[PATCH] media: vb2: always set buffer cache sync hints

[PATCH] media: vb2: always set buffer cache sync hints

[Sergey Senozhatsky]

We need to always set ->need_cache_sync_on_prepare and
->need_cache_sync_on_finish when we initialize vb2 buffer.

Currently these flags are set/adjusted only in V4L2's
vb2_queue_or_prepare_buf(), which means that for the code
paths that don't use V4L2 vb2 will always tell videobuf2
core to skip ->prepare() and ->finish() cache syncs/flushes.

This is a quick solution that should do the trick. The
proper fix, however, is much more complicated and requires
a rather big videobuf2 refactoring - we need to move cache
sync/flush decision making out of core videobuf2 to the
allocators.

Reported-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/media/common/videobuf2/videobuf2-core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index 5499013cf82e..14a26888a892 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -414,6 +414,8 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
 		vb->index = q->num_buffers + buffer;
 		vb->type = q->type;
 		vb->memory = memory;
+		vb->need_cache_sync_on_prepare = 1;
+		vb->need_cache_sync_on_finish = 1;
 		for (plane = 0; plane < num_planes; ++plane) {
 			vb->planes[plane].length = plane_sizes[plane];
 			vb->planes[plane].min_length = plane_sizes[plane];
-- 
2.29.2

Re: [PATCH] media: vb2: always set buffer cache sync hints

[Sergey Senozhatsky]

On (20/11/27 18:41), Sergey Senozhatsky wrote:
> We need to always set ->need_cache_sync_on_prepare and
> ->need_cache_sync_on_finish when we initialize vb2 buffer.
> 
> Currently these flags are set/adjusted only in V4L2's
> vb2_queue_or_prepare_buf(), which means that for the code
> paths that don't use V4L2 vb2 will always tell videobuf2
> core to skip ->prepare() and ->finish() cache syncs/flushes.
> 
> This is a quick solution that should do the trick. The
> proper fix, however, is much more complicated and requires
> a rather big videobuf2 refactoring - we need to move cache
> sync/flush decision making out of core videobuf2 to the
> allocators.
> 
> Reported-by: Tomasz Figa <tfiga@chromium.org>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>

Fixes: f5f5fa73fbfb ("media: videobuf2: handle V4L2 buffer cache flags")

	-ss

Re: [PATCH] media: vb2: always set buffer cache sync hints

[Hans Verkuil]

On 27/11/2020 10:41, Sergey Senozhatsky wrote:
> We need to always set ->need_cache_sync_on_prepare and
> ->need_cache_sync_on_finish when we initialize vb2 buffer.
> 
> Currently these flags are set/adjusted only in V4L2's
> vb2_queue_or_prepare_buf(), which means that for the code
> paths that don't use V4L2 vb2 will always tell videobuf2
> core to skip ->prepare() and ->finish() cache syncs/flushes.
> 
> This is a quick solution that should do the trick. The
> proper fix, however, is much more complicated and requires
> a rather big videobuf2 refactoring - we need to move cache
> sync/flush decision making out of core videobuf2 to the
> allocators.
> 
> Reported-by: Tomasz Figa <tfiga@chromium.org>
> Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
> ---
>  drivers/media/common/videobuf2/videobuf2-core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 5499013cf82e..14a26888a892 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -414,6 +414,8 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
>  		vb->index = q->num_buffers + buffer;
>  		vb->type = q->type;
>  		vb->memory = memory;
> +		vb->need_cache_sync_on_prepare = 1;
> +		vb->need_cache_sync_on_finish = 1;

I think this needs a comment, basically explaining what you said in
the commit log. It's not obvious from the code that this is a
workaround.

Regards,

	Hans

>  		for (plane = 0; plane < num_planes; ++plane) {
>  			vb->planes[plane].length = plane_sizes[plane];
>  			vb->planes[plane].min_length = plane_sizes[plane];
> 

Re: [PATCH] media: vb2: always set buffer cache sync hints

[Sergey Senozhatsky]

On (20/11/27 13:59), Hans Verkuil wrote:
> 
> I think this needs a comment, basically explaining what you said in
> the commit log. It's not obvious from the code that this is a
> workaround.

Something like this?

