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: > > 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@xxxxxxxxxxxx> > > --- > > 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 > > > > > /*********************************************/ > > -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
