Re: [PATCH v3 1/2] media: videobuf2: Fix dmabuf cache sync/flush in dma-contig

[Mikhail Rudenko <mike.rudenko@gmail.com>]

On 2025-02-28 at 19:25 +09, Tomasz Figa <tfiga@chromium.org> wrote:

> On Fri, Feb 28, 2025 at 12:30 PM Nicolas Dufresne <nicolas@ndufresne.ca> wrote:
>>
>> Le mardi 28 janvier 2025 à 23:35 +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@vger.kernel.org
>> > Signed-off-by: Mikhail Rudenko <mike.rudenko@gmail.com>
>> > ---
>> >  .../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
>> > bb0b7fa67b539aa73ad5ccf3c3bc318e26f8a4cb..146d7997a0da5989fb081a6f28c
>> > e0641fe726e63 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);
>>
>> Am I correct that this is mostly to prevent the kernel from reading
>> back old data from the cache after an application or other driver did
>> CPU writes ? If so, can't we restrict that to DMA_TO_DEVICE and
>> DMA_BIDIRECTIONAL ?
>
> Note that this function must also synchronize between the user-space
> and kernel mappings, where the DMA direction doesn't really matter.
> Also it's unlikely for it to be called when not needed - why would one
> begin a CPU access before the DMA, when the DMA is FROM_DEVICE?
>
>>
>> As for pending kernel writes, they should have been flushed before the
>> buffer is made available for dequeue.
>
> There is no implicit flushing for imported DMA-bufs. All the flushing
> needs to be executed directly by the CPU accessors by surrounding the
> access with begin and end CPU access, be it in the kernel or
> userspace.
>
>> And any access while a buffer is
>> queued is concurrent access, which is expected to have undefined
>> behaviour.
>>
>
> Correct.
>
>> > +
>> > +     dma_sync_sgtable_for_cpu(buf->dev, sgt, buf->dma_dir);
>>
>> Isn't there a link to make between buf->dma_dir and direcction before
>> calling this ? Also, shouldn't we use direction insead of buf->dma_dir
>> to possibly limit the scope ?
>
> Oh, yes, that's a good catch.

Indeed, will fix this in v4.

> It should be |direction| passed here and
> not |buf->dma_dir|, since the former determines what CPU access will
> be done.
>
>>
>> > +
>> >       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, buf->dma_dir);
>>
>> Similar questions for the end_cpu_access implementation.
>
> Yeah, same here.

Noted, will fix.

>>
>> Nicolas
>>
>> > +
>> >       return 0;
>> >  }
>> >
>> >
>>


--
Best regards,
Mikhail Rudenko