On Tue, Mar 4, 2025 at 12:24 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote:
>
> Hi Mikhail,
>
> Le lundi 03 mars 2025 à 14:40 +0300, Mikhail Rudenko a écrit :
> > When support for V4L2_FLAG_MEMORY_NON_CONSISTENT was removed in
> > commit 129134e5415d ("media: media/v4l2: remove
> > V4L2_FLAG_MEMORY_NON_CONSISTENT flag"),
> > vb2_dc_dmabuf_ops_{begin,end}_cpu_access() functions were made
> > no-ops. Later, when support for V4L2_MEMORY_FLAG_NON_COHERENT was
> > introduced in commit c0acf9cfeee0 ("media: videobuf2: handle
> > V4L2_MEMORY_FLAG_NON_COHERENT flag"), the above functions remained
> > no-ops, making cache maintenance for non-coherent dmabufs allocated
> > by
> > dma-contig impossible.
> >
> > Fix this by reintroducing dma_sync_sgtable_for_{cpu,device} and
> > {flush,invalidate}_kernel_vmap_range calls to
> > vb2_dc_dmabuf_ops_{begin,end}_cpu_access() functions for non-coherent
> > buffers.
> >
> > Fixes: c0acf9cfeee0 ("media: videobuf2: handle
> > V4L2_MEMORY_FLAG_NON_COHERENT flag")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Mikhail Rudenko <mike.rudenko@xxxxxxxxx>
> > ---
> > .../media/common/videobuf2/videobuf2-dma-contig.c | 22
> > ++++++++++++++++++++++
> > 1 file changed, 22 insertions(+)
> >
> > diff --git a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> > b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> > index
> > a13ec569c82f6da2d977222b94af32e74c6c6c82..d41095fe5bd21faf815d6b035d7
> > bc888a84a95d5 100644
> > --- a/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> > +++ b/drivers/media/common/videobuf2/videobuf2-dma-contig.c
> > @@ -427,6 +427,17 @@ static int
> > vb2_dc_dmabuf_ops_begin_cpu_access(struct dma_buf *dbuf,
> > enum dma_data_direction
> > direction)
> > {
> > + struct vb2_dc_buf *buf = dbuf->priv;
> > + struct sg_table *sgt = buf->dma_sgt;
> > +
> > + if (!buf->non_coherent_mem)
> > + return 0;
> > +
> > + if (buf->vaddr)
> > + invalidate_kernel_vmap_range(buf->vaddr, buf->size);
>
> What would make me a lot more confortable with this change is if you
> enable kernel mappings for one test. This will ensure you cover the
> call to "invalidate" in your testing. I'd like to know about the
> performance impact. With this implementation it should be identical to
> the VB2 one.
I agree that it would be good to test that path as well. I wonder if
we could somehow do it with one of the vi* drivers...
>
> What I was trying to say in previous comments, is that my impression is
> that we can skip this for CPU read access, since we don't guaranty
> concurrent access anyway. Both address space can keep their cache in
> that case. Though, I see RKISP does not use kernel mapping plus I'm not
> reporting a bug, but checking if we should leave a comment for possible
> users of kernel mapping in the future ?
We can't skip it for CPU read access, because it may be the first read
after the DMA writing to the buffer, so we need to invalidate the
caches.
That said, on majority of systems this will be a no-op, because it
only applies to VIVT and VIPT aliasing caches + only when the kernel
mapping is actually used (the buf->vaddr mapping is created on
demand).
>
> > +
> > + dma_sync_sgtable_for_cpu(buf->dev, sgt, direction);
> > +
> > return 0;
> > }
> >
> > @@ -434,6 +445,17 @@ static int
> > vb2_dc_dmabuf_ops_end_cpu_access(struct dma_buf *dbuf,
> > enum dma_data_direction direction)
> > {
> > + struct vb2_dc_buf *buf = dbuf->priv;
> > + struct sg_table *sgt = buf->dma_sgt;
> > +
> > + if (!buf->non_coherent_mem)
> > + return 0;
> > +
> > + if (buf->vaddr)
> > + flush_kernel_vmap_range(buf->vaddr, buf->size);
> > +
> > + dma_sync_sgtable_for_device(buf->dev, sgt, direction);
> > +
> > return 0;
> > }
> >
> >
>
