Messages in this thread | | | Subject | Re: [RESEND PATCH] media: vb2: Fix vb2_dc_prepare do not correct sync data to device | From | Robin Murphy <> | Date | Tue, 22 Sep 2015 16:37:17 +0100 |
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Hi Hans,
On 21/09/15 14:13, Hans Verkuil wrote: > Hi Tiffany! > > On 21-09-15 14:26, Tiffany Lin wrote: >> vb2_dc_prepare use the number of SG entries dma_map_sg_attrs return. >> But in dma_sync_sg_for_device, it use lengths of each SG entries >> before dma_map_sg_attrs. dma_map_sg_attrs will concatenate >> SGs until dma length > dma seg bundary. sgt->nents will less than >> sgt->orig_nents. Using SG entries after dma_map_sg_attrs >> in vb2_dc_prepare will make some SGs are not sync to device. >> After add DMA_ATTR_SKIP_CPU_SYNC in vb2_dc_get_userptr to remove >> sync data to device twice. Device randomly get incorrect data because >> some SGs are not sync to device. Change to use number of SG entries >> before dma_map_sg_attrs in vb2_dc_prepare to prevent this issue. >> >> Signed-off-by: Tiffany Lin <tiffany.lin@mediatek.com> >> --- >> drivers/media/v4l2-core/videobuf2-dma-contig.c | 4 ++-- >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/drivers/media/v4l2-core/videobuf2-dma-contig.c b/drivers/media/v4l2-core/videobuf2-dma-contig.c >> index 2397ceb..c5d00bd 100644 >> --- a/drivers/media/v4l2-core/videobuf2-dma-contig.c >> +++ b/drivers/media/v4l2-core/videobuf2-dma-contig.c >> @@ -100,7 +100,7 @@ static void vb2_dc_prepare(void *buf_priv) >> if (!sgt || buf->db_attach) >> return; >> >> - dma_sync_sg_for_device(buf->dev, sgt->sgl, sgt->nents, buf->dma_dir); >> + dma_sync_sg_for_device(buf->dev, sgt->sgl, sgt->orig_nents, buf->dma_dir); >> } >> >> static void vb2_dc_finish(void *buf_priv) >> @@ -112,7 +112,7 @@ static void vb2_dc_finish(void *buf_priv) >> if (!sgt || buf->db_attach) >> return; >> >> - dma_sync_sg_for_cpu(buf->dev, sgt->sgl, sgt->nents, buf->dma_dir); >> + dma_sync_sg_for_cpu(buf->dev, sgt->sgl, sgt->orig_nents, buf->dma_dir); >> } > > I don't really understand it. I am assuming that this happens on an arm and that > the dma_map_sg_attrs and dma_sync_sg_* functions used are arm_iommu_map_sg() and > arm_iommu_sync_sg_* as implemented in arch/arm/mm/dma-mapping.c. > > Now, as I understand it (and my understanding may very well be flawed!) the map_sg > function concatenates SG entries if possible, so it may return fewer entries. But > the dma_sync_sg functions use those updated SG entries, so the full buffer should > be covered by this. Using orig_nents will actually sync parts of the buffer twice! > The first nents entries already cover the full buffer so any remaining entries up > to orig_nents will just duplicate parts of the buffer.
As Documentation/DMA-API.txt says, the parameters to dma_sync_sg_* must be the same as those originally passed into dma_map_sg. The segments are only merged *from the point of view of the device*: if I have a scatterlist of two discontiguous 4K segments, I can remap them with an IOMMU so the device sees them as a single 8K buffer, and tell it as such. If on the other hand I want to do maintenance from the CPU side (i.e. any DMA API call), then those DMA addresses mean nothing and I can only operate on the CPU addresses of the underlying pages, which are still very much discontiguous in the linear map; ergo I still need to iterate over the original entries.
Whilst I can't claim much familiarity with v4l itself, from a brief look over the existing code this patch does look to be doing the right thing.
Robin.
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