linux-kernel - Re: [PATCH v2 3/6] media: v4l2-mem2mem: add stateless_(try_)decoder

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Date:   Mon, 7 Oct 2019 10:32:59 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Jernej Škrabec <jernej.skrabec@...l.net>
Cc:     mchehab@...nel.org, paul.kocialkowski@...tlin.com,
        mripard@...nel.org, pawel@...iak.com, m.szyprowski@...sung.com,
        kyungmin.park@...sung.com, tfiga@...omium.org, wens@...e.org,
        gregkh@...uxfoundation.org, boris.brezillon@...labora.com,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        devel@...verdev.osuosl.org, linux-arm-kernel@...ts.infradead.org,
        ezequiel@...labora.com, jonas@...boo.se
Subject: Re: [PATCH v2 3/6] media: v4l2-mem2mem: add
 stateless_(try_)decoder_cmd ioctl helpers

On 10/7/19 8:02 AM, Jernej Škrabec wrote:
> Dne petek, 04. oktober 2019 ob 11:21:12 CEST je Hans Verkuil napisal(a):
>> On 9/29/19 10:00 PM, Jernej Skrabec wrote:
>>> These helpers are used by stateless codecs when they support multiple
>>> slices per frame and hold capture buffer flag is set. It's expected that
>>> all such codecs will use this code.
>>>
>>> Signed-off-by: Jernej Skrabec <jernej.skrabec@...l.net>
>>> ---
>>>
>>>  drivers/media/v4l2-core/v4l2-mem2mem.c | 35 ++++++++++++++++++++++++++
>>>  include/media/v4l2-mem2mem.h           |  4 +++
>>>  2 files changed, 39 insertions(+)
>>>
>>> diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c
>>> b/drivers/media/v4l2-core/v4l2-mem2mem.c index 19937dd3c6f6..2677a07e4c9b
>>> 100644
>>> --- a/drivers/media/v4l2-core/v4l2-mem2mem.c
>>> +++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
>>> @@ -1154,6 +1154,41 @@ int v4l2_m2m_ioctl_try_decoder_cmd(struct file
>>> *file, void *fh,> 
>>>  }
>>>  EXPORT_SYMBOL_GPL(v4l2_m2m_ioctl_try_decoder_cmd);
>>>
>>> +int v4l2_m2m_ioctl_stateless_try_decoder_cmd(struct file *file, void *fh,
>>> +					     struct 
> v4l2_decoder_cmd *dc)
>>> +{
>>> +	if (dc->cmd != V4L2_DEC_CMD_FLUSH)
>>> +		return -EINVAL;
>>> +
>>> +	dc->flags = 0;
>>> +
>>> +	return 0;
>>> +}
>>> +EXPORT_SYMBOL_GPL(v4l2_m2m_ioctl_stateless_try_decoder_cmd);
>>> +
>>> +int v4l2_m2m_ioctl_stateless_decoder_cmd(struct file *file, void *priv,
>>> +					 struct 
> v4l2_decoder_cmd *dc)
>>> +{
>>> +	struct v4l2_fh *fh = file->private_data;
>>> +	struct vb2_v4l2_buffer *out_vb, *cap_vb;
>>> +	int ret;
>>> +
>>> +	ret = v4l2_m2m_ioctl_stateless_try_decoder_cmd(file, priv, dc);
>>> +	if (ret < 0)
>>> +		return ret;
>>> +
>>> +	out_vb = v4l2_m2m_last_src_buf(fh->m2m_ctx);
>>> +	cap_vb = v4l2_m2m_last_dst_buf(fh->m2m_ctx);
>>
>> I think this should be v4l2_m2m_next_dst_buf. If multiple capture buffers
>> were queued up, then it can only be the first capture buffer that can be
>> 'HELD'.
> 
> I'm pretty sure v4l2_m2m_last_dst_buf() is correct. We want to affect last job 
> in the queue, all jobs before are already properly handled by comparing 
> timestamps.

You're absolutely right.

> 
>>
>> This might solve the race condition you saw with ffmpeg.
> 
> This actually doesn't change anything. ffmpeg currently queues only one job and 
> then waits until it's executed. In this case it actually doesn't matter if 
> "last" or "next" variant is used.

Can you debug this a bit further? I don't want to merge this unless we know what's
going wrong with ffmpeg.

I suspect it is a race condition. I'll reply to patch 6/6 with more info.

Regards,

	Hans

> 
> Best regards,
> Jernej
> 
>>
>> Regards,
>>
>> 	Hans
>>
>>> +
>>> +	if (out_vb)
>>> +		out_vb->flags &= ~V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF;
>>> +	else if (cap_vb && cap_vb->is_held)
>>> +		v4l2_m2m_buf_done(cap_vb, VB2_BUF_STATE_DONE);
>>> +
>>> +	return 0;
>>> +}
>>> +EXPORT_SYMBOL_GPL(v4l2_m2m_ioctl_stateless_decoder_cmd);
>>> +
>>>
>>>  /*
>>>  
>>>   * v4l2_file_operations helpers. It is assumed here same lock is used
>>>   * for the output and the capture buffer queue.
>>>
>>> diff --git a/include/media/v4l2-mem2mem.h b/include/media/v4l2-mem2mem.h
>>> index c9fa96c8eed1..8ae2f56c7fa3 100644
>>> --- a/include/media/v4l2-mem2mem.h
>>> +++ b/include/media/v4l2-mem2mem.h
>>> @@ -714,6 +714,10 @@ int v4l2_m2m_ioctl_try_encoder_cmd(struct file *file,
>>> void *fh,> 
>>>  				   struct v4l2_encoder_cmd *ec);
>>>  
>>>  int v4l2_m2m_ioctl_try_decoder_cmd(struct file *file, void *fh,
>>>  
>>>  				   struct v4l2_decoder_cmd *dc);
>>>
>>> +int v4l2_m2m_ioctl_stateless_try_decoder_cmd(struct file *file, void *fh,
>>> +					     struct 
> v4l2_decoder_cmd *dc);
>>> +int v4l2_m2m_ioctl_stateless_decoder_cmd(struct file *file, void *priv,
>>> +					 struct 
> v4l2_decoder_cmd *dc);
>>>
>>>  int v4l2_m2m_fop_mmap(struct file *file, struct vm_area_struct *vma);
>>>  __poll_t v4l2_m2m_fop_poll(struct file *file, poll_table *wait);
> 
> 
> 
>