Re: [RESEND PATCH] media: vb2: Fix vb2_dc_prepare do not correct sync data to device

[tiffany lin]

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
> 
> >  
> >  /*********************************************/
> > 


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