Re: [PATCH] [media] vb2: move dma-buf unmap from __vb2_dqbuf() to vb2_buffer_done()

[Javier Martinez Canillas]

Hello Sakari,

On 08/16/2016 05:13 PM, Sakari Ailus wrote:
> Hi Javier,
> 
> Javier Martinez Canillas wrote:
>> Hello Sakari,
>>
>> On 08/16/2016 04:47 PM, Sakari Ailus wrote:
>>> Hi Javier,
>>>
>>> Javier Martinez Canillas wrote:
>>>> Hello Hans,
>>>>
>>>> Thanks a lot for your feedback.
>>>>
>>>> On 08/13/2016 09:47 AM, Hans Verkuil wrote:
>>>>> On 07/20/2016 08:22 PM, Javier Martinez Canillas wrote:
>>>>>> Currently the dma-buf is unmapped when the buffer is dequeued by userspace
>>>>>> but it's not used anymore after the driver finished processing the buffer.
>>>>>>
>>>>>> So instead of doing the dma-buf unmapping in __vb2_dqbuf(), it can be made
>>>>>> in vb2_buffer_done() after the driver notified that buf processing is done.
>>>>>>
>>>>>> Decoupling the buffer dequeue from the dma-buf unmapping has also the side
>>>>>> effect of making possible to add dma-buf fence support in the future since
>>>>>> the buffer could be dequeued even before the driver has finished using it.
>>>>>>
>>>>>> Signed-off-by: Javier Martinez Canillas <javier@xxxxxxxxxxxxxxx>
>>>>>>
>>>>>> ---
>>>>>> Hello,
>>>>>>
>>>>>> I've tested this patch doing DMA buffer sharing between a
>>>>>> vivid input and output device with both v4l2-ctl and gst:
>>>>>>
>>>>>> $ v4l2-ctl -d0 -e1 --stream-dmabuf --stream-out-mmap
>>>>>> $ v4l2-ctl -d0 -e1 --stream-mmap --stream-out-dmabuf
>>>>>> $ gst-launch-1.0 v4l2src device=/dev/video0 io-mode=dmabuf ! v4l2sink device=/dev/video1 io-mode=dmabuf-import
>>>>>>
>>>>>> And I didn't find any issues but more testing will be appreciated.
>>>>>>
>>>>>> Best regards,
>>>>>> Javier
>>>>>>
>>>>>>  drivers/media/v4l2-core/videobuf2-core.c | 34 +++++++++++++++++++++-----------
>>>>>>  1 file changed, 22 insertions(+), 12 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/media/v4l2-core/videobuf2-core.c b/drivers/media/v4l2-core/videobuf2-core.c
>>>>>> index 7128b09810be..973331efaf79 100644
>>>>>> --- a/drivers/media/v4l2-core/videobuf2-core.c
>>>>>> +++ b/drivers/media/v4l2-core/videobuf2-core.c
>>>>>> @@ -958,6 +958,22 @@ void *vb2_plane_cookie(struct vb2_buffer *vb, unsigned int plane_no)
>>>>>>  EXPORT_SYMBOL_GPL(vb2_plane_cookie);
>>>>>>  
>>>>>>  /**
>>>>>> + * __vb2_unmap_dmabuf() - unmap dma-buf attached to buffer planes
>>>>>> + */
>>>>>> +static void __vb2_unmap_dmabuf(struct vb2_buffer *vb)
>>>>>> +{
>>>>>> +	int i;
>>>>>> +
>>>>>> +	for (i = 0; i < vb->num_planes; ++i) {
>>>>>> +		if (!vb->planes[i].dbuf_mapped)
>>>>>> +			continue;
>>>>>> +		call_void_memop(vb, unmap_dmabuf,
>>>>>> +				vb->planes[i].mem_priv);
>>>>>
>>>>> Does unmap_dmabuf work in interrupt context? Since vb2_buffer_done can be called from
>>>>> an irq handler this is a concern.
>>>>>
>>>>
>>>> Good point, I believe it shouldn't be called from atomic context since both
>>>> the dma_buf_vunmap() and dma_buf_unmap_attachment() functions can sleep.
>>>>  
>>>>> That said, vb2_buffer_done already calls call_void_memop(vb, finish, vb->planes[plane].mem_priv);
>>>>> to sync buffers, and that can take a long time as well. So it is not a good idea to
>>>>> have this in vb2_buffer_done.
>>>>>
>>>>
>>>> I see.
>>>>
>>>>> What I would like to see is to have vb2 handle this finish() call and the vb2_unmap_dmabuf
>>>>> in some workthread or equivalent.
>>>>>
>>>>> It would complicate matters somewhat in vb2, but it would simplify drivers since these
>>>>> actions would not longer take place in interrupt context.
>>>>>
>>>>> I think this patch makes sense, but I would prefer that this is moved out of the interrupt
>>>>> context.
>>>>>
>>>>
>>>> Ok, I can take a look to this and handle the finish() and unmap_dmabuf()
>>>> out of interrupt context as you suggested.
>>>
>>> I have a patch doing the former which is a part of my cache management
>>> fix patchset:
>>>
>>> <URL:http://git.retiisi.org.uk/?p=~sailus/linux.git;a=commitdiff;h=b57f937627abda158ada01a3297dbb0f0a57b515>
>>> <URL:http://git.retiisi.org.uk/?p=~sailus/linux.git;a=shortlog;h=refs/heads/vb2-dc-noncoherent>
>>>
>>
>> Interesting, thanks for the links.
>>  
>>> There were a few drivers doing nasty things with memory that I couldn't
>>> quite fix back then. Just FYI.
>>>
>>
>> Did you mean that there were issues with moving finish mem op call to DQBUF?
>>
>> Do you recall what these drivers were or what were doing that caused problems?
> 
> Not any particular drivers --- the problem is that flushing the cache

Ah, you were explaining the rationale of the change, not that you had
issues after doing the mentioned change, sorry for my confusion.

> simply takes a lot of time, often milliseconds depending on the machine.
> There's also no reason to do it in interrupt context. It kills realtime
> performance, too.
>

Yes, I understand why calling finish() in vb2_buffer_done() is bad :)
 
>>
>> In any case, what Hans proposed AFAIU is not to change when the finish call
>> happens but to split the vb2_buffer_done() function and defer part of it to
>> a workqueue or kthread. I'll give a try to that approach probably tomorrow.
> 
> There's also the context of the user space process calling DQBUF, too.
> Why not to use that one instead?
> 

The idea of $SUBJECT was to do the operations as soon as possible instead of
waiting for these to happen when user-space calls DQBUF. There isn't a reason
to wait until DQBUF to call finish() AFAICT, besides making vb2 more simpler
of course (and this is also true for dma-buf unmap, it can be done sooner).

The reason why I posted this patch is that I'm exploring the possibility to
add dma-buf implicit fence support to vb2, and one option is to use the fence
as sync point instead of requiring user-space to call DQBUF and QBUF. So in
that case, it would be good for the finish and dma-buf unmap operations to
happen as soon as the driver is done processing the buffer.

Best regards,
-- 
Javier Martinez Canillas
Open Source Group
Samsung Research America