diff mbox series

[12/12] drm/modifiers: Enforce consistency between the cap an IN_FORMATS

Message ID 20210413094904.3736372-12-daniel.vetter@ffwll.ch (mailing list archive)
State New
Headers show
Series [01/12] drm/arm: Don't set allow_fb_modifiers explicitly | expand

Commit Message

Daniel Vetter April 13, 2021, 9:49 a.m. UTC
It's very confusing for userspace to have to deal with inconsistencies
here, and some drivers screwed this up a bit. Most just ommitted the
format list when they meant to say that only linear modifier is
allowed, but some also meant that only implied modifiers are
acceptable (because actually none of the planes registered supported
modifiers).

Now that this is all done consistently across all drivers, document
the rules and enforce it in the drm core.

Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
---
 drivers/gpu/drm/drm_plane.c   | 16 +++++++++++++++-
 include/drm/drm_mode_config.h |  2 ++
 2 files changed, 17 insertions(+), 1 deletion(-)

Comments

Lucas Stach April 13, 2021, 11:54 a.m. UTC | #1
Am Dienstag, dem 13.04.2021 um 11:49 +0200 schrieb Daniel Vetter:
> It's very confusing for userspace to have to deal with inconsistencies
> here, and some drivers screwed this up a bit. Most just ommitted the
> format list when they meant to say that only linear modifier is
> allowed, but some also meant that only implied modifiers are
> acceptable (because actually none of the planes registered supported
> modifiers).

For a lot of the embedded display drivers that never had any out-of-
band tiling meta shared with the GPU part, the implied modifier is
actually DRM_FORMAT_MOD_LINEAR, so maybe that's where some of the
confusion about needing to specify the modifier list comes from.

> Now that this is all done consistently across all drivers, document
> the rules and enforce it in the drm core.

This clarification looks good to me.

Reviewed-by: Lucas Stach <l.stach@pengutronix.de>

> Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> ---
>  drivers/gpu/drm/drm_plane.c   | 16 +++++++++++++++-
>  include/drm/drm_mode_config.h |  2 ++
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 0dd43882fe7c..16a7e3e57f7f 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -128,6 +128,11 @@
>   *     pairs supported by this plane. The blob is a struct
>   *     drm_format_modifier_blob. Without this property the plane doesn't
>   *     support buffers with modifiers. Userspace cannot change this property.
> + *
> + *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
> + *     capability for general modifier support. If this flag is set then every
> + *     plane will have the IN_FORMATS property, even when it only supports
> + *     DRM_FORMAT_MOD_LINEAR.
>   */
>  
> 
> 
> 
> 
> 
> 
> 
>  static unsigned int drm_num_planes(struct drm_device *dev)
> @@ -277,8 +282,14 @@ static int __drm_universal_plane_init(struct drm_device *dev,
>  			format_modifier_count++;
>  	}
>  
> 
> 
> 
> 
> 
> 
> 
> -	if (format_modifier_count)
> +	/* autoset the cap and check for consistency across all planes */
> +	if (format_modifier_count) {
> +		WARN_ON(!config->allow_fb_modifiers &&
> +			!list_empty(&config->plane_list));
>  		config->allow_fb_modifiers = true;
> +	} else {
> +		WARN_ON(config->allow_fb_modifiers);
> +	}
>  
> 
> 
> 
> 
> 
> 
> 
>  	plane->modifier_count = format_modifier_count;
>  	plane->modifiers = kmalloc_array(format_modifier_count,
> @@ -360,6 +371,9 @@ static int __drm_universal_plane_init(struct drm_device *dev,
>   * drm_universal_plane_init() to let the DRM managed resource infrastructure
>   * take care of cleanup and deallocation.
>   *
> + * Drivers supporting modifiers must set @format_modifiers on all their planes,
> + * even those that only support DRM_FORMAT_MOD_LINEAR.
> + *
>   * Returns:
>   * Zero on success, error code on failure.
>   */
> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index ab424ddd7665..1ddf7783fdf7 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -909,6 +909,8 @@ struct drm_mode_config {
>  	 * @allow_fb_modifiers:
>  	 *
>  	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> +	 * Note that drivers should not set this directly, it is automatically
> +	 * set in drm_universal_plane_init().
>  	 *
>  	 * IMPORTANT:
>  	 *
Pekka Paalanen April 13, 2021, 11:56 a.m. UTC | #2
On Tue, 13 Apr 2021 11:49:03 +0200
Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> It's very confusing for userspace to have to deal with inconsistencies
> here, and some drivers screwed this up a bit. Most just ommitted the
> format list when they meant to say that only linear modifier is
> allowed, but some also meant that only implied modifiers are
> acceptable (because actually none of the planes registered supported
> modifiers).
> 
> Now that this is all done consistently across all drivers, document
> the rules and enforce it in the drm core.
> 
> Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> ---
>  drivers/gpu/drm/drm_plane.c   | 16 +++++++++++++++-
>  include/drm/drm_mode_config.h |  2 ++
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 0dd43882fe7c..16a7e3e57f7f 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -128,6 +128,11 @@
>   *     pairs supported by this plane. The blob is a struct
>   *     drm_format_modifier_blob. Without this property the plane doesn't
>   *     support buffers with modifiers. Userspace cannot change this property.
> + *
> + *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
> + *     capability for general modifier support. If this flag is set then every
> + *     plane will have the IN_FORMATS property, even when it only supports
> + *     DRM_FORMAT_MOD_LINEAR.

