On 2022-11-08 10:51, Catalin Marinas wrote:
On Mon, Nov 07, 2022 at 01:26:21PM +0000, Robin Murphy wrote:
On 2022-11-07 10:54, Catalin Marinas wrote:
On Mon, Nov 07, 2022 at 10:46:03AM +0100, Christoph Hellwig wrote:
+static inline bool dma_sg_kmalloc_needs_bounce(struct device *dev,
+ struct scatterlist *sg, int nents,
+ enum dma_data_direction dir)
+{
+ struct scatterlist *s;
+ int i;
+
+ if (!IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) ||
+ dir == DMA_TO_DEVICE || dev_is_dma_coherent(dev))
+ return false;
This part should be shared with dma-direct in a well documented helper.
+ for_each_sg(sg, s, nents, i) {
+ if (dma_kmalloc_needs_bounce(dev, s->length, dir))
+ return true;
+ }
And for this loop iteration I'd much prefer it to be out of line, and
also not available in a global helper.
But maybe someone can come up with a nice tweak to the dma-iommu
code to not require the extra sglist walk anyway.
An idea: we could add another member to struct scatterlist to track the
bounced address. We can then do the bouncing in a similar way to
iommu_dma_map_sg_swiotlb() but without the iova allocation. The latter
would be a common path for both the bounced and non-bounced cases.
FWIW I spent a little time looking at this as well; I'm pretty confident
it can be done without the extra walk if the iommu-dma bouncing is
completely refactored (and it might want a SWIOTLB helper to retrieve
the original page from a bounced address).
Doesn't sg_page() provide the original page already? Either way, the
swiotlb knows it as it needs to do the copying between buffers.
For the part where we temporarily rewrite the offsets and lengths to
pass to iommu_map_sg(), we'd also have to swizzle any relevant page
pointers so that that picks up the physical addresses of the bounce
buffer slots rather than the original pages, but then we need to put
them back straight afterwards. Since SWIOTLB keeps track of that
internally, it'll be a lot neater and more efficient to simply ask for
it than to allocate more temporary storage to remember it independently
(like I did for that horrible erratum thing to keep it self-contained).
That's going to be a bigger
job than I'll be able to finish this cycle, and I concluded that this
in-between approach wouldn't be worth posting for its own sake, but as
part of this series I think it's a reasonable compromise.
I'll drop my hack once you have something. Happy to carry it as part of
this series.
Cool, I can't promise how soon I'll get there, but like I said if all
the other objections are worked out in the meantime I have no issue with
landing this approach and improving on it later.
Thanks,
Robin.
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 375a5e90d86a..87aaf8b5cdb4 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -16,7 +16,7 @@ struct scatterlist {
#ifdef CONFIG_NEED_SG_DMA_LENGTH
unsigned int dma_length;
#endif
-#ifdef CONFIG_PCI_P2PDMA
+#ifdef CONFIG_NEED_SG_DMA_FLAGS
unsigned int dma_flags;
#endif
I initially had something similar but I decided it's overkill for a
patch that I expected to be NAK'ed.
I'll include your patch in my series in the meantime.
