linux-kernel - Re: [PATCH v9 17/22] media: docs: Document the behaviour of uvcdriver

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Date:   Sat, 27 Mar 2021 12:19:00 +0100
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Ricardo Ribalda <ribalda@...omium.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        tfiga@...omium.org
Subject: Re: [PATCH v9 17/22] media: docs: Document the behaviour of uvcdriver

On 26/03/2021 10:58, Ricardo Ribalda wrote:
> The uvc driver relies on the camera firmware to keep the control states
> and therefore is not capable of changing an inactive control.
> 
> Allow returning -EACESS in those cases.

-EACCES

> 
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  Documentation/userspace-api/media/v4l/vidioc-g-ctrl.rst      | 5 +++++
>  Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst | 5 +++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/Documentation/userspace-api/media/v4l/vidioc-g-ctrl.rst b/Documentation/userspace-api/media/v4l/vidioc-g-ctrl.rst
> index 4f1bed53fad5..8c0a203385c2 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-g-ctrl.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-g-ctrl.rst
> @@ -95,3 +95,8 @@ EBUSY
>  
>  EACCES
>      Attempt to set a read-only control or to get a write-only control.
> +
> +    Or if there is an attempt to set an inactive control and the driver is
> +    not capable of keeping the new value until the control is active again.

keeping: 'caching' or 'storing' are better words, I think.

> +    This is the case for drivers that do not use the standard control
> +    framework and rely purely on the hardware to keep the controls' state.

I would drop that last sentence. It is not relevant information to the users of
the API.

> diff --git a/Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst b/Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst
> index b9c62affbb5a..bb7de7a25241 100644
> --- a/Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst
> +++ b/Documentation/userspace-api/media/v4l/vidioc-g-ext-ctrls.rst
> @@ -438,3 +438,8 @@ EACCES
>  
>      Or the ``which`` field was set to ``V4L2_CTRL_WHICH_REQUEST_VAL`` but the
>      device does not support requests.
> +
> +    Or if there is an attempt to set an inactive control and the driver is
> +    not capable of keeping the new value until the control is active again.
> +    This is the case for drivers that do not use the standard control
> +    framework and rely purely on the hardware to keep the controls' state.

Same comments as above.

> 

Regards,

	Hans