Re: [PATCH 2/3] dma: Introduce dma_max_mapping_size()

[Joerg Roedel]

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
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Re: [PATCH 2/3] dma: Introduce dma_max_mapping_size()

[Christoph Hellwig]

> +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.
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[PATCH 2/3] dma: Introduce dma_max_mapping_size()

[Joerg Roedel]

From: Joerg Roedel 

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 
---
 include/linux/dma-mapping.h | 16 
 kernel/dma/direct.c | 10 ++
 2 files changed, 26 insertions(+)

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 
 
@@ -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);
+}
-- 
2.17.1

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