Ooh, that's even better. But isn't that changing the meaning of the
cap? Isn't the cap older than IN_FORMATS?

What about the opposite? Is it allowed to have even a single IN_FORMATS
if you don't have the cap?

>   */
>  
>  static unsigned int drm_num_planes(struct drm_device *dev)
> @@ -277,8 +282,14 @@ static int __drm_universal_plane_init(struct drm_device *dev,
>  			format_modifier_count++;
>  	}
>  
> -	if (format_modifier_count)
> +	/* autoset the cap and check for consistency across all planes */
> +	if (format_modifier_count) {
> +		WARN_ON(!config->allow_fb_modifiers &&
> +			!list_empty(&config->plane_list));

What does this mean?

>  		config->allow_fb_modifiers = true;
> +	} else {
> +		WARN_ON(config->allow_fb_modifiers);
> +	}
>  
>  	plane->modifier_count = format_modifier_count;
>  	plane->modifiers = kmalloc_array(format_modifier_count,
> @@ -360,6 +371,9 @@ static int __drm_universal_plane_init(struct drm_device *dev,
>   * drm_universal_plane_init() to let the DRM managed resource infrastructure
>   * take care of cleanup and deallocation.
>   *
> + * Drivers supporting modifiers must set @format_modifiers on all their planes,
> + * even those that only support DRM_FORMAT_MOD_LINEAR.

Good.

> + *
>   * Returns:
>   * Zero on success, error code on failure.
>   */
> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index ab424ddd7665..1ddf7783fdf7 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -909,6 +909,8 @@ struct drm_mode_config {
>  	 * @allow_fb_modifiers:
>  	 *
>  	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> +	 * Note that drivers should not set this directly, it is automatically
> +	 * set in drm_universal_plane_init().
>  	 *
>  	 * IMPORTANT:
>  	 *

Thanks,
pq
Daniel Vetter April 13, 2021, 2:11 p.m. UTC | #3
On Tue, Apr 13, 2021 at 02:56:02PM +0300, Pekka Paalanen wrote:
> On Tue, 13 Apr 2021 11:49:03 +0200
> Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> 
> > It's very confusing for userspace to have to deal with inconsistencies
> > here, and some drivers screwed this up a bit. Most just ommitted the
> > format list when they meant to say that only linear modifier is
> > allowed, but some also meant that only implied modifiers are
> > acceptable (because actually none of the planes registered supported
> > modifiers).
> > 
> > Now that this is all done consistently across all drivers, document
> > the rules and enforce it in the drm core.
> > 
> > Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Maxime Ripard <mripard@kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: David Airlie <airlied@linux.ie>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > ---
> >  drivers/gpu/drm/drm_plane.c   | 16 +++++++++++++++-
> >  include/drm/drm_mode_config.h |  2 ++
> >  2 files changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> > index 0dd43882fe7c..16a7e3e57f7f 100644
> > --- a/drivers/gpu/drm/drm_plane.c
> > +++ b/drivers/gpu/drm/drm_plane.c
> > @@ -128,6 +128,11 @@
> >   *     pairs supported by this plane. The blob is a struct
> >   *     drm_format_modifier_blob. Without this property the plane doesn't
> >   *     support buffers with modifiers. Userspace cannot change this property.
> > + *
> > + *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
> > + *     capability for general modifier support. If this flag is set then every
> > + *     plane will have the IN_FORMATS property, even when it only supports
> > + *     DRM_FORMAT_MOD_LINEAR.
> 
> Ooh, that's even better. But isn't that changing the meaning of the
> cap? Isn't the cap older than IN_FORMATS?

