[2/3] dma: Introduce dma_max_mapping_size()

Commit Message

Joerg Roedel Jan. 15, 2019, 1:22 p.m. UTC

From: Joerg Roedel <[e-mail]>

The function returns the maximum size that can be mapped
using DMA-API functions. The patch also adds the
implementation for direct DMA and a new dma_map_ops pointer
so that other implementations can expose their limit.

Signed-off-by: Joerg Roedel <[e-mail]>
---
 include/linux/dma-mapping.h | 16 ++++++++++++++++
 kernel/dma/direct.c         | 10 ++++++++++
 2 files changed, 26 insertions(+)

Comments

Christoph Hellwig Jan. 15, 2019, 1:37 p.m. UTC | #1

> +size_t dma_direct_max_mapping_size(struct device *dev)
> +{
> +	/*
> +	 * Return the minimum of the direct DMA limit and the SWIOTLB limit.
> +	 * Since direct DMA has no limit, it is fine to just return the SWIOTLB
> +	 * limit.
> +	 */
> +	return swiotlb_max_mapping_size(dev);

Well, if we don't actually use the swiotlb buffers despite it being
compiled in or even allocated we don't need the limit.

Joerg Roedel Jan. 15, 2019, 4:23 p.m. UTC | #2

On Tue, Jan 15, 2019 at 02:37:54PM +0100, Christoph Hellwig wrote:
> > +size_t dma_direct_max_mapping_size(struct device *dev)
> > +{
> > +	/*
> > +	 * Return the minimum of the direct DMA limit and the SWIOTLB limit.
> > +	 * Since direct DMA has no limit, it is fine to just return the SWIOTLB
> > +	 * limit.
> > +	 */
> > +	return swiotlb_max_mapping_size(dev);
> 
> Well, if we don't actually use the swiotlb buffers despite it being
> compiled in or even allocated we don't need the limit.

Right, I thought about that too, but didn't find a generic way to check
for all the cases. There are various checks that could be done:

	1) Check if SWIOTLB is initialized at all, if not, return
	   SIZE_MAX as the limit. This can't be checked from dma-direct
	   code right now, but could be easily implemented.

	2) Check for swiotlb=force needs to be done.

	3) Check whether the device can access all of available RAM. I
	   have no idea how to check that in an architecture independent
	   way. It also has to take memory hotplug into account as well
	   as the DMA mask of the device.

	   An easy approximation could be to omit the limit if the
	   dma-mask covers all of the physical address bits available
	   on the platform. It would require to pass the dma-mask as an
	   additional parameter like it is done in dma_supported().

Any better ideas for how to implement 3)?

Regards,

	Joerg

Christoph Hellwig Jan. 15, 2019, 5:54 p.m. UTC | #3

On Tue, Jan 15, 2019 at 05:23:22PM +0100, Joerg Roedel wrote:
> Right, I thought about that too, but didn't find a generic way to check
> for all the cases. There are various checks that could be done:
> 
> 	1) Check if SWIOTLB is initialized at all, if not, return
> 	   SIZE_MAX as the limit. This can't be checked from dma-direct
> 	   code right now, but could be easily implemented.

Yes, this is the low hanging fruit.

> 	2) Check for swiotlb=force needs to be done.
> 
> 	3) Check whether the device can access all of available RAM. I
> 	   have no idea how to check that in an architecture independent
> 	   way. It also has to take memory hotplug into account as well
> 	   as the DMA mask of the device.
> 
> 	   An easy approximation could be to omit the limit if the
> 	   dma-mask covers all of the physical address bits available
> 	   on the platform. It would require to pass the dma-mask as an
> 	   additional parameter like it is done in dma_supported().
> 
> Any better ideas for how to implement 3)?

And yeah, this is hard.  So I'd just go for the low hanging fruit
for now and only implement 1) with a comment mentioning that
we are a little pessimistic.

Patch

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index f6ded992c183..a3ca8a71a704 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h
@@ -130,6 +130,7 @@  struct dma_map_ops {
 			enum dma_data_direction direction);
 	int (*dma_supported)(struct device *dev, u64 mask);
 	u64 (*get_required_mask)(struct device *dev);
+	size_t (*max_mapping_size)(struct device *dev);
 };
 
 #define DMA_MAPPING_ERROR		(~(dma_addr_t)0)
@@ -257,6 +258,8 @@  static inline void dma_direct_sync_sg_for_cpu(struct device *dev,
 }
 #endif
 
+size_t dma_direct_max_mapping_size(struct device *dev);
+
 #ifdef CONFIG_HAS_DMA
 #include <asm/dma-mapping.h>
 
@@ -440,6 +443,19 @@  static inline int dma_mapping_error(struct device *dev, dma_addr_t dma_addr)
 	return 0;
 }
 
+static inline size_t dma_max_mapping_size(struct device *dev)
+{
+	const struct dma_map_ops *ops = get_dma_ops(dev);
+	size_t size = SIZE_MAX;
+
+	if (dma_is_direct(ops))
+		size = dma_direct_max_mapping_size(dev);
+	else if (ops && ops->max_mapping_size)
+		size = ops->max_mapping_size(dev);
+
+	return size;
+}
+
 void *dma_alloc_attrs(struct device *dev, size_t size, dma_addr_t *dma_handle,
 		gfp_t flag, unsigned long attrs);
 void dma_free_attrs(struct device *dev, size_t size, void *cpu_addr,
diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 355d16acee6d..84917e1003c4 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -380,3 +380,13 @@  int dma_direct_supported(struct device *dev, u64 mask)
 	 */
 	return mask >= __phys_to_dma(dev, min_mask);
 }
+
+size_t dma_direct_max_mapping_size(struct device *dev)
+{
+	/*
+	 * Return the minimum of the direct DMA limit and the SWIOTLB limit.
+	 * Since direct DMA has no limit, it is fine to just return the SWIOTLB
+	 * limit.
+	 */
+	return swiotlb_max_mapping_size(dev);
+}