Re: [RFT PATCH v3 5/6] uvcvideo: queue: Support asynchronous buffer handling

[Kieran Bingham]

Hi Laurent,

On 30/07/2018 21:39, Laurent Pinchart wrote:
> Hi Kieran
> 
> I think your v4 doesn't take the review comments below into account.

I'm sorry for missing them.
Lets not miss it for v5 :)


> 
> On Wednesday, 17 January 2018 01:45:33 EEST Laurent Pinchart wrote:
>> On Friday, 12 January 2018 11:19:26 EET Kieran Bingham wrote:
>>> The buffer queue interface currently operates sequentially, processing
>>> buffers after they have fully completed.
>>>
>>> In preparation for supporting parallel tasks operating on the buffers,
>>> we will need to support buffers being processed on multiple CPUs.
>>>
>>> Adapt the uvc_queue_next_buffer() such that a reference count tracks the
>>> active use of the buffer, returning the buffer to the VB2 stack at
>>> completion.
>>>
>>> Signed-off-by: Kieran Bingham <kieran.bingham@xxxxxxxxxxxxxxxx>
>>> ---
>>>
>>>  drivers/media/usb/uvc/uvc_queue.c | 61 ++++++++++++++++++++++++++------
>>>  drivers/media/usb/uvc/uvcvideo.h  |  4 ++-
>>>  2 files changed, 54 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/media/usb/uvc/uvc_queue.c
>>> b/drivers/media/usb/uvc/uvc_queue.c index ddac4d89a291..5a9987e547d3
>>> 100644
>>> --- a/drivers/media/usb/uvc/uvc_queue.c
>>> +++ b/drivers/media/usb/uvc/uvc_queue.c
>>> @@ -131,6 +131,7 @@ static void uvc_buffer_queue(struct vb2_buffer *vb)
>>>
>>>  	spin_lock_irqsave(&queue->irqlock, flags);
>>>  	if (likely(!(queue->flags & UVC_QUEUE_DISCONNECTED))) {
>>>
>>> +		kref_init(&buf->ref);
>>>
>>>  		list_add_tail(&buf->queue, &queue->irqqueue);
>>>  	
>>>  	} else {
>>>  	
>>>  		/* If the device is disconnected return the buffer to userspace
>>>
>>> @@ -424,28 +425,66 @@ struct uvc_buffer
>>> *uvc_queue_get_current_buffer(struct uvc_video_queue *queue) return
>>> nextbuf;
>>>
>>>  }
>>>
>>> -struct uvc_buffer *uvc_queue_next_buffer(struct uvc_video_queue *queue,
>>> +/*
>>> + * uvc_queue_requeue: Requeue a buffer on our internal irqqueue
>>> + *
>>> + * Reuse a buffer through our internal queue without the need to
>>> 'prepare'
>>> + * The buffer will be returned to userspace through the uvc_buffer_queue
>>> call if
>>> + * the device has been disconnected
> 
> Additionally, periods are messing at the end of sentences.


Fixed.

> 
>>> + */
>>> +static void uvc_queue_requeue(struct uvc_video_queue *queue,
>>>
>>>  		struct uvc_buffer *buf)
>>>  
>>>  {
>>>
>>> -	struct uvc_buffer *nextbuf;
>>> -	unsigned long flags;
>>> +	buf->error = 0;
>>> +	buf->state = UVC_BUF_STATE_QUEUED;
>>> +	buf->bytesused = 0;
>>> +	vb2_set_plane_payload(&buf->buf.vb2_buf, 0, 0);
>>> +
>>> +	uvc_buffer_queue(&buf->buf.vb2_buf);
>>> +}
>>
>> You could have inlined this in uvc_queue_buffer_complete(), but the above
>> documentation is useful, so I'm fine if you prefer keeping it as a separate
>> function. Maybe you could call it uvc_queue_buffer_requeue() to be
>> consistent with the other functions ?
>>

Yes, it could be inlined - but it is as you say below it's somewhat of a
change in functionality. Instead of returning the buffer, it is
re-queued - and I felt that warranted it's own function in a way such
that this was self-documenting.

Thus, I'd like to keep the simple helper function. I'm sure the compiler
will inline it anyway - but I think it makes the code much more readable.

I've renamed it to uvc_queue_buffer_requeue() as requested.

>>> +static void uvc_queue_buffer_complete(struct kref *ref)
>>> +{
>>> +	struct uvc_buffer *buf = container_of(ref, struct uvc_buffer, ref);
>>> +	struct vb2_buffer *vb = &buf->buf.vb2_buf;
>>> +	struct uvc_video_queue *queue = vb2_get_drv_priv(vb->vb2_queue);
>>>
>>>  	if ((queue->flags & UVC_QUEUE_DROP_CORRUPTED) && buf->error) {
>>>
>>> -		buf->error = 0;
>>> -		buf->state = UVC_BUF_STATE_QUEUED;
>>> -		buf->bytesused = 0;
>>> -		vb2_set_plane_payload(&buf->buf.vb2_buf, 0, 0);
>>> -		return buf;
>>> +		uvc_queue_requeue(queue, buf);
>>> +		return;
>>
>> This changes the behaviour of the driver. Currently when an erroneous buffer
>> is encountered, it will be immediately reused. With this patch applied it
>> will be pushed to the back of the queue for later reuse. This will result
>> in buffers being reordered, possibly causing issues later when we'll
>> implement fences support (or possibly even today).

Correct - but because the buffers can now complete asynchronously -
there is no alternative except to put it back on the queue. (I don't
think ?)

We can't even sort the buffers on the queue as that would just be racy.

I didn't think there was any guarantee on the order that buffers were
returned back to userspace ? So I didn't think this should be a problem?
(After all - isn't that why the buffers have their index values for
mapping them?)


>> I think the whole drop corrupted buffers mechanism was a bad idea in the
>> first place and I'd like to remove it at some point, buffers in the error
>> state should be handled by applications. However, until that's done, I
>> wonder whether it would be possible to keep the current order. I
>> unfortunately don't see an easy option to do so at the moment, but maybe
>> you would. Otherwise I suppose we'll have to leave it as is.

No - I'm afraid I don't see any easy option to maintain order - and keep
the buffers asynchronous.

Perhaps we could do a 'sort' at some stage in the pipeline, but I'm not
convinced this would add value yet.

Perhaps not the best way to approach this - but maybe lets think about
sort ordering if we get any bug reports.


>> I'm tempted to flip the driver to not drop corrupted buffers by default.
>> I've done so on my computer, I'll see if I run into any issue. It could be
>> useful if you could set the nodrop parameter to 1 on your systems too when
>> performing tests.
>>

Or that :) - but that's more of a policy change.

I think you're right though - the userspace applications should be able
to decide if they want to present corrupt buffers or not.

>>>  	}
>>>
>>> +	buf->state = buf->error ? UVC_BUF_STATE_ERROR : UVC_BUF_STATE_DONE;
>>> +	vb2_set_plane_payload(&buf->buf.vb2_buf, 0, buf->bytesused);
>>> +	vb2_buffer_done(&buf->buf.vb2_buf, VB2_BUF_STATE_DONE);
>>> +}
>>> +
>>> +/*
>>> + * Release a reference on the buffer. Complete the buffer when the last
>>> + * reference is released
>>> + */
>>> +void uvc_queue_buffer_release(struct uvc_buffer *buf)
>>> +{
>>> +	kref_put(&buf->ref, uvc_queue_buffer_complete);
>>> +}
>>> +
>>> +/*
>>> + * Remove this buffer from the queue. Lifetime will persist while async
>>> actions
>>> + * are still running (if any), and uvc_queue_buffer_release will give the
>>> buffer
>>> + * back to VB2 when all users have completed.
>>> + */
>>> +struct uvc_buffer *uvc_queue_next_buffer(struct uvc_video_queue *queue,
>>> +		struct uvc_buffer *buf)
>>> +{
>>> +	struct uvc_buffer *nextbuf;
>>> +	unsigned long flags;
>>> +
>>>
>>>  	spin_lock_irqsave(&queue->irqlock, flags);
>>>  	list_del(&buf->queue);
>>>  	nextbuf = __uvc_queue_get_current_buffer(queue);
>>>  	spin_unlock_irqrestore(&queue->irqlock, flags);
>>>
>>> -	buf->state = buf->error ? UVC_BUF_STATE_ERROR : UVC_BUF_STATE_DONE;
>>> -	vb2_set_plane_payload(&buf->buf.vb2_buf, 0, buf->bytesused);
>>> -	vb2_buffer_done(&buf->buf.vb2_buf, VB2_BUF_STATE_DONE);
>>> +	uvc_queue_buffer_release(buf);
>>>
>>>  	return nextbuf;
>>>  
>>>  }
>>>
>>> diff --git a/drivers/media/usb/uvc/uvcvideo.h
>>> b/drivers/media/usb/uvc/uvcvideo.h index 5caa1f4de3cb..6a18dbfc3e5b 100644
>>> --- a/drivers/media/usb/uvc/uvcvideo.h
>>> +++ b/drivers/media/usb/uvc/uvcvideo.h
>>> @@ -404,6 +404,9 @@ struct uvc_buffer {
>>>
>>>  	unsigned int bytesused;
>>>  	
>>>  	u32 pts;
>>>
>>> +
>>> +	/* asynchronous buffer handling */
>>
>> Please capitalize the first word to match other comments in the driver.

Fixed.

>>
>>> +	struct kref ref;
>>>
>>>  };
>>>  
>>>  #define UVC_QUEUE_DISCONNECTED		(1 << 0)
>>>
>>> @@ -696,6 +699,7 @@ extern struct uvc_buffer *
>>>
>>>  		uvc_queue_get_current_buffer(struct uvc_video_queue *queue);
>>>  
>>>  extern struct uvc_buffer *uvc_queue_next_buffer(struct uvc_video_queue
>>>
>>> *queue, struct uvc_buffer *buf);
>>> +extern void uvc_queue_buffer_release(struct uvc_buffer *buf);
>>
>> No need for the extern keyboard. I'll submit a patch to drop it for all
>> functions.
>>

Looks like this was successfully removed already :)


>>>  extern int uvc_queue_mmap(struct uvc_video_queue *queue,
>>>  
>>>  		struct vm_area_struct *vma);
>>>  
>>>  extern unsigned int uvc_queue_poll(struct uvc_video_queue *queue,
> 

-- 
Regards
--
Kieran