Hm indeed. But also how exactly are you going to user modifiers without
IN_FORMATS ... it's a bit hard. I think this is all because we've enabled
modifiers piece-by-piece and never across the entire thing (e.g. with
compositor and protocols), so the missing pieces only became apparent
later on.

I'm not sure whether compositors really want to support this, I guess
worst case we could disable the cap on these old kernels.

> What about the opposite? Is it allowed to have even a single IN_FORMATS
> if you don't have the cap?

That direction is enforced since 5.1, because some drivers screwed it up
and confusion in userspace ensued.

Should I add a bug that on kernels older than 5.1 the situation is more
murky and there's lots of bugs?

> 
> >   */
> >  
> >  static unsigned int drm_num_planes(struct drm_device *dev)
> > @@ -277,8 +282,14 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> >  			format_modifier_count++;
> >  	}
> >  
> > -	if (format_modifier_count)
> > +	/* autoset the cap and check for consistency across all planes */
> > +	if (format_modifier_count) {
> > +		WARN_ON(!config->allow_fb_modifiers &&
> > +			!list_empty(&config->plane_list));
> 
> What does this mean?

If allow_fb_modifiers isn't set yet (we do that in the line below) and we
are _not_ the first plane that gets added to the driver (that's done
towards the end of the function) then that means there's already a plane
registered without modifiers and hence IN_FORMAT. Which we then warn
about.

> 
> >  		config->allow_fb_modifiers = true;
> > +	} else {
> > +		WARN_ON(config->allow_fb_modifiers);

This warning here checks the other case of an earlier plane with
modifiers, but the one we're adding now doesn't have them.
-Daniel

> > +	}
> >  
> >  	plane->modifier_count = format_modifier_count;
> >  	plane->modifiers = kmalloc_array(format_modifier_count,
> > @@ -360,6 +371,9 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> >   * drm_universal_plane_init() to let the DRM managed resource infrastructure
> >   * take care of cleanup and deallocation.
> >   *
> > + * Drivers supporting modifiers must set @format_modifiers on all their planes,
> > + * even those that only support DRM_FORMAT_MOD_LINEAR.
> 
> Good.
> 
> > + *
> >   * Returns:
> >   * Zero on success, error code on failure.
> >   */
> > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> > index ab424ddd7665..1ddf7783fdf7 100644
> > --- a/include/drm/drm_mode_config.h
> > +++ b/include/drm/drm_mode_config.h
> > @@ -909,6 +909,8 @@ struct drm_mode_config {
> >  	 * @allow_fb_modifiers:
> >  	 *
> >  	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> > +	 * Note that drivers should not set this directly, it is automatically
> > +	 * set in drm_universal_plane_init().
> >  	 *
> >  	 * IMPORTANT:
> >  	 *
> 
> Thanks,
> pq
Daniel Vetter April 13, 2021, 2:17 p.m. UTC | #4
On Tue, Apr 13, 2021 at 01:54:00PM +0200, Lucas Stach wrote:
> Am Dienstag, dem 13.04.2021 um 11:49 +0200 schrieb Daniel Vetter:
> > It's very confusing for userspace to have to deal with inconsistencies
> > here, and some drivers screwed this up a bit. Most just ommitted the
> > format list when they meant to say that only linear modifier is
> > allowed, but some also meant that only implied modifiers are
> > acceptable (because actually none of the planes registered supported
> > modifiers).
> 
> For a lot of the embedded display drivers that never had any out-of-
> band tiling meta shared with the GPU part, the implied modifier is
> actually DRM_FORMAT_MOD_LINEAR, so maybe that's where some of the
> confusion about needing to specify the modifier list comes from.

