linux-kernel - Re: [PATCH] media: vidioc-queryctrl.rst: clarify combining READ_ONLY with WRITE

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Message-ID: <a7e82545-141b-7496-f0f1-dadbe05565f0@collabora.com>
Date:   Wed, 25 Sep 2019 13:29:06 -0300
From:   Helen Koike <helen.koike@...labora.com>
To:     Hans Verkuil <hverkuil-cisco@...all.nl>, mchehab@...nel.org,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com
Subject: Re: [PATCH] media: vidioc-queryctrl.rst: clarify combining READ_ONLY
 with WRITE_ONLY

Hi Hans

On 9/25/19 12:17 PM, Hans Verkuil wrote:
> On 9/25/19 5:09 PM, Helen Koike wrote:
>> Make it clear in the documentation that V4L2_CTRL_FLAG_READ_ONLY doesn't
>> conflict with V4L2_CTRL_FLAG_WRITE_ONLY. Also make it clear that
>> if both are combined then the control has read and write permissions.
> 
> That doesn't look right.
> 
> This is the test in v4l2-compliance:
> 
> __u32 rw_mask = V4L2_CTRL_FLAG_READ_ONLY | V4L2_CTRL_FLAG_WRITE_ONLY;
> 
> if ((fl & rw_mask) == rw_mask)
> 	return fail("can't read nor write this control\n");
> 
> If both flags are set, then a fail is reported.
> 
> Did you mis-read?
> 
> Setting both flags makes no sense.

Maybe I misunderstood the V4L2_CTRL_TYPE_CTRL_CLASS, from v4l2-compliance:

	case V4L2_CTRL_TYPE_CTRL_CLASS:
		if (fl != (V4L2_CTRL_FLAG_READ_ONLY | V4L2_CTRL_FLAG_WRITE_ONLY))
			return fail("invalid flags for control class\n");

I was confused why vivid was using both flags in vivid_ctrl_class (and also the debayer patch that was following this model).

Why both flags are expected when this type is used?

Thanks,
Helen

> 
> Regards,
> 
> 	Hans
> 
>>
>> Signed-off-by: Helen Koike <helen.koike@...labora.com>
>>
>> ---
>> Hi,
>>
>> v4l2-compliance expects both flags for read and write permissions, so I
>> would like to make it clear in the docs.
>> Please let me know if this is not the case.
>>
>> Thanks
>> Helen
>> ---
>>  Documentation/media/uapi/v4l/vidioc-queryctrl.rst | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/Documentation/media/uapi/v4l/vidioc-queryctrl.rst b/Documentation/media/uapi/v4l/vidioc-queryctrl.rst
>> index a3d56ffbf4cc..1d3aecdf679f 100644
>> --- a/Documentation/media/uapi/v4l/vidioc-queryctrl.rst
>> +++ b/Documentation/media/uapi/v4l/vidioc-queryctrl.rst
>> @@ -500,6 +500,11 @@ See also the examples in :ref:`control`.
>>        - 0x0004
>>        - This control is permanently readable only. Any attempt to change
>>  	the control will result in an ``EINVAL`` error code.
>> +
>> +	.. note::
>> +
>> +	   If combined with ``V4L2_CTRL_FLAG_WRITE_ONLY``, then both, read
>> +	   and write are allowed.
>>      * - ``V4L2_CTRL_FLAG_UPDATE``
>>        - 0x0008
>>        - A hint that changing this control may affect the value of other
>> @@ -523,6 +528,11 @@ See also the examples in :ref:`control`.
>>  	where writing a value will cause the device to carry out a given
>>  	action (e. g. motor control) but no meaningful value can be
>>  	returned.
>> +
>> +	.. note::
>> +
>> +	   If combined with ``V4L2_CTRL_FLAG_READ_ONLY``, then both, read
>> +	   and write are allowed.
>>      * - ``V4L2_CTRL_FLAG_VOLATILE``
>>        - 0x0080
>>        - This control is volatile, which means that the value of the
>>
>