Hi Hans,

On Mon, 2015-09-21 at 15:13 +0200, Hans Verkuil wrote:

Hi Tiffany!

On 21-09-15 14:26, Tiffany Lin wrote:

quoted

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.

We are using __iommu_* implemented in "arch/arm64/mm/dma-mapping.c" in
review patch
http://lists.linuxfoundation.org/pipermail/iommu/2015-July/013898.html
Without patch "[media] vb2: use dma_map_sg_attrs to prevent unnecessary
sync", vb2 will sync data to device twice.
One is from "dma_map_sg" in "vb2_dc_get_userptr", the other is from
"dma_sync_sg_for_device" in "vb2_dc_prepare". dma_map_sg use orig_nents,
and dma_sync_sg_for_device use nents."

We do not run in 32bits mode, but check "arm_dma_sync_sg_for_device" in
"arch/arm/mm/dma-mapping.c", 
ops->sync_single_for_device(dev, sg_dma_address(s), s->length, dir);
It looks like has same issue.

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.

I found that in __iommu_sync_sg_for_device, it use sg->length , do not
cover full buffer.
By adding log in " __iommu_sync_sg_for_device" without patch "[media]
vb2: use dma_map_sg_attrs to prevent unnecessary sync", we could see
total synced size are different between called from dma_map_sg and
dma_sync_sg_for_device.
__iommu_sync_sg_for_device called from dma_sync_sg_for_device got
updated SG entries number but it use un-updated sg length.
After using "DMA_ATTR_SKIP_CPU_SYNC" to skip sync in vb2_dc_get_userptr,
we got some part of the buffer not sync.

So this patch makes no sense in the current code.

If I understand your log text correctly this patch goes on top of Sakari Ailus' vb2
sync patch series. So if it wasn't needed before, but it is needed after his patch
series, then the problem is in that patch series.

This patch goes on top of these two patchs
https://www.mail-archive.com/linux-media%40vger.kernel.org/msg82143.html
http://lists.linuxfoundation.org/pipermail/iommu/2015-July/013898.html

In any case, I need some help understanding this patch.

And *if* this patch is correct, then the same thing should likely be done for
videobuf2-dma-sg.c.

Yes, if this patch correct, same thing should be done for
videobuf2-dma-sg.c

Regards,

	Hans

quoted

 
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