Yeah that entire discussion last few days is why I wanted to audit all the
drivers and make sure that going forward we're more explicit&consistent
with all this. There's huge confusion around implied modifier vs "no
IN_FORMATS blob property" and what that exactly means.
-Daniel

> > Now that this is all done consistently across all drivers, document
> > the rules and enforce it in the drm core.
> 
> This clarification looks good to me.
> 
> Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
> 
> > Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Maxime Ripard <mripard@kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: David Airlie <airlied@linux.ie>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > ---
> >  drivers/gpu/drm/drm_plane.c   | 16 +++++++++++++++-
> >  include/drm/drm_mode_config.h |  2 ++
> >  2 files changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> > index 0dd43882fe7c..16a7e3e57f7f 100644
> > --- a/drivers/gpu/drm/drm_plane.c
> > +++ b/drivers/gpu/drm/drm_plane.c
> > @@ -128,6 +128,11 @@
> >   *     pairs supported by this plane. The blob is a struct
> >   *     drm_format_modifier_blob. Without this property the plane doesn't
> >   *     support buffers with modifiers. Userspace cannot change this property.
> > + *
> > + *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
> > + *     capability for general modifier support. If this flag is set then every
> > + *     plane will have the IN_FORMATS property, even when it only supports
> > + *     DRM_FORMAT_MOD_LINEAR.
> >   */
> >  
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >  static unsigned int drm_num_planes(struct drm_device *dev)
> > @@ -277,8 +282,14 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> >  			format_modifier_count++;
> >  	}
> >  
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> > -	if (format_modifier_count)
> > +	/* autoset the cap and check for consistency across all planes */
> > +	if (format_modifier_count) {
> > +		WARN_ON(!config->allow_fb_modifiers &&
> > +			!list_empty(&config->plane_list));
> >  		config->allow_fb_modifiers = true;
> > +	} else {
> > +		WARN_ON(config->allow_fb_modifiers);
> > +	}
> >  
> > 
> > 
> > 
> > 
> > 
> > 
> > 
> >  	plane->modifier_count = format_modifier_count;
> >  	plane->modifiers = kmalloc_array(format_modifier_count,
> > @@ -360,6 +371,9 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> >   * drm_universal_plane_init() to let the DRM managed resource infrastructure
> >   * take care of cleanup and deallocation.
> >   *
> > + * Drivers supporting modifiers must set @format_modifiers on all their planes,
> > + * even those that only support DRM_FORMAT_MOD_LINEAR.
> > + *
> >   * Returns:
> >   * Zero on success, error code on failure.
> >   */
> > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> > index ab424ddd7665..1ddf7783fdf7 100644
> > --- a/include/drm/drm_mode_config.h
> > +++ b/include/drm/drm_mode_config.h
> > @@ -909,6 +909,8 @@ struct drm_mode_config {
> >  	 * @allow_fb_modifiers:
> >  	 *
> >  	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> > +	 * Note that drivers should not set this directly, it is automatically
> > +	 * set in drm_universal_plane_init().
> >  	 *
> >  	 * IMPORTANT:
> >  	 *
> 
>
Daniel Vetter April 13, 2021, 2:19 p.m. UTC | #5
On Tue, Apr 13, 2021 at 04:11:29PM +0200, Daniel Vetter wrote:
> On Tue, Apr 13, 2021 at 02:56:02PM +0300, Pekka Paalanen wrote:
> > On Tue, 13 Apr 2021 11:49:03 +0200
> > Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > 
> > > It's very confusing for userspace to have to deal with inconsistencies
> > > here, and some drivers screwed this up a bit. Most just ommitted the
> > > format list when they meant to say that only linear modifier is
> > > allowed, but some also meant that only implied modifiers are
> > > acceptable (because actually none of the planes registered supported
> > > modifiers).
> > > 
> > > Now that this is all done consistently across all drivers, document
> > > the rules and enforce it in the drm core.
> > > 
> > > Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > > Cc: Maxime Ripard <mripard@kernel.org>
> > > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > > Cc: David Airlie <airlied@linux.ie>
> > > Cc: Daniel Vetter <daniel@ffwll.ch>
> > > ---
> > >  drivers/gpu/drm/drm_plane.c   | 16 +++++++++++++++-
> > >  include/drm/drm_mode_config.h |  2 ++
> > >  2 files changed, 17 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> > > index 0dd43882fe7c..16a7e3e57f7f 100644
> > > --- a/drivers/gpu/drm/drm_plane.c
> > > +++ b/drivers/gpu/drm/drm_plane.c
> > > @@ -128,6 +128,11 @@
> > >   *     pairs supported by this plane. The blob is a struct
> > >   *     drm_format_modifier_blob. Without this property the plane doesn't
> > >   *     support buffers with modifiers. Userspace cannot change this property.
> > > + *
> > > + *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
> > > + *     capability for general modifier support. If this flag is set then every
> > > + *     plane will have the IN_FORMATS property, even when it only supports
> > > + *     DRM_FORMAT_MOD_LINEAR.
> > 
> > Ooh, that's even better. But isn't that changing the meaning of the
> > cap? Isn't the cap older than IN_FORMATS?
> 
> Hm indeed. But also how exactly are you going to user modifiers without
> IN_FORMATS ... it's a bit hard. I think this is all because we've enabled
> modifiers piece-by-piece and never across the entire thing (e.g. with
> compositor and protocols), so the missing pieces only became apparent
> later on.

