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

[Sakari Ailus]

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>

There were a few drivers doing nasty things with memory that I couldn't
quite fix back then. Just FYI.

-- 
Regards,

Sakari Ailus
sakari.ailus@xxxxxx