---

diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
index 5499013cf82e..21b2b0ae3629 100644
--- a/drivers/media/common/videobuf2/videobuf2-core.c
+++ b/drivers/media/common/videobuf2/videobuf2-core.c
@@ -414,6 +414,16 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
                vb->index = q->num_buffers + buffer;
                vb->type = q->type;
                vb->memory = memory;
+               /*
+                * A workaround fix. We need to set these flags here so that
+                * videobuf2 core will always call ->prepare()/->finish()
+                * cache sync/flush on vb2 buffers. Otherwise, for backends
+                * that don't rely on V4L2 (perhaps dvb) these flags will
+                * always be false and, hence, videobuf2 core will skip cache
+                * sync/flush operations.
+                */
+               vb->need_cache_sync_on_prepare = 1;
+               vb->need_cache_sync_on_finish = 1;
                for (plane = 0; plane < num_planes; ++plane) {
                        vb->planes[plane].length = plane_sizes[plane];
                        vb->planes[plane].min_length = plane_sizes[plane];

Re: [PATCH] media: vb2: always set buffer cache sync hints

[Hans Verkuil]

On 27/11/2020 15:30, Sergey Senozhatsky wrote:
> On (20/11/27 13:59), Hans Verkuil wrote:
>>
>> I think this needs a comment, basically explaining what you said in
>> the commit log. It's not obvious from the code that this is a
>> workaround.
> 
> Something like this?

Yes.

BTW, wouldn't it be sufficient to change this code to:

	if (!q->allow_cache_hints && q->memory != VB2_MEMORY_DMABUF) {
		vb->need_cache_sync_on_prepare = 1;
		vb->need_cache_sync_on_finish = 1;
	}

Or am I missing something?

DVB drivers do not set this flag, and even for DVB cache sync shouldn't be
needed for DMABUF, right?

Regards,

	Hans

> 
> ---
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 5499013cf82e..21b2b0ae3629 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -414,6 +414,16 @@ static int __vb2_queue_alloc(struct vb2_queue *q, enum vb2_memory memory,
>                 vb->index = q->num_buffers + buffer;
>                 vb->type = q->type;
>                 vb->memory = memory;
> +               /*
> +                * A workaround fix. We need to set these flags here so that
> +                * videobuf2 core will always call ->prepare()/->finish()
> +                * cache sync/flush on vb2 buffers. Otherwise, for backends
> +                * that don't rely on V4L2 (perhaps dvb) these flags will
> +                * always be false and, hence, videobuf2 core will skip cache
> +                * sync/flush operations.
> +                */
> +               vb->need_cache_sync_on_prepare = 1;
> +               vb->need_cache_sync_on_finish = 1;
>                 for (plane = 0; plane < num_planes; ++plane) {
>                         vb->planes[plane].length = plane_sizes[plane];
>                         vb->planes[plane].min_length = plane_sizes[plane];
> 

Re: [PATCH] media: vb2: always set buffer cache sync hints

[Sergey Senozhatsky]

On (20/11/27 15:56), Hans Verkuil wrote:
> Yes.
> 
> BTW, wouldn't it be sufficient to change this code to:
> 
> 	if (!q->allow_cache_hints && q->memory != VB2_MEMORY_DMABUF) {
> 		vb->need_cache_sync_on_prepare = 1;
> 		vb->need_cache_sync_on_finish = 1;
> 	}

I think it would be sufficient.

	-ss

Re: [PATCH] media: vb2: always set buffer cache sync hints

[Tomasz Figa]

On Sat, Nov 28, 2020 at 1:35 AM Sergey Senozhatsky
<sergey.senozhatsky@gmail.com> wrote:
>
> On (20/11/27 15:56), Hans Verkuil wrote:
> > Yes.
> >
> > BTW, wouldn't it be sufficient to change this code to:
> >
> >       if (!q->allow_cache_hints && q->memory != VB2_MEMORY_DMABUF) {
> >               vb->need_cache_sync_on_prepare = 1;
> >               vb->need_cache_sync_on_finish = 1;
> >       }
>
> I think it would be sufficient.

Does it matter at this point if allow_cache_hints is set or not?

Best regards,
Tomasz

Re: [PATCH] media: vb2: always set buffer cache sync hints

[Sergey Senozhatsky]

On (20/11/28 01:50), Tomasz Figa wrote:
> On Sat, Nov 28, 2020 at 1:35 AM Sergey Senozhatsky
> <sergey.senozhatsky@gmail.com> wrote:
> >
> > On (20/11/27 15:56), Hans Verkuil wrote:
> > > Yes.
> > >
> > > BTW, wouldn't it be sufficient to change this code to:
> > >
> > >       if (!q->allow_cache_hints && q->memory != VB2_MEMORY_DMABUF) {
> > >               vb->need_cache_sync_on_prepare = 1;
> > >               vb->need_cache_sync_on_finish = 1;
> > >       }
> >
> > I think it would be sufficient.
> 
> Does it matter at this point if allow_cache_hints is set or not?

That's a good question. I'd say that it'll probably make sense to set
need_cache_sync for as many buffers as possible, regardless the queue
configuration (except for ->memory type), just to stay on the safe side.
I can spin another patch version.

	-ss