On Thu, Apr 09, 2015 at 04:34:08PM +0200, Thierry Reding wrote:
diff --git a/drivers/gpu/drm/armada/armada_gem.c b/drivers/gpu/drm/armada/armada_gem.c index 580e10acaa3a..c2d4414031ab 100644 --- a/drivers/gpu/drm/armada/armada_gem.c +++ b/drivers/gpu/drm/armada/armada_gem.c @@ -453,19 +453,14 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach, sg_set_page(sg, page, PAGE_SIZE, 0); }
if (dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir) == 0) {num = sgt->nents;goto release;}
drm_clflush_sg(sgt);} else if (dobj->page) { /* Single contiguous page */ if (sg_alloc_table(sgt, 1, GFP_KERNEL)) goto free_sgt;
sg_set_page(sgt->sgl, dobj->page, dobj->obj.size, 0);
if (dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir) == 0)goto free_table;
drm_clflush_sg(sgt);@@ -480,7 +475,6 @@ armada_gem_prime_map_dma_buf(struct dma_buf_attachment *attach, release: for_each_sg(sgt->sgl, sg, num, i) page_cache_release(sg_page(sg));
- free_table: sg_free_table(sgt); free_sgt: kfree(sgt);
@@ -494,9 +488,6 @@ static void armada_gem_prime_unmap_dma_buf(struct dma_buf_attachment *attach, struct armada_gem_object *dobj = drm_to_armada_gem(obj); int i;
- if (!dobj->linear)
dma_unmap_sg(attach->dev, sgt->sgl, sgt->nents, dir);
I'm really wonder where this is the right thing to do.
DMA coherency on ARMv6 and ARMv7 CPUs is not just a case of "do something just before DMA" - it's more complicated than that because of the speculative prefetching.
What you must remember is this:
Any memory which is readable to the CPU may be speculatively prefetched by the CPU, and cache lines allocated into the L1 and L2 caches.
What this means is that if you're doing this:
Flush caches Perform DMA to buffer Read buffer from CPU
You may or may not see the data you expect in the buffer - it's indeterminant, depending on how aggressive the CPU has been at prefetching data.