linux-kernel - Re: [PATCH v17 8/8] media: verisilicon: Support deleting buffers on capture queue

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Message-ID: <326db4c8-4a54-4346-b443-c4cae4196da9@xs4all.nl>
Date: Fri, 26 Jan 2024 10:17:10 +0100
From: Hans Verkuil <hverkuil@...all.nl>
To: Nicolas Dufresne <nicolas@...fresne.ca>,
 Benjamin Gaignard <benjamin.gaignard@...labora.com>, mchehab@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
 kernel@...labora.com
Subject: Re: [PATCH v17 8/8] media: verisilicon: Support deleting buffers on
 capture queue

On 25/01/2024 17:27, Nicolas Dufresne wrote:
> Hi,
> 
> Le mercredi 24 janvier 2024 à 16:35 +0100, Benjamin Gaignard a écrit :
>> Le 24/01/2024 à 13:52, Hans Verkuil a écrit :
>>> On 19/01/2024 10:49, Benjamin Gaignard wrote:
>>>> Allow to delete buffers on capture queue because it the one which
>>>> own the decoded buffers. After a dynamic resolution change lot of
>>>> them could remain allocated but won't be used anymore so deleting
>>>> them save memory.
>>>> Do not add this feature on output queue because the buffers are
>>>> smaller, fewer and always recycled even after a dynamic resolution
>>>> change.
>>>>
>>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
>>>> ---
>>>>   drivers/media/platform/verisilicon/hantro_drv.c  | 1 +
>>>>   drivers/media/platform/verisilicon/hantro_v4l2.c | 1 +
>>>>   2 files changed, 2 insertions(+)
>>>>
>>>> diff --git a/drivers/media/platform/verisilicon/hantro_drv.c b/drivers/media/platform/verisilicon/hantro_drv.c
>>>> index db3df6cc4513..f6b0a676a740 100644
>>>> --- a/drivers/media/platform/verisilicon/hantro_drv.c
>>>> +++ b/drivers/media/platform/verisilicon/hantro_drv.c
>>>> @@ -248,6 +248,7 @@ queue_init(void *priv, struct vb2_queue *src_vq, struct vb2_queue *dst_vq)
>>>>   	dst_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
>>>>   	dst_vq->lock = &ctx->dev->vpu_mutex;
>>>>   	dst_vq->dev = ctx->dev->v4l2_dev.dev;
>>>> +	src_vq->supports_delete_bufs = true;
>>> As I mentioned, I remain unconvinced by this. It is just making the API inconsistent
>>> since if you support delete_bufs, then why support it for one queue only and not both?
>>
>> Because the both queues don't handle the same type of data.
>> For example for a stateless decoder, for me, it makes sense to allow delete decoded frames
>> if they won't be used anymore but that won't makes sense for bitstream buffers.
> 
> I personally think that for stateless and stateful decoder bitstream queue this
> can be useful. We currently guess the size we need, and there is no way to
> allocate bigger ones without the driver forgetting about reference frames.
> 
> In stateful, some drivers allow to split the bitstream (I tested wave5 notably),
> but I was told this is not always the case. A bit of a gray zone in that API,
> with lack of capabilities to describe it. On stateless, the entire bitstream
> slice must be in one buffer.
> 
> Though, for the asymmetry, most stateful decoders won't allow delete bufs on
> capture, because the buffers are registered in the firmware ahead of time. wave5
> can't even implement create_bufs on capture. We had an argument about having
> create_bufs on only one queue for that specific driver, as we wanted something
> upstream, we flex to removing create bufs completely. I think the all or nothing
> rule on per queue create/delete_bufs is not aligned with the reality.

I think the default should be that it supports DELETE_BUFS for both queues, but
it should still be possible to only have it on one queue.

When v18 is posted I want to play around with that, because I am not certain
what the easiest way is to implement this.

Another thing that needs to be added is a check that DELETE_BUFS is only enabled
if CREATE_BUFS is also present, it makes no sense otherwise.

Finally I want to take another look at the work required to make a CREATE_BUFS
replacement since that ioctl is horrible. Whether that will become part of this
series or done as a follow-up I am not sure about, but this series should definitely
make it possible to cleanly integrate it.

Regards,

	Hans

> 
> Nicolas
>>
>>>
>>>>   
>>>>   	return vb2_queue_init(dst_vq);
>>>>   }
>>>> diff --git a/drivers/media/platform/verisilicon/hantro_v4l2.c b/drivers/media/platform/verisilicon/hantro_v4l2.c
>>>> index 941fa23c211a..34eab90e8a42 100644
>>>> --- a/drivers/media/platform/verisilicon/hantro_v4l2.c
>>>> +++ b/drivers/media/platform/verisilicon/hantro_v4l2.c
>>>> @@ -756,6 +756,7 @@ const struct v4l2_ioctl_ops hantro_ioctl_ops = {
>>>>   	.vidioc_dqbuf = v4l2_m2m_ioctl_dqbuf,
>>>>   	.vidioc_prepare_buf = v4l2_m2m_ioctl_prepare_buf,
>>>>   	.vidioc_create_bufs = v4l2_m2m_ioctl_create_bufs,
>>>> +	.vidioc_delete_bufs = v4l2_m2m_ioctl_delete_bufs,
>>>>   	.vidioc_expbuf = v4l2_m2m_ioctl_expbuf,
>>>>   
>>>>   	.vidioc_subscribe_event = v4l2_ctrl_subscribe_event,
>>> In my view setting vidioc_delete_bufs should enable this feature, and if
>>> for some strange reason only one queue support it, then make a wrapper
>>> callback that returns an error when used with the wrong queue.
>>>
>>> Also note that patch 6/8 never checks for q->supports_delete_bufs in
>>> vb2_core_delete_bufs(), which is wrong!
>>
>> I will fix that in next version.
>> Regards,
>> Benjamin
>>
>>>
>>> Regards,
>>>
>>> 	Hans
>>>
>>
>