Ok I worked git log -Gallow_fb_modifiers and there's 3 drivers which
enabled modifiers before IN_FORMATS was merged:
- i915
- msm/mdp4 (for the tiled NV12 format thing)
- tegra

> I'm not sure whether compositors really want to support this, I guess
> worst case we could disable the cap on these old kernels.
> 
> > What about the opposite? Is it allowed to have even a single IN_FORMATS
> > if you don't have the cap?
> 
> That direction is enforced since 5.1, because some drivers screwed it up
> and confusion in userspace ensued.
> 
> Should I add a bug that on kernels older than 5.1 the situation is more
> murky and there's lots of bugs?

I guess we should recommend to userspace that if they spot an
inconsistency between IN_FORMATS across planes and the cap then maybe they
want to disable modifier support because it might be all kinds of broken?
-Daniel

> 
> > 
> > >   */
> > >  
> > >  static unsigned int drm_num_planes(struct drm_device *dev)
> > > @@ -277,8 +282,14 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> > >  			format_modifier_count++;
> > >  	}
> > >  
> > > -	if (format_modifier_count)
> > > +	/* autoset the cap and check for consistency across all planes */
> > > +	if (format_modifier_count) {
> > > +		WARN_ON(!config->allow_fb_modifiers &&
> > > +			!list_empty(&config->plane_list));
> > 
> > What does this mean?
> 
> If allow_fb_modifiers isn't set yet (we do that in the line below) and we
> are _not_ the first plane that gets added to the driver (that's done
> towards the end of the function) then that means there's already a plane
> registered without modifiers and hence IN_FORMAT. Which we then warn
> about.
> 
> > 
> > >  		config->allow_fb_modifiers = true;
> > > +	} else {
> > > +		WARN_ON(config->allow_fb_modifiers);
> 
> This warning here checks the other case of an earlier plane with
> modifiers, but the one we're adding now doesn't have them.
> -Daniel
> 
> > > +	}
> > >  
> > >  	plane->modifier_count = format_modifier_count;
> > >  	plane->modifiers = kmalloc_array(format_modifier_count,
> > > @@ -360,6 +371,9 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> > >   * drm_universal_plane_init() to let the DRM managed resource infrastructure
> > >   * take care of cleanup and deallocation.
> > >   *
> > > + * Drivers supporting modifiers must set @format_modifiers on all their planes,
> > > + * even those that only support DRM_FORMAT_MOD_LINEAR.
> > 
> > Good.
> > 
> > > + *
> > >   * Returns:
> > >   * Zero on success, error code on failure.
> > >   */
> > > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> > > index ab424ddd7665..1ddf7783fdf7 100644
> > > --- a/include/drm/drm_mode_config.h
> > > +++ b/include/drm/drm_mode_config.h
> > > @@ -909,6 +909,8 @@ struct drm_mode_config {
> > >  	 * @allow_fb_modifiers:
> > >  	 *
> > >  	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> > > +	 * Note that drivers should not set this directly, it is automatically
> > > +	 * set in drm_universal_plane_init().
> > >  	 *
> > >  	 * IMPORTANT:
> > >  	 *
> > 
> > Thanks,
> > pq
> 
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
Simon Ser April 13, 2021, 3:22 p.m. UTC | #6
On Tuesday, April 13th, 2021 at 11:49 AM, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:

> + *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver

Prepend an ampersand like so and a hyperlink will be rendered:

    Note that userspace can check the &DRM_CAP_ADDFB2_MODIFIERS driver
Pekka Paalanen April 14, 2021, 7:45 a.m. UTC | #7
On Tue, 13 Apr 2021 16:11:29 +0200
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Tue, Apr 13, 2021 at 02:56:02PM +0300, Pekka Paalanen wrote:
> > On Tue, 13 Apr 2021 11:49:03 +0200
> > Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> >   
> > > It's very confusing for userspace to have to deal with inconsistencies
> > > here, and some drivers screwed this up a bit. Most just ommitted the
> > > format list when they meant to say that only linear modifier is
> > > allowed, but some also meant that only implied modifiers are
> > > acceptable (because actually none of the planes registered supported
> > > modifiers).
> > > 
> > > Now that this is all done consistently across all drivers, document
> > > the rules and enforce it in the drm core.
> > > 
> > > Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > > Cc: Maxime Ripard <mripard@kernel.org>
> > > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > > Cc: David Airlie <airlied@linux.ie>
> > > Cc: Daniel Vetter <daniel@ffwll.ch>
> > > ---
> > >  drivers/gpu/drm/drm_plane.c   | 16 +++++++++++++++-
> > >  include/drm/drm_mode_config.h |  2 ++
> > >  2 files changed, 17 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> > > index 0dd43882fe7c..16a7e3e57f7f 100644
> > > --- a/drivers/gpu/drm/drm_plane.c
> > > +++ b/drivers/gpu/drm/drm_plane.c
> > > @@ -128,6 +128,11 @@
> > >   *     pairs supported by this plane. The blob is a struct
> > >   *     drm_format_modifier_blob. Without this property the plane doesn't
> > >   *     support buffers with modifiers. Userspace cannot change this property.
> > > + *
> > > + *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
> > > + *     capability for general modifier support. If this flag is set then every
> > > + *     plane will have the IN_FORMATS property, even when it only supports
> > > + *     DRM_FORMAT_MOD_LINEAR.  
> > 
> > Ooh, that's even better. But isn't that changing the meaning of the
> > cap? Isn't the cap older than IN_FORMATS?  
> 
> Hm indeed. But also how exactly are you going to user modifiers without
> IN_FORMATS ... it's a bit hard.

Easy for at least one specific case, as Daniel Stone said in IRC. Use
GBM to allocate using the no-modifiers API but specify USE_LINEAR. That
basically gives you MOD_LINEAR buffer. Then you can try to make a DRM
FB for it using AddFB2-with-modifiers.

Does anyone do this, I have no idea.

Actually, I think this semantic change is fine. Old userspace did not
know that the cap means all planes have IN_FORMATS, so they can deal
with IN_FORMATS missing, but if it is never missing, no problem.

It could be a problem with new userspace and old kernel, but that's by
definition not a kernel bug, right? Just... inconvenient for userspace
as they can't make full use of the flag and need to keep the fallback
path for missing IN_FORMATS.

As long as there are KMS drivers that don't support modifiers, generic
userspace probably needs the fallback path anyway.

> I think this is all because we've enabled
> modifiers piece-by-piece and never across the entire thing (e.g. with
> compositor and protocols), so the missing pieces only became apparent
> later on.
> 
> I'm not sure whether compositors really want to support this, I guess
> worst case we could disable the cap on these old kernels.
> 
> > What about the opposite? Is it allowed to have even a single IN_FORMATS
> > if you don't have the cap?  
> 
> That direction is enforced since 5.1, because some drivers screwed it up
> and confusion in userspace ensued.
> 
> Should I add a bug that on kernels older than 5.1 the situation is more
> murky and there's lots of bugs?

Yes, that would help to set expectations.

I'm currently on Debian stable FWIW, so 4.19 based kernel with I don't
know what patches.

On Tue, 13 Apr 2021 16:19:10 +0200
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Tue, Apr 13, 2021 at 04:11:29PM +0200, Daniel Vetter wrote:
> > 
> > Should I add a bug that on kernels older than 5.1 the situation is more
> > murky and there's lots of bugs?  
> 
> I guess we should recommend to userspace that if they spot an
> inconsistency between IN_FORMATS across planes and the cap then maybe they
> want to disable modifier support because it might be all kinds of broken?

