Re: [RFC PATCH v4 09/11] media: uapi: Add V4L2_CID_USER_IMX_ASRC_RATIO_MOD control

[Hans Verkuil]

On 21/09/2023 08:55, Shengjiu Wang wrote:
> On Wed, Sep 20, 2023 at 6:19 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>>
>> On 20/09/2023 11:32, Shengjiu Wang wrote:
>>> The input clock and output clock may not be the accurate
>>> rate as the sample rate, there is some drift, so the convert
>>> ratio of i.MX ASRC module need to be changed according to
>>> actual clock rate.
>>>
>>> Add V4L2_CID_USER_IMX_ASRC_RATIO_MOD control for user to
>>> adjust the ratio.
>>>
>>> Signed-off-by: Shengjiu Wang <shengjiu.wang@xxxxxxx>
>>> ---
>>>  Documentation/userspace-api/media/v4l/control.rst | 5 +++++
>>>  drivers/media/v4l2-core/v4l2-ctrls-defs.c         | 1 +
>>>  include/uapi/linux/v4l2-controls.h                | 1 +
>>>  3 files changed, 7 insertions(+)
>>>
>>> diff --git a/Documentation/userspace-api/media/v4l/control.rst b/Documentation/userspace-api/media/v4l/control.rst
>>> index 4463fce694b0..2bc175900a34 100644
>>> --- a/Documentation/userspace-api/media/v4l/control.rst
>>> +++ b/Documentation/userspace-api/media/v4l/control.rst
>>> @@ -318,6 +318,11 @@ Control IDs
>>>      depending on particular custom controls should check the driver name
>>>      and version, see :ref:`querycap`.
>>>
>>> +.. _v4l2-audio-imx:
>>> +
>>> +``V4L2_CID_USER_IMX_ASRC_RATIO_MOD``
>>> +    sets the rasampler ratio modifier of i.MX asrc module.
>>
>> rasampler -> resampler (I think?)
>>
>> This doesn't document at all what the type of the control is or how to interpret it.
>>
>>> +
>>>  Applications can enumerate the available controls with the
>>>  :ref:`VIDIOC_QUERYCTRL` and
>>>  :ref:`VIDIOC_QUERYMENU <VIDIOC_QUERYCTRL>` ioctls, get and set a
>>> diff --git a/drivers/media/v4l2-core/v4l2-ctrls-defs.c b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
>>> index 8696eb1cdd61..16f66f66198c 100644
>>> --- a/drivers/media/v4l2-core/v4l2-ctrls-defs.c
>>> +++ b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
>>> @@ -1242,6 +1242,7 @@ const char *v4l2_ctrl_get_name(u32 id)
>>>       case V4L2_CID_COLORIMETRY_CLASS:        return "Colorimetry Controls";
>>>       case V4L2_CID_COLORIMETRY_HDR10_CLL_INFO:               return "HDR10 Content Light Info";
>>>       case V4L2_CID_COLORIMETRY_HDR10_MASTERING_DISPLAY:      return "HDR10 Mastering Display";
>>> +     case V4L2_CID_USER_IMX_ASRC_RATIO_MOD:                  return "ASRC RATIO MOD";
>>
>> Let's stay consistent with the other control names:
>>
>> "ASRC Ratio Modifier"
>>
>> But if this is a driver specific control, then this doesn't belong here.
>>
>> Driver specific controls are defined in the driver itself, including this
>> description.
>>
>> Same for the control documentation: if it is driver specific, then that
>> typically is documented either in a driver-specific public header, or
>> possibly in driver-specific documentation (Documentation/admin-guide/media/).
>>
>> But is this imx specific? Wouldn't other similar devices need this?
> 
> It is imx specific.

Why? I'm not opposed to this, but I wonder if you looked at datasheets of
similar devices from other vendors: would they use something similar?

And the very short description you gave in the commit log refers to input
and output clock: how would userspace know those clock frequencies? In
other words, what information does userspace need in order to set this
control correctly? And is that information actually available? How would
you use this control?

I don't really understand how this is supposed to be used.

> 
> Does this mean that I need to create a header file in include/uapi/linux
> folder to put this definition?  I just hesitate if this is necessary.

Yes, put it there. There are some examples of this already:

include/uapi/linux/aspeed-video.h
include/uapi/linux/max2175.h

> 
> There is folder Documentation/userspace-api/media/drivers/ for drivers
> Should this document in this folder, not in the
> Documentation/admin-guide/media/?

Yes, you are correct. For the headers above, the corresponding documentation
is in:

Documentation/userspace-api/media/drivers/aspeed-video.rst
Documentation/userspace-api/media/drivers/max2175.rst

So you have some examples as reference.

Frankly, what is in admin-guide and in userspace-api is a bit random, it
probably could use a cleanup.

Regards,

	Hans

> 
> Best regards
> Wang shengjiu
>>
>>>       default:
>>>               return NULL;
>>>       }
>>> diff --git a/include/uapi/linux/v4l2-controls.h b/include/uapi/linux/v4l2-controls.h
>>> index c3604a0a3e30..b1c319906d12 100644
>>> --- a/include/uