 17d252dc25e2..84a5fc28c48d 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -200,6 +200,12 @@ config DRM_TTM
>   	  GPU memory types. Will be enabled automatically if a device driver
>   	  uses it.
>   
> +config DRM_EXEC
> +	tristate
> +	depends on DRM
> +	help
> +	  Execution context for command submissions
> +
>   config DRM_BUDDY
>   	tristate
>   	depends on DRM
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index ab4460fcd63f..d40defbb0347 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -78,6 +78,8 @@ obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o
>   #
>   # Memory-management helpers
>   #
> +#
> +obj-$(CONFIG_DRM_EXEC) += drm_exec.o
>   
>   obj-$(CONFIG_DRM_BUDDY) += drm_buddy.o
>   
> diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
> new file mode 100644
> index 000000000000..df546cc5a227
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_exec.c
> @@ -0,0 +1,249 @@
> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
> +
> +#include <drm/drm_exec.h>
> +#include <drm/drm_gem.h>
> +#include <linux/dma-resv.h>
> +
> +/**
> + * DOC: Overview
> + *
> + * This component mainly abstracts the retry loop necessary for locking
> + * multiple GEM objects while preparing hardware operations (e.g. command
> + * submissions, page table updates etc..).
> + *
> + * If a contention is detected while locking a GEM object the cleanup procedure
> + * unlocks all previously locked GEM objects and locks the contended one first
> + * before locking any further objects.
> + *
> + * After an object is locked fences slots can optionally be reserved on the
> + * dma_resv object inside the GEM object.
> + *
> + * A typical usage pattern should look like this::
> + *
> + *	struct drm_gem_object *obj;
> + *	struct drm_exec exec;
> + *	unsigned long index;
> + *	int ret;
> + *
> + *	drm_exec_init(&exec, true);
> + *	drm_exec_while_not_all_locked(&exec) {
> + *		ret = drm_exec_prepare_obj(&exec, boA, 1);
> + *		drm_exec_continue_on_contention(&exec);
> + *		if (ret)
> + *			goto error;
> + *
> + *		ret = drm_exec_lock(&exec, boB, 1);

This function doesn't seem to exist (anymore).

> + *		drm_exec_continue_on_contention(&exec);
> + *		if (ret)
> + *			goto error;
> + *	}
> + *
> + *	drm_exec_for_each_locked_object(&exec, index, obj) {
> + *		dma_resv_add_fence(obj->resv, fence, DMA_RESV_USAGE_READ);
> + *		...
> + *	}
> + *	drm_exec_fini(&exec);
> + *
> + * See struct dma_exec for more details.
> + */
> +
> +/* Dummy value used to initially enter the retry loop */
> +#define DRM_EXEC_DUMMY (void*)~0
> +
> +/* Unlock all objects and drop references */
> +static void drm_exec_unlock_all(struct drm_exec *exec)
> +{
> +	struct drm_gem_object *obj;
> +	unsigned long index;
> +
> +	drm_exec_for_each_locked_object(exec, index, obj) {
> +		dma_resv_unlock(obj->resv);
> +		drm_gem_object_put(obj);
> +	}
> +
> +	if (exec->prelocked) {
> +		dma_resv_unlock(exec->prelocked->resv);
> +		drm_gem_object_put(exec->prelocked);
> +		exec->prelocked = NULL;
> +	}

Let's say we try to lock 3 objects A, B and C in chronological order and 
in the first "drm_exec_cleanup() iteration" C is contended. Firstly, we 
lock C in the next iteration. If now A or B is contended, we never set 
exec->prelocked to NULL in drm_exec_prepare_obj(), since we did not yet 
reach C.

Hence, this causes a double unlock, since the prelocked object is also 
unlocked in the above loop.

Maybe I miss a detail, but to me it looks like setting exec->prelocked 
to NULL and dropping the reference should be enough.

I found that through one of my tests causing the following warning while 
porting over to your v3.

[   26.507866] =====================================
[   26.507867] WARNING: bad unlock balance detected!
[   26.507868] 6.2.0-rc6-vmbind-0.2+ #118 Tainted: G        W
[   26.507868] -------------------------------------
[   26.507869] nouveau_dma_cop/1724 is trying to release lock 
(reservation_ww_class_mutex) at:
[   26.507870] [<ffffffffc05f113a>] drm_exec_unlock_all+0x7a/0xc0 [drm_exec]
[   26.507873] but there are no more locks to release!
<snip>
[   26.508146]
                stack backtrace:
[   26.508146] CPU: 10 PID: 1724 Comm: nouveau_dma_cop Tainted: G 
W          6.2.0-rc6-vmbind-0.2+ #118
[   26.508147] Hardware name: ASUS System Product Name/PRIME Z690-A, 
BIOS 2103 09/30/2022
[   26.508148] Call Trace:
[   26.508148]  <TASK>
[   26.508149]  dump_stack_lvl+0x5b/0x77
[   26.508151]  lock_release.cold+0x45/0x4a
[   26.508154]  __mutex_unlock_slowpath+0x2a/0x260
[   26.508157]  drm_exec_unlock_all+0x7a/0xc0 [drm_exec]
[   26.508159]  drm_exec_cleanup+0x153/0x15c [drm_exec]
[   26.508162]  nouveau_exec_job_submit+0x6b/0x1b5 [nouveau]
[   26.508230]  nouveau_job_submit+0x60/0x450 [nouveau]
[   26.508307]  ? drm_sched_job_init+0x56/0x90 [gpu_sched]
[   26.508311]  nouveau_exec_ioctl_exec+0x228/0x2b0 [nouveau]
[   26.508386]  ? __pfx_nouveau_exec_ioctl_exec+0x10/0x10 [nouveau]
[   26.508457]  drm_ioctl_kernel+0xa9/0x160
[   26.508459]  drm_ioctl+0x1f7/0x4b0
<snip>

> +}
> +
> +/**
> + * drm_exec_init - initialize a drm_exec object
> + * @exec: the drm_exec object to initialize
> + * @interruptible: if locks should be acquired interruptible
> + *
> + * Initialize the object and make sure that we can track locked and duplicate

Duplicate tracking was removed.

> + * objects.
> + */
> +void drm_exec_init(struct drm_exec *exec, bool interruptible)
> +{
> +	exec->interruptible = interruptible;
> +	exec->objects = kmalloc(PAGE_SIZE, GFP_KERNEL);
> +
> +	/* If allocation here fails, just delay that till the first use */
> +	exec->max_objects = exec->objects ? PAGE_SIZE / sizeof(void *) : 0;
> +	exec->num_objects = 0;
> +	exec->contended = DRM_EXEC_DUMMY;
> +	exec->prelocked = NULL;
> +}
> +EXPORT_SYMBOL(drm_exec_init);
> +
> +/**
> + * drm_exec_fini - finalize a drm_exec object
> + * @exec: the drm_exec object to finilize
> + *
> + * Unlock all locked objects, drop the references to objects and free all memory
> + * used for tracking the state.
> + */
> +void drm_exec_fini(struct drm_exec *exec)
> +{
> +	drm_exec_unlock_all(exec);
> +	kvfree(exec->objects);
> +	if (exec->contended != DRM_EXEC_DUMMY) {
> +		drm_gem_object_put(exec->contended);
> +		ww_acquire_fini(&exec->ticket);
> +	}
> +}
> +EXPORT_SYMBOL(drm_exec_fini);
> +
> +/**
> + * drm_exec_cleanup - cleanup when contention is detected
> + * @exec: the drm_exec object to cleanup
> + *
> + * Cleanup the current state and return true if we should stay inside the retry
> + * loop, false if there wasn't any contention detected and we can keep the
> + * objects locked.
> + */
> +bool drm_exec_cleanup(struct drm_exec *exec)
> +{
> +	if (likely(!exec->contended)) {
> +		ww_acquire_done(&exec->ticket);
> +		return false;
> +	}
> +
> +	if (likely(exec->contended == DRM_EXEC_DUMMY)) {
> +		exec->contended = NULL;
> +		ww_acquire_init(&exec->ticket, &reservation_ww_class);
> +		return true;
> +	}
> +
> +	drm_exec_unlock_all(exec);
> +	exec->num_objects = 0;
> +	return true;
> +}
> +EXPORT_SYMBOL(drm_exec_cleanup);
> +
> +/* Track the locked object in the xa and reserve fences */
> +static int drm_exec_obj_locked(struct drm_exec *exec,
> +			       struct drm_gem_object *obj)
> +{
> +	if (unlikely(exec->num_objects == exec->max_objects)) {
> +		size_t size = exec->max_objects * sizeof(void *);
> +		void *tmp;
> +
> +		tmp = kvrealloc(exec->objects, size, size + PAGE_SIZE,
> +				GFP_KERNEL);
> +		if (!tmp)
> +			return -ENOMEM;
> +
> +		exec->objects = tmp;
> +		exec->max_objects += PAGE_SIZE / sizeof(void *);
> +	}
> +	drm_gem_object_get(obj);
> +	exec->objects[exec->num_objects++] = obj;
> +
> +	return 0;
> +}
> +
> +/* Make sure the contended object is locked first */
> +static int drm_exec_lock_contended(struct drm_exec *exec)
> +{
> +	struct drm_gem_object *obj = exec->contended;
> +	int ret;
> +
> +	if (likely(!obj))
> +		return 0;
> +
> +	if (exec->interruptible) {
> +		ret = dma_resv_lock_slow_interruptible(obj->resv,
> +						       &exec->ticket);
> +		if (unlikely(ret))
> +			goto error_dropref;
> +	} else {
> +		dma_resv_lock_slow(obj->resv, &exec->ticket);
> +	}
> +
> +	ret = drm_exec_obj_locked(exec, obj);
> +	if (unlikely(ret)) {
> +		dma_resv_unlock(obj->resv);
> +		goto error_dropref;
> +	}
> +
> +	swap(exec->prelocked, obj);
> +
> +error_dropref:
> +	/* Always cleanup the contention so that error handling can kick in */
> +	drm_gem_object_put(obj);
> +	exec->contended = NULL;
> +	return ret;
> +}
> +
> +/**
> + * drm_exec_prepare_obj - prepare a GEM object for use
> + * @exec: the drm_exec object with the state
> + * @obj: the GEM object to prepare
> + * @num_fences: how many fences to reserve
> + *
> + * Prepare a GEM object for use by locking it and reserving fence slots. All
> + * succesfully locked objects are put into the locked container. Duplicates

"successfully"

> + * detected as well and automatically moved into the duplicates container.

This comment needs update, duplicates are not tracked anymore.

> + *
> + * Returns: -EDEADLK if a contention is detected, -ENOMEM when memory
> + * allocation failed and zero for success.

You may also want to document that this function returns -EALREADY for 
duplicates and that the caller needs to handle it accordingly.

> + */
> +int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
> +			 unsigned int num_fences)
> +{
> +	int ret;
> +
> +	ret = drm_exec_lock_contended(exec);
> +	if (unlikely(ret))
> +		return ret;
> +
> +	if (exec->prelocked == obj) {
> +		drm_gem_object_put(exec->prelocked);
> +		exec->prelocked = NULL;
> +
> +		return dma_resv_reserve_fences(obj->resv, num_fences);
> +	}
> +
> +	if (exec->interruptible)
> +		ret = dma_resv_lock_interruptible(obj->resv, &exec->ticket);
> +	else
> +		ret = dma_resv_lock(obj->resv, &exec->ticket);
> +
> +	if (unlikely(ret == -EDEADLK)) {
> +		drm_gem_object_get(obj);
> +		exec->contended = obj;
> +		return -EDEADLK;
> +	}
> +
> +	if (unlikely(ret))
> +		return ret;
> +
> +	ret = drm_exec_obj_locked(exec, obj);
> +	if (ret)
> +		goto error_unlock;
> +
> +	/* Keep locked when reserving fences fails */
> +	return dma_resv_reserve_fences(obj->resv, num_fences);
> +
> +error_unlock:
> +	dma_resv_unlock(obj->resv);
> +	return ret;
> +}
> +EXPORT_SYMBOL(drm_exec_prepare_obj);
> +
> +MODULE_DESCRIPTION("DRM execution context");
> +MODULE_LICENSE("Dual MIT/GPL");
> diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
> new file mode 100644
> index 000000000000..65e518c01db3
> --- /dev/null
> +++ b/include/drm/drm_exec.h
> @@ -0,0 +1,115 @@
> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
> +
> +#ifndef __DRM_EXEC_H__
> +#define __DRM_EXEC_H__
> +
> +#include <linux/ww_mutex.h>
> +
> +struct drm_gem_object;
> +
> +/**
> + * struct drm_exec - Execution context
> + */
> +struct drm_exec {
> +	/**
> +	 * @interruptible: If locks should be taken interruptible
> +	 */
> +	bool			interruptible;
> +
> +	/**
> +	 * @ticket: WW ticket used for acquiring locks
> +	 */
> +	struct ww_acquire_ctx	ticket;
> +
> +	/**
> +	 * @num_objects: number of objects locked
> +	 */
> +	unsigned int		num_objects;
> +
> +	/**
> +	 * @max_objects: maximum objects in array
> +	 */
> +	unsigned int		max_objects;
> +
> +	/**
> +	 * @objects: array of the locked objects
> +	 */
> +	struct drm_gem_object	**objects;
> +
> +	/**
> +	 * @contended: contended GEM object we backet of for

"backed off"

> +	 */
> +	struct drm_gem_object	*contended;
> +
> +	/**
> +	 * @prelocked: already locked GEM object because of contention

"due to"

> +	 */
> +	struct drm_gem_object *prelocked;
> +};
> +
> +/**
> + * drm_exec_for_each_locked_object - iterate over all the locked objects
> + * @exec: drm_exec object
> + * @index: unsigned long index for the iteration
> + * @obj: the current GEM object
> + *
> + * Iterate over all the locked GEM objects inside the drm_exec object.
> + */
> +#define drm_exec_for_each_locked_object(exec, index, obj)	\
> +	for (index = 0, obj = (exec)->objects[0];		\
> +	     index < (exec)->num_objects;			\
> +	     ++index, obj = (exec)->objects[index])
> +
> +/**
> + * drm_exec_while_not_all_locked - loop until all GEM objects are prepared
> + * @exec: drm_exec object
> + *
> + * Core functionality of the drm_exec object. Loops until all GEM objects are
> + * prepared and no more contention exists.
> + *
> + * At the beginning of the loop it is guaranteed that no GEM object is locked.
> + */
> +#define drm_exec_while_not_all_locked(exec)	\
> +	while (drm_exec_cleanup(exec))
> +
> +/**
> + * drm_exec_continue_on_contention - continue the loop when we need to cleanup
> + * @exec: drm_exec object
> + *
> + * Control flow helper to continue when a contention was detected and we need to
> + * clean up and re-start the loop to prepare all GEM objects.
> + */
> +#define drm_exec_continue_on_contention(exec)		\
> +	if (unlikely(drm_exec_is_contended(exec)))	\
> +		continue
> +
> +/**
> + * drm_exec_break_on_contention - break a subordinal loop on contention
> + * @exec: drm_exec object
> + *
> + * Control flow helper to break a subordinal loop when a contention was detected
> + * and we need to clean up and re-start the loop to prepare all GEM objects.
> + */
> +#define drm_exec_break_on_contention(exec)		\
> +	if (unlikely(drm_exec_is_contended(exec)))	\
> +		break
> +
> +/**
> + * drm_exec_is_contended - check for contention
> + * @exec: drm_exec object
> + *
> + * Returns true if the drm_exec object has run into some contention while
> + * locking a GEM object and needs to clean up.
> + */
> +static inline bool drm_exec_is_contended(struct drm_exec *exec)
> +{
> +	return !!exec->contended;
> +}
> +
> +void drm_exec_init(struct drm_exec *exec, bool interruptible);
> +void drm_exec_fini(struct drm_exec *exec);
> +bool drm_exec_cleanup(struct drm_exec *exec);
> +int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
> +			 unsigned int num_fences);
> +
> +#endif

Christian König March 2, 2023, 7:49 a.m. UTC | #2

Am 28.02.23 um 20:13 schrieb Danilo Krummrich:
> [SNIP]
>> +    if (exec->prelocked) {
>> +        dma_resv_unlock(exec->prelocked->resv);
>> +        drm_gem_object_put(exec->prelocked);
>> +        exec->prelocked = NULL;
>> +    }
>
> Let's say we try to lock 3 objects A, B and C in chronological order 
> and in the first "drm_exec_cleanup() iteration" C is contended. 
> Firstly, we lock C in the next iteration. If now A or B is contended, 
> we never set exec->prelocked to NULL in drm_exec_prepare_obj(), since 
> we did not yet reach C.
>
> Hence, this causes a double unlock, since the prelocked object is also 
> unlocked in the above loop.
>
> Maybe I miss a detail, but to me it looks like setting exec->prelocked 
> to NULL and dropping the reference should be enough.

Ah, yes of course. That wasn't correct and my test cases didn't covered it.

Going to fix this and all the comments you pointed out and update the 
test cases, should be done by next week.

Thanks,
Christian.

Luben Tuikov March 3, 2023, 1:45 p.m. UTC | #3

On 2023-02-28 14:13, Danilo Krummrich wrote:
> On 2/28/23 09:33, Christian König wrote:
>> This adds the infrastructure for an execution context for GEM buffers
>> which is similar to the existinc TTMs execbuf util and intended to replace
> 
> "existing"
> 
>> it in the long term.
>>
>> The basic functionality is that we abstracts the necessary loop to lock
>> many different GEM buffers with automated deadlock and duplicate handling.
>>
>> v2: drop xarray and use dynamic resized array instead, the locking
>>      overhead is unecessary and measureable.
> 
> "unecessary", "measurable"

"unnecessary".

Luben Tuikov March 3, 2023, 2:11 p.m. UTC | #4

On 2023-02-28 03:33, Christian König wrote:
> This adds the infrastructure for an execution context for GEM buffers
> which is similar to the existinc TTMs execbuf util and intended to replace
> it in the long term.
> 
> The basic functionality is that we abstracts the necessary loop to lock
> many different GEM buffers with automated deadlock and duplicate handling.
> 
> v2: drop xarray and use dynamic resized array instead, the locking
>     overhead is unecessary and measureable.
> v3: drop duplicate tracking, radeon is really the only one needing that.
> 
> Signed-off-by: Christian König <[e-mail]>
> ---
>  Documentation/gpu/drm-mm.rst |  12 ++
>  drivers/gpu/drm/Kconfig      |   6 +
>  drivers/gpu/drm/Makefile     |   2 +
>  drivers/gpu/drm/drm_exec.c   | 249 +++++++++++++++++++++++++++++++++++
>  include/drm/drm_exec.h       | 115 ++++++++++++++++
>  5 files changed, 384 insertions(+)
>  create mode 100644 drivers/gpu/drm/drm_exec.c
>  create mode 100644 include/drm/drm_exec.h
> 
> diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
> index a79fd3549ff8..a52e6f4117d6 100644
> --- a/Documentation/gpu/drm-mm.rst
> +++ b/Documentation/gpu/drm-mm.rst
> @@ -493,6 +493,18 @@ DRM Sync Objects
>  .. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
>     :export:
>  
> +DRM Execution context
> +=====================
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_exec.c
> +   :doc: Overview
> +
> +.. kernel-doc:: include/drm/drm_exec.h
> +   :internal:
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_exec.c
> +   :export:
> +
>  GPU Scheduler
>  =============
>  
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index 17d252dc25e2..84a5fc28c48d 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -200,6 +200,12 @@ config DRM_TTM
>  	  GPU memory types. Will be enabled automatically if a device driver
>  	  uses it.
>  
> +config DRM_EXEC
> +	tristate
> +	depends on DRM
> +	help
> +	  Execution context for command submissions
> +
>  config DRM_BUDDY
>  	tristate
>  	depends on DRM
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index ab4460fcd63f..d40defbb0347 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -78,6 +78,8 @@ obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o
>  #
>  # Memory-management helpers
>  #
> +#
> +obj-$(CONFIG_DRM_EXEC) += drm_exec.o
>  
>  obj-$(CONFIG_DRM_BUDDY) += drm_buddy.o
>  
> diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
> new file mode 100644
> index 000000000000..df546cc5a227
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_exec.c
> @@ -0,0 +1,249 @@
> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
> +
> +#include <drm/drm_exec.h>
> +#include <drm/drm_gem.h>
> +#include <linux/dma-resv.h>
> +
> +/**
> + * DOC: Overview
> + *
> + * This component mainly abstracts the retry loop necessary for locking
> + * multiple GEM objects while preparing hardware operations (e.g. command
> + * submissions, page table updates etc..).
> + *
> + * If a contention is detected while locking a GEM object the cleanup procedure
> + * unlocks all previously locked GEM objects and locks the contended one first
> + * before locking any further objects.
> + *
> + * After an object is locked fences slots can optionally be reserved on the
> + * dma_resv object inside the GEM object.
> + *
> + * A typical usage pattern should look like this::
> + *
> + *	struct drm_gem_object *obj;
> + *	struct drm_exec exec;
> + *	unsigned long index;
> + *	int ret;
> + *
> + *	drm_exec_init(&exec, true);
> + *	drm_exec_while_not_all_locked(&exec) {
> + *		ret = drm_exec_prepare_obj(&exec, boA, 1);
> + *		drm_exec_continue_on_contention(&exec);
> + *		if (ret)
> + *			goto error;
> + *
> + *		ret = drm_exec_lock(&exec, boB, 1);
> + *		drm_exec_continue_on_contention(&exec);
> + *		if (ret)
> + *			goto error;
> + *	}
> + *
> + *	drm_exec_for_each_locked_object(&exec, index, obj) {
> + *		dma_resv_add_fence(obj->resv, fence, DMA_RESV_USAGE_READ);
> + *		...
> + *	}
> + *	drm_exec_fini(&exec);

Maybe add the error: label here to show how recovery is to be had.

> + *
> + * See struct dma_exec for more details.
> + */
> +
> +/* Dummy value used to initially enter the retry loop */
> +#define DRM_EXEC_DUMMY (void*)~0
> +
> +/* Unlock all objects and drop references */
> +static void drm_exec_unlock_all(struct drm_exec *exec)
> +{
> +	struct drm_gem_object *obj;
> +	unsigned long index;
> +
> +	drm_exec_for_each_locked_object(exec, index, obj) {
> +		dma_resv_unlock(obj->resv);
> +		drm_gem_object_put(obj);
> +	}
> +
> +	if (exec->prelocked) {
> +		dma_resv_unlock(exec->prelocked->resv);
> +		drm_gem_object_put(exec->prelocked);
> +		exec->prelocked = NULL;
> +	}
> +}
> +
> +/**
> + * drm_exec_init - initialize a drm_exec object
> + * @exec: the drm_exec object to initialize
> + * @interruptible: if locks should be acquired interruptible
> + *
> + * Initialize the object and make sure that we can track locked and duplicate
> + * objects.
> + */
> +void drm_exec_init(struct drm_exec *exec, bool interruptible)
> +{
> +	exec->interruptible = interruptible;
> +	exec->objects = kmalloc(PAGE_SIZE, GFP_KERNEL);
> +
> +	/* If allocation here fails, just delay that till the first use */
> +	exec->max_objects = exec->objects ? PAGE_SIZE / sizeof(void *) : 0;

I didn't see a subsequent allocation for objects. Does it make
sense to continue here if we weren't able to allocate? It seems
that the "first use" is most likely immediately after drm_exec_init(),
and if kmalloc() failed, perhaps things are only about to get worse.

(Pet peeve :-) "till" --> "until".)

> +	exec->num_objects = 0;
> +	exec->contended = DRM_EXEC_DUMMY;
> +	exec->prelocked = NULL;
> +}
> +EXPORT_SYMBOL(drm_exec_init);
> +
> +/**
> + * drm_exec_fini - finalize a drm_exec object
> + * @exec: the drm_exec object to finilize
> + *
> + * Unlock all locked objects, drop the references to objects and free all memory
> + * used for tracking the state.
> + */
> +void drm_exec_fini(struct drm_exec *exec)
> +{
> +	drm_exec_unlock_all(exec);
> +	kvfree(exec->objects);
> +	if (exec->contended != DRM_EXEC_DUMMY) {
> +		drm_gem_object_put(exec->contended);
> +		ww_acquire_fini(&exec->ticket);
> +	}
> +}
> +EXPORT_SYMBOL(drm_exec_fini);
> +
> +/**
> + * drm_exec_cleanup - cleanup when contention is detected
> + * @exec: the drm_exec object to cleanup
> + *
> + * Cleanup the current state and return true if we should stay inside the retry
> + * loop, false if there wasn't any contention detected and we can keep the
> + * objects locked.
> + */
> +bool drm_exec_cleanup(struct drm_exec *exec)
> +{
> +	if (likely(!exec->contended)) {
> +		ww_acquire_done(&exec->ticket);
> +		return false;
> +	}
> +
> +	if (likely(exec->contended == DRM_EXEC_DUMMY)) {
> +		exec->contended = NULL;
> +		ww_acquire_init(&exec->ticket, &reservation_ww_class);
> +		return true;
> +	}
> +
> +	drm_exec_unlock_all(exec);
> +	exec->num_objects = 0;
> +	return true;
> +}
> +EXPORT_SYMBOL(drm_exec_cleanup);
> +
> +/* Track the locked object in the xa and reserve fences */
> +static int drm_exec_obj_locked(struct drm_exec *exec,
> +			       struct drm_gem_object *obj)
> +{
> +	if (unlikely(exec->num_objects == exec->max_objects)) {
> +		size_t size = exec->max_objects * sizeof(void *);
> +		void *tmp;
> +
> +		tmp = kvrealloc(exec->objects, size, size + PAGE_SIZE,
> +				GFP_KERNEL);
> +		if (!tmp)
> +			return -ENOMEM;
> +
> +		exec->objects = tmp;
> +		exec->max_objects += PAGE_SIZE / sizeof(void *);
> +	}
> +	drm_gem_object_get(obj);
> +	exec->objects[exec->num_objects++] = obj;
> +
> +	return 0;
> +}
> +
> +/* Make sure the contended object is locked first */
> +static int drm_exec_lock_contended(struct drm_exec *exec)
> +{
> +	struct drm_gem_object *obj = exec->contended;
> +	int ret;
> +
> +	if (likely(!obj))
> +		return 0;
> +
> +	if (exec->interruptible) {
> +		ret = dma_resv_lock_slow_interruptible(obj->resv,
> +						       &exec->ticket);
> +		if (unlikely(ret))
> +			goto error_dropref;
> +	} else {
> +		dma_resv_lock_slow(obj->resv, &exec->ticket);
> +	}
> +
> +	ret = drm_exec_obj_locked(exec, obj);
> +	if (unlikely(ret)) {
> +		dma_resv_unlock(obj->resv);
> +		goto error_dropref;
> +	}
> +
> +	swap(exec->prelocked, obj);
> +
> +error_dropref:
> +	/* Always cleanup the contention so that error handling can kick in */
> +	drm_gem_object_put(obj);
> +	exec->contended = NULL;
> +	return ret;
> +}
> +
> +/**
> + * drm_exec_prepare_obj - prepare a GEM object for use
> + * @exec: the drm_exec object with the state
> + * @obj: the GEM object to prepare
> + * @num_fences: how many fences to reserve
> + *
> + * Prepare a GEM object for use by locking it and reserving fence slots. All
> + * succesfully locked objects are put into the locked container. Duplicates
> + * detected as well and automatically moved into the duplicates container.
> + *
> + * Returns: -EDEADLK if a contention is detected, -ENOMEM when memory
> + * allocation failed and zero for success.
> + */
> +int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
> +			 unsigned int num_fences)
> +{
> +	int ret;
> +
> +	ret = drm_exec_lock_contended(exec);
> +	if (unlikely(ret))
> +		return ret;
> +
> +	if (exec->prelocked == obj) {
> +		drm_gem_object_put(exec->prelocked);
> +		exec->prelocked = NULL;
> +
> +		return dma_resv_reserve_fences(obj->resv, num_fences);
> +	}
> +
> +	if (exec->interruptible)
> +		ret = dma_resv_lock_interruptible(obj->resv, &exec->ticket);
> +	else
> +		ret = dma_resv_lock(obj->resv, &exec->ticket);
> +
> +	if (unlikely(ret == -EDEADLK)) {
> +		drm_gem_object_get(obj);
> +		exec->contended = obj;
> +		return -EDEADLK;
> +	}
> +
> +	if (unlikely(ret))
> +		return ret;
> +
> +	ret = drm_exec_obj_locked(exec, obj);
> +	if (ret)
> +		goto error_unlock;
> +
> +	/* Keep locked when reserving fences fails */
> +	return dma_resv_reserve_fences(obj->resv, num_fences);
> +
> +error_unlock:
> +	dma_resv_unlock(obj->resv);
> +	return ret;
> +}
> +EXPORT_SYMBOL(drm_exec_prepare_obj);
> +
> +MODULE_DESCRIPTION("DRM execution context");
> +MODULE_LICENSE("Dual MIT/GPL");
> diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
> new file mode 100644
> index 000000000000..65e518c01db3
> --- /dev/null
> +++ b/include/drm/drm_exec.h
> @@ -0,0 +1,115 @@
> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
> +
> +#ifndef __DRM_EXEC_H__
> +#define __DRM_EXEC_H__
> +
> +#include <linux/ww_mutex.h>
> +
> +struct drm_gem_object;
> +
> +/**
> + * struct drm_exec - Execution context
> + */
> +struct drm_exec {
> +	/**
> +	 * @interruptible: If locks should be taken interruptible
> +	 */
> +	bool			interruptible;
> +
> +	/**
> +	 * @ticket: WW ticket used for acquiring locks
> +	 */
> +	struct ww_acquire_ctx	ticket;
> +
> +	/**
> +	 * @num_objects: number of objects locked
> +	 */
> +	unsigned int		num_objects;

"num_objects" may be confused with the size of "objects" array.
Maybe "num_locked" would be clearer?

> +
> +	/**
> +	 * @max_objects: maximum objects in array
> +	 */
> +	unsigned int		max_objects;
> +
> +	/**
> +	 * @objects: array of the locked objects
> +	 */
> +	struct drm_gem_object	**objects;
> +
> +	/**
> +	 * @contended: contended GEM object we backet of for
> +	 */
> +	struct drm_gem_object	*contended;

Perhaps "backed off for" in the comment?

> +
> +	/**
> +	 * @prelocked: already locked GEM object because of contention
> +	 */
> +	struct drm_gem_object *prelocked;

Could this be a subset of the objects array as opposed to a single object?

Thomas Hellström (Intel) March 10, 2023, 10:42 a.m. UTC | #5

Hi Christian

On 2/28/23 09:33, Christian König wrote:
> This adds the infrastructure for an execution context for GEM buffers
> which is similar to the existinc TTMs execbuf util and intended to replace
> it in the long term.
>
> The basic functionality is that we abstracts the necessary loop to lock
> many different GEM buffers with automated deadlock and duplicate handling.
>
> v2: drop xarray and use dynamic resized array instead, the locking
>      overhead is unecessary and measureable.
> v3: drop duplicate tracking, radeon is really the only one needing that.
>
> Signed-off-by: Christian König <[e-mail]>

Nice. This seems to have all we need for now for Xe as well, although 
not for i915 ATM.

I see future exension of this for things like sleeping lock evictions 
(where locks may go out-of-scope during the locking loop), including 
other resv containers, passing through dma-buf move_notify() etc, what 
are your thoughts around that? Extend this or build a different 
functionality and make this a specialization of that?

What about the drm_gem_lock_reservations() interface? While a bit less 
capable it's confusing having two apis doing essentially the same thing, 
could we deprecate that?

Finally a comment below:


> ---
>   Documentation/gpu/drm-mm.rst |  12 ++
>   drivers/gpu/drm/Kconfig      |   6 +
>   drivers/gpu/drm/Makefile     |   2 +
>   drivers/gpu/drm/drm_exec.c   | 249 +++++++++++++++++++++++++++++++++++
>   include/drm/drm_exec.h       | 115 ++++++++++++++++
>   5 files changed, 384 insertions(+)
>   create mode 100644 drivers/gpu/drm/drm_exec.c
>   create mode 100644 include/drm/drm_exec.h
>
> diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
> index a79fd3549ff8..a52e6f4117d6 100644
> --- a/Documentation/gpu/drm-mm.rst
> +++ b/Documentation/gpu/drm-mm.rst
> @@ -493,6 +493,18 @@ DRM Sync Objects
>   .. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
>      :export:
>   
> +DRM Execution context
> +=====================
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_exec.c
> +   :doc: Overview
> +
> +.. kernel-doc:: include/drm/drm_exec.h
> +   :internal:
> +
> +.. kernel-doc:: drivers/gpu/drm/drm_exec.c
> +   :export:
> +
>   GPU Scheduler
>   =============
>   
> diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
> index 17d252dc25e2..84a5fc28c48d 100644
> --- a/drivers/gpu/drm/Kconfig
> +++ b/drivers/gpu/drm/Kconfig
> @@ -200,6 +200,12 @@ config DRM_TTM
>   	  GPU memory types. Will be enabled automatically if a device driver
>   	  uses it.
>   
> +config DRM_EXEC
> +	tristate
> +	depends on DRM
> +	help
> +	  Execution context for command submissions
> +
>   config DRM_BUDDY
>   	tristate
>   	depends on DRM
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index ab4460fcd63f..d40defbb0347 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -78,6 +78,8 @@ obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o
>   #
>   # Memory-management helpers
>   #
> +#
> +obj-$(CONFIG_DRM_EXEC) += drm_exec.o
>   
>   obj-$(CONFIG_DRM_BUDDY) += drm_buddy.o
>   
> diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
> new file mode 100644
> index 000000000000..df546cc5a227
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_exec.c
> @@ -0,0 +1,249 @@
> +/* SPDX-License-Identifier: GPL-2.0 OR MIT */
> +
> +#include <drm/drm_exec.h>
> +#include <drm/drm_gem.h>
> +#include <linux/dma-resv.h>
> +
> +/**
> + * DOC: Overview
> + *
> + * This component mainly abstracts the retry loop necessary for locking
> + * multiple GEM objects while preparing hardware operations (e.g. command
> + * submissions, page table updates etc..).
> + *
> + * If a contention is detected while locking a GEM object the cleanup procedure
> + * unlocks all previously locked GEM objects and locks the contended one first
> + * before locking any further objects.
> + *
> + * After an object is locked fences slots can optionally be reserved on the
> + * dma_resv object inside the GEM object.
> + *
> + * A typical usage pattern should look like this::
> + *
> + *	struct drm_gem_object *obj;
> + *	struct drm_exec exec;
> + *	unsigned long index;
> + *	int ret;
> + *
> + *	drm_exec_init(&exec, true);
> + *	drm_exec_while_not_all_locked(&exec) {
> + *		ret = drm_exec_prepare_obj(&exec, boA, 1);
> + *		drm_exec_continue_on_contention(&exec);
> + *		if (ret)
> + *			goto error;
> + *
> + *		ret = drm_exec_lock(&exec, boB, 1);

Should be drm_exec_prepare_obj()?

> + *		drm_exec_continue_on_contention(&exec);
> + *		if (ret)
> + *			goto error;
> + *	}
> + *
> + *	drm_exec_for_each_locked_object(&exec, index, obj) {
> + *		dma_resv_add_fence(obj->resv, fence, DMA_RESV_USAGE_READ);
> + *		...
> + *	}
> + *	drm_exec_fini(&exec);
> + *
> + * See struct dma_exec for more details.
> + */
> +
> +/* Dummy value used to initially enter the retry loop */
> +#define DRM_EXEC_DUMMY (void*)~0
> +
> +/* Unlock all objects and drop references */
> +static void drm_exec_unlock_all(struct drm_exec *exec)
> +{
> +	struct drm_gem_object *obj;
> +	unsigned long index;
> +
> +	drm_exec_for_each_locked_object(exec, index, obj) {
> +		dma_resv_unlock(obj->resv);
> +		drm_gem_object_put(obj);
> +	}
> +
> +	if (exec->prelocked) {
> +		dma_resv_unlock(exec->prelocked->resv);
> +		drm_gem_object_put(exec->prelocked);
> +		exec->prelocked = NULL;
> +	}
> +}
> +
> +/**
> + * drm_exec_init - initialize a drm_exec object
> + * @exec: the drm_exec object to initialize
> + * @interruptible: if locks should be acquired interruptible
> + *
> + * Initialize the object and make sure that we can track locked and duplicate
> + * objects.
> + */
> +void drm_exec_init(struct drm_exec *exec, bool interruptible)
> +{
> +	exec->interruptible = interruptible;
> +	exec->objects = kmalloc(PAGE_SIZE, GFP_KERNEL);
> +
> +	/* If allocation here fails, just delay that till the first use */
> +	exec->max_objects = exec->objects ? PAGE_SIZE / sizeof(void *) : 0;
> +	exec->num_objects = 0;
> +	exec->contended = DRM_EXEC_DUMMY;
> +	exec->prelocked = NULL;
> +}
> +EXPORT_SYMBOL(drm_exec_init);
> +
> +/**
> + * drm_exec_fini - finalize a drm_exec object
> + * @exec: the drm_exec object to finilize
> + *
> + * Unlock all locked objects, drop the references to objects and free all memory
> + * used for tracking the state.
> + */
> +void drm_exec_fini(struct drm_exec *exec)
> +{
> +	drm_exec_unlock_all(exec);
> +	kvfree(exec->objects);
> +	if (exec->contended != DRM_EXEC_DUMMY) {
> +		drm_gem_object_put(exec->contended);
> +		ww_acquire_fini(&exec->ticket);
> +	}
> +}
> +EXPORT_SYMBOL(drm_exec_fini);
> +
> +/**
> + * drm_exec_cleanup - cleanup when contention is detected
> + * @exec: the drm_exec object to cleanup
> + *
> + * Cleanup the current state and return true if we should stay inside the retry
> + * loop, false if there wasn't any contention detected and we can keep the
> + * objects locked.
> + */
> +bool drm_exec_cleanup(struct drm_exec *exec)
> +{
> +	if (likely(!exec->contended)) {
> +		ww_acquire_done(&exec->ticket);
> +		return false;
> +	}
> +
> +	if (likely(exec->contended == DRM_EXEC_DUMMY)) {
> +		exec->contended = NULL;
> +		ww_acquire_init(&exec->ticket, &reservation_ww_class);
> +		return true;
> +	}
> +
> +	drm_exec_unlock_all(exec);
> +	exec->num_objects = 0;
> +	return true;
> +}
> +EXPORT_SYMBOL(drm_exec_cleanup);
> +
> +/* Track the locked object in the xa and reserve fences */
> +static int drm_exec_obj_locked(struct drm_exec *exec,
> +			       struct drm_gem_object *obj)
> +{
> +	if (unlikely(exec->num_objects == exec->max_objects)) {
> +		size_t size = exec->max_objects * sizeof(void *);
> +		void *tmp;
> +
> +		tmp = kvrealloc(exec->objects, size, size + PAGE_SIZE,
> +				GFP_KERNEL);
> +		if (!tmp)
> +			return -ENOMEM;
> +
> +		exec->objects = tmp;
> +		exec->max_objects += PAGE_SIZE / sizeof(void *);
> +	}
> +	drm_gem_object_get(obj);
> +	exec->objects[exec->num_objects++] = obj;
> +
> +	return 0;
> +}
> +
> +/* Make sure the contended object is locked first */
> +static int drm_exec_lock_contended(struct drm_exec *exec)
> +{
> +	struct drm_gem_object *obj = exec->contended;
> +	int ret;
> +
> +	if (likely(!obj))
> +		return 0;
> +
> +	if (exec->interruptible) {
> +		ret = dma_resv_lock_slow_interruptible(obj->resv,
> +						       &exec->ticket);
> +		if (unlikely(ret))
> +			goto error_dropref;
> +	} else {
> +		dma_resv_lock_slow(obj->resv, &exec->ticket);
> +	}
> +
> +	ret = drm_exec_obj_locked(exec, obj);
> +	if (unlikely(ret)) {
> +		dma_resv_unlock(obj->resv);
> +		goto error_dropref;
> +	}
> +
> +	swap(exec->prelocked, obj);
> +
> +error_dropref:
> +	/* Always cleanup the contention so that error handling can kick in */
> +	drm_gem_object_put(obj);
> +	exec->contended = NULL;
> +	return ret;
> +}
> +
> +/**
> + * drm_exec_prepare_obj - prepare a GEM object for use
> + * @exec: the drm_exec object with the state
> + * @obj: the GEM object to prepare
> + * @num_fences: how many fences to reserve
> + *
> + * Prepare a GEM object for use by locking it and reserving fence slots. All
> + * succesfully locked objects are put into the locked container. Duplicates
> + * detected as well and automatically moved into the duplicates container.
> + *
> + * Returns: -EDEADLK if a contention is detected, -ENOMEM when memory
> + * allocation failed and zero for success.
> + */
> +int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
> +			 unsigned int num_fences)
> +{
> +	int ret;
> +
> +	ret = drm_exec_lock_contended(exec);
> +	if (unlikely(ret))
> +		return ret;
> +
> +	if (exec->prelocked == obj) {
> +		drm_gem_object_put(exec->prelocked);
> +		exec->prelocked = NULL;
> +
> +		return dma_resv_reserve_fences(obj->resv, num_fences);
> +	}
> +
> +	if (exec->interruptible)
> +		ret = dma_resv_lock_interruptible(obj->resv, &exec->ticket);
> +	else
> +		ret = dma_resv_lock(obj->resv, &exec->ticket);
> +
> +	if (unlikely(ret == -EDEADLK)) {
> +		drm_gem_object_get(obj);
> +		exec->contended = obj;
> +		return -EDEADLK;
> +	}
> +
> +	if (unlikely(ret))
> +		return ret;
> +
> +	ret = drm_exec_obj_locked(exec, obj);
> +	if (ret)
> +		goto error_unlock;
> +
> +	/* Keep locked when reserving fences fails */
> +	return dma_resv_reserve_fences(obj->resv, num_fences);

IMHO, keeping the object locked on a certain error code but not on 
others is hard to follow both for humans and static analyzers and will 
most certainly result in driver errors due to lacking -ENOMEM coverage. 
In the unlikely -ENOMEM situation, could we have the caller handle that 
explicitly by, for example, issue a new call with 0 fences reserved?

Thanks,
/Thomas

Francois Dugast April 14, 2023, 2:29 p.m. UTC | #6

Hi Christian, hi Thomas,

On Fri, Mar 10, 2023 at 11:42:56AM +0100, Thomas Hellström (Intel) wrote:
> Nice. This seems to have all we need for now for Xe as well, although not
> for i915 ATM.

A series to use drm_exec in Xe has been sent for review:
https://patchwork.freedesktop.org/series/116477/

Thanks,
Francois

Patch

diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
index a79fd3549ff8..a52e6f4117d6 100644
--- a/Documentation/gpu/drm-mm.rst
+++ b/Documentation/gpu/drm-mm.rst
@@ -493,6 +493,18 @@  DRM Sync Objects
 .. kernel-doc:: drivers/gpu/drm/drm_syncobj.c
    :export:
 
+DRM Execution context
+=====================
+
+.. kernel-doc:: drivers/gpu/drm/drm_exec.c
+   :doc: Overview
+
+.. kernel-doc:: include/drm/drm_exec.h
+   :internal:
+
+.. kernel-doc:: drivers/gpu/drm/drm_exec.c
+   :export:
+
 GPU Scheduler
 =============
 
diff --git a/drivers/gpu/drm/Kconfig b/drivers/gpu/drm/Kconfig
index 17d252dc25e2..84a5fc28c48d 100644
--- a/drivers/gpu/drm/Kconfig
+++ b/drivers/gpu/drm/Kconfig
@@ -200,6 +200,12 @@  config DRM_TTM
 	  GPU memory types. Will be enabled automatically if a device driver
 	  uses it.
 
+config DRM_EXEC
+	tristate
+	depends on DRM
+	help
+	  Execution context for command submissions
+
 config DRM_BUDDY
 	tristate
 	depends on DRM
diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index ab4460fcd63f..d40defbb0347 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -78,6 +78,8 @@  obj-$(CONFIG_DRM_PANEL_ORIENTATION_QUIRKS) += drm_panel_orientation_quirks.o
 #
 # Memory-management helpers
 #
+#
+obj-$(CONFIG_DRM_EXEC) += drm_exec.o
 
 obj-$(CONFIG_DRM_BUDDY) += drm_buddy.o
 
diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
new file mode 100644
index 000000000000..df546cc5a227
--- /dev/null
+++ b/drivers/gpu/drm/drm_exec.c
@@ -0,0 +1,249 @@ 
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
+
+#include <drm/drm_exec.h>
+#include <drm/drm_gem.h>
+#include <linux/dma-resv.h>
+
+/**
+ * DOC: Overview
+ *
+ * This component mainly abstracts the retry loop necessary for locking
+ * multiple GEM objects while preparing hardware operations (e.g. command
+ * submissions, page table updates etc..).
+ *
+ * If a contention is detected while locking a GEM object the cleanup procedure
+ * unlocks all previously locked GEM objects and locks the contended one first
+ * before locking any further objects.
+ *
+ * After an object is locked fences slots can optionally be reserved on the
+ * dma_resv object inside the GEM object.
+ *
+ * A typical usage pattern should look like this::
+ *
+ *	struct drm_gem_object *obj;
+ *	struct drm_exec exec;
+ *	unsigned long index;
+ *	int ret;
+ *
+ *	drm_exec_init(&exec, true);
+ *	drm_exec_while_not_all_locked(&exec) {
+ *		ret = drm_exec_prepare_obj(&exec, boA, 1);
+ *		drm_exec_continue_on_contention(&exec);
+ *		if (ret)
+ *			goto error;
+ *
+ *		ret = drm_exec_lock(&exec, boB, 1);
+ *		drm_exec_continue_on_contention(&exec);
+ *		if (ret)
+ *			goto error;
+ *	}
+ *
+ *	drm_exec_for_each_locked_object(&exec, index, obj) {
+ *		dma_resv_add_fence(obj->resv, fence, DMA_RESV_USAGE_READ);
+ *		...
+ *	}
+ *	drm_exec_fini(&exec);
+ *
+ * See struct dma_exec for more details.
+ */
+
+/* Dummy value used to initially enter the retry loop */
+#define DRM_EXEC_DUMMY (void*)~0
+
+/* Unlock all objects and drop references */
+static void drm_exec_unlock_all(struct drm_exec *exec)
+{
+	struct drm_gem_object *obj;
+	unsigned long index;
+
+	drm_exec_for_each_locked_object(exec, index, obj) {
+		dma_resv_unlock(obj->resv);
+		drm_gem_object_put(obj);
+	}
+
+	if (exec->prelocked) {
+		dma_resv_unlock(exec->prelocked->resv);
+		drm_gem_object_put(exec->prelocked);
+		exec->prelocked = NULL;
+	}
+}
+
+/**
+ * drm_exec_init - initialize a drm_exec object
+ * @exec: the drm_exec object to initialize
+ * @interruptible: if locks should be acquired interruptible
+ *
+ * Initialize the object and make sure that we can track locked and duplicate
+ * objects.
+ */
+void drm_exec_init(struct drm_exec *exec, bool interruptible)
+{
+	exec->interruptible = interruptible;
+	exec->objects = kmalloc(PAGE_SIZE, GFP_KERNEL);
+
+	/* If allocation here fails, just delay that till the first use */
+	exec->max_objects = exec->objects ? PAGE_SIZE / sizeof(void *) : 0;
+	exec->num_objects = 0;
+	exec->contended = DRM_EXEC_DUMMY;
+	exec->prelocked = NULL;
+}
+EXPORT_SYMBOL(drm_exec_init);
+
+/**
+ * drm_exec_fini - finalize a drm_exec object
+ * @exec: the drm_exec object to finilize
+ *
+ * Unlock all locked objects, drop the references to objects and free all memory
+ * used for tracking the state.
+ */
+void drm_exec_fini(struct drm_exec *exec)
+{
+	drm_exec_unlock_all(exec);
+	kvfree(exec->objects);
+	if (exec->contended != DRM_EXEC_DUMMY) {
+		drm_gem_object_put(exec->contended);
+		ww_acquire_fini(&exec->ticket);
+	}
+}
+EXPORT_SYMBOL(drm_exec_fini);
+
+/**
+ * drm_exec_cleanup - cleanup when contention is detected
+ * @exec: the drm_exec object to cleanup
+ *
+ * Cleanup the current state and return true if we should stay inside the retry
+ * loop, false if there wasn't any contention detected and we can keep the
+ * objects locked.
+ */
+bool drm_exec_cleanup(struct drm_exec *exec)
+{
+	if (likely(!exec->contended)) {
+		ww_acquire_done(&exec->ticket);
+		return false;
+	}
+
+	if (likely(exec->contended == DRM_EXEC_DUMMY)) {
+		exec->contended = NULL;
+		ww_acquire_init(&exec->ticket, &reservation_ww_class);
+		return true;
+	}
+
+	drm_exec_unlock_all(exec);
+	exec->num_objects = 0;
+	return true;
+}
+EXPORT_SYMBOL(drm_exec_cleanup);
+
+/* Track the locked object in the xa and reserve fences */
+static int drm_exec_obj_locked(struct drm_exec *exec,
+			       struct drm_gem_object *obj)
+{
+	if (unlikely(exec->num_objects == exec->max_objects)) {
+		size_t size = exec->max_objects * sizeof(void *);
+		void *tmp;
+
+		tmp = kvrealloc(exec->objects, size, size + PAGE_SIZE,
+				GFP_KERNEL);
+		if (!tmp)
+			return -ENOMEM;
+
+		exec->objects = tmp;
+		exec->max_objects += PAGE_SIZE / sizeof(void *);
+	}
+	drm_gem_object_get(obj);
+	exec->objects[exec->num_objects++] = obj;
+
+	return 0;
+}
+
+/* Make sure the contended object is locked first */
+static int drm_exec_lock_contended(struct drm_exec *exec)
+{
+	struct drm_gem_object *obj = exec->contended;
+	int ret;
+
+	if (likely(!obj))
+		return 0;
+
+	if (exec->interruptible) {
+		ret = dma_resv_lock_slow_interruptible(obj->resv,
+						       &exec->ticket);
+		if (unlikely(ret))
+			goto error_dropref;
+	} else {
+		dma_resv_lock_slow(obj->resv, &exec->ticket);
+	}
+
+	ret = drm_exec_obj_locked(exec, obj);
+	if (unlikely(ret)) {
+		dma_resv_unlock(obj->resv);
+		goto error_dropref;
+	}
+
+	swap(exec->prelocked, obj);
+
+error_dropref:
+	/* Always cleanup the contention so that error handling can kick in */
+	drm_gem_object_put(obj);
+	exec->contended = NULL;
+	return ret;
+}
+
+/**
+ * drm_exec_prepare_obj - prepare a GEM object for use
+ * @exec: the drm_exec object with the state
+ * @obj: the GEM object to prepare
+ * @num_fences: how many fences to reserve
+ *
+ * Prepare a GEM object for use by locking it and reserving fence slots. All
+ * succesfully locked objects are put into the locked container. Duplicates
+ * detected as well and automatically moved into the duplicates container.
+ *
+ * Returns: -EDEADLK if a contention is detected, -ENOMEM when memory
+ * allocation failed and zero for success.
+ */
+int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
+			 unsigned int num_fences)
+{
+	int ret;
+
+	ret = drm_exec_lock_contended(exec);
+	if (unlikely(ret))
+		return ret;
+
+	if (exec->prelocked == obj) {
+		drm_gem_object_put(exec->prelocked);
+		exec->prelocked = NULL;
+
+		return dma_resv_reserve_fences(obj->resv, num_fences);
+	}
+
+	if (exec->interruptible)
+		ret = dma_resv_lock_interruptible(obj->resv, &exec->ticket);
+	else
+		ret = dma_resv_lock(obj->resv, &exec->ticket);
+
+	if (unlikely(ret == -EDEADLK)) {
+		drm_gem_object_get(obj);
+		exec->contended = obj;
+		return -EDEADLK;
+	}
+
+	if (unlikely(ret))
+		return ret;
+
+	ret = drm_exec_obj_locked(exec, obj);
+	if (ret)
+		goto error_unlock;
+
+	/* Keep locked when reserving fences fails */
+	return dma_resv_reserve_fences(obj->resv, num_fences);
+
+error_unlock:
+	dma_resv_unlock(obj->resv);
+	return ret;
+}
+EXPORT_SYMBOL(drm_exec_prepare_obj);
+
+MODULE_DESCRIPTION("DRM execution context");
+MODULE_LICENSE("Dual MIT/GPL");
diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
new file mode 100644
index 000000000000..65e518c01db3
--- /dev/null
+++ b/include/drm/drm_exec.h
@@ -0,0 +1,115 @@ 
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
+
+#ifndef __DRM_EXEC_H__
+#define __DRM_EXEC_H__
+
+#include <linux/ww_mutex.h>
+
+struct drm_gem_object;
+
+/**
+ * struct drm_exec - Execution context
+ */
+struct drm_exec {
+	/**
+	 * @interruptible: If locks should be taken interruptible
+	 */
+	bool			interruptible;
+
+	/**
+	 * @ticket: WW ticket used for acquiring locks
+	 */
+	struct ww_acquire_ctx	ticket;
+
+	/**
+	 * @num_objects: number of objects locked
+	 */
+	unsigned int		num_objects;
+
+	/**
+	 * @max_objects: maximum objects in array
+	 */
+	unsigned int		max_objects;
+
+	/**
+	 * @objects: array of the locked objects
+	 */
+	struct drm_gem_object	**objects;
+
+	/**
+	 * @contended: contended GEM object we backet of for
+	 */
+	struct drm_gem_object	*contended;
+
+	/**
+	 * @prelocked: already locked GEM object because of contention
+	 */
+	struct drm_gem_object *prelocked;
+};
+
+/**
+ * drm_exec_for_each_locked_object - iterate over all the locked objects
+ * @exec: drm_exec object
+ * @index: unsigned long index for the iteration
+ * @obj: the current GEM object
+ *
+ * Iterate over all the locked GEM objects inside the drm_exec object.
+ */
+#define drm_exec_for_each_locked_object(exec, index, obj)	\
+	for (index = 0, obj = (exec)->objects[0];		\
+	     index < (exec)->num_objects;			\
+	     ++index, obj = (exec)->objects[index])
+
+/**
+ * drm_exec_while_not_all_locked - loop until all GEM objects are prepared
+ * @exec: drm_exec object
+ *
+ * Core functionality of the drm_exec object. Loops until all GEM objects are
+ * prepared and no more contention exists.
+ *
+ * At the beginning of the loop it is guaranteed that no GEM object is locked.
+ */
+#define drm_exec_while_not_all_locked(exec)	\
+	while (drm_exec_cleanup(exec))
+
+/**
+ * drm_exec_continue_on_contention - continue the loop when we need to cleanup
+ * @exec: drm_exec object
+ *
+ * Control flow helper to continue when a contention was detected and we need to
+ * clean up and re-start the loop to prepare all GEM objects.
+ */
+#define drm_exec_continue_on_contention(exec)		\
+	if (unlikely(drm_exec_is_contended(exec)))	\
+		continue
+
+/**
+ * drm_exec_break_on_contention - break a subordinal loop on contention
+ * @exec: drm_exec object
+ *
+ * Control flow helper to break a subordinal loop when a contention was detected
+ * and we need to clean up and re-start the loop to prepare all GEM objects.
+ */
+#define drm_exec_break_on_contention(exec)		\
+	if (unlikely(drm_exec_is_contended(exec)))	\
+		break
+
+/**
+ * drm_exec_is_contended - check for contention
+ * @exec: drm_exec object
+ *
+ * Returns true if the drm_exec object has run into some contention while
+ * locking a GEM object and needs to clean up.
+ */
+static inline bool drm_exec_is_contended(struct drm_exec *exec)
+{
+	return !!exec->contended;
+}
+
+void drm_exec_init(struct drm_exec *exec, bool interruptible);
+void drm_exec_fini(struct drm_exec *exec);
+bool drm_exec_cleanup(struct drm_exec *exec);
+int drm_exec_prepare_obj(struct drm_exec *exec, struct drm_gem_object *obj,
+			 unsigned int num_fences);
+
+#endif