Yes please!


------

> >   
> > >   */
> > >  
> > >  static unsigned int drm_num_planes(struct drm_device *dev)
> > > @@ -277,8 +282,14 @@ static int __drm_universal_plane_init(struct drm_device *dev,
> > >  			format_modifier_count++;
> > >  	}
> > >  
> > > -	if (format_modifier_count)
> > > +	/* autoset the cap and check for consistency across all planes */
> > > +	if (format_modifier_count) {
> > > +		WARN_ON(!config->allow_fb_modifiers &&
> > > +			!list_empty(&config->plane_list));  
> > 
> > What does this mean?  
> 
> If allow_fb_modifiers isn't set yet (we do that in the line below) and we
> are _not_ the first plane that gets added to the driver (that's done
> towards the end of the function) then that means there's already a plane
> registered without modifiers and hence IN_FORMAT. Which we then warn
> about.

Ah, ok! Would have taken a while for me to decipher that, and
impossible with just this patch context.

> >   
> > >  		config->allow_fb_modifiers = true;
> > > +	} else {
> > > +		WARN_ON(config->allow_fb_modifiers);  
> 
> This warning here checks the other case of an earlier plane with
> modifiers, but the one we're adding now doesn't have them.

Cool.


Thanks,
pq
Maxime Ripard April 14, 2021, 8:52 a.m. UTC | #8
On Tue, Apr 13, 2021 at 11:49:03AM +0200, Daniel Vetter wrote:
> It's very confusing for userspace to have to deal with inconsistencies
> here, and some drivers screwed this up a bit. Most just ommitted the
> format list when they meant to say that only linear modifier is
> allowed, but some also meant that only implied modifiers are
> acceptable (because actually none of the planes registered supported
> modifiers).
> 
> Now that this is all done consistently across all drivers, document
> the rules and enforce it in the drm core.
> 
> Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>

Acked-by: Maxime Ripard <maxime@cerno.tech>

Maxime
diff mbox series

Patch

diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
index 0dd43882fe7c..16a7e3e57f7f 100644
--- a/drivers/gpu/drm/drm_plane.c
+++ b/drivers/gpu/drm/drm_plane.c
@@ -128,6 +128,11 @@ 
  *     pairs supported by this plane. The blob is a struct
  *     drm_format_modifier_blob. Without this property the plane doesn't
  *     support buffers with modifiers. Userspace cannot change this property.
+ *
+ *     Note that userspace can check the DRM_CAP_ADDFB2_MODIFIERS driver
+ *     capability for general modifier support. If this flag is set then every
+ *     plane will have the IN_FORMATS property, even when it only supports
+ *     DRM_FORMAT_MOD_LINEAR.
  */
 
 static unsigned int drm_num_planes(struct drm_device *dev)
@@ -277,8 +282,14 @@  static int __drm_universal_plane_init(struct drm_device *dev,
 			format_modifier_count++;
 	}
 
-	if (format_modifier_count)
+	/* autoset the cap and check for consistency across all planes */
+	if (format_modifier_count) {
+		WARN_ON(!config->allow_fb_modifiers &&
+			!list_empty(&config->plane_list));
 		config->allow_fb_modifiers = true;
+	} else {
+		WARN_ON(config->allow_fb_modifiers);
+	}
 
 	plane->modifier_count = format_modifier_count;
 	plane->modifiers = kmalloc_array(format_modifier_count,
@@ -360,6 +371,9 @@  static int __drm_universal_plane_init(struct drm_device *dev,
  * drm_universal_plane_init() to let the DRM managed resource infrastructure
  * take care of cleanup and deallocation.
  *
+ * Drivers supporting modifiers must set @format_modifiers on all their planes,
+ * even those that only support DRM_FORMAT_MOD_LINEAR.
+ *
  * Returns:
  * Zero on success, error code on failure.
  */
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index ab424ddd7665..1ddf7783fdf7 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -909,6 +909,8 @@  struct drm_mode_config {
 	 * @allow_fb_modifiers:
 	 *
 	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
+	 * Note that drivers should not set this directly, it is automatically
+	 * set in drm_universal_plane_init().
 	 *
 	 * IMPORTANT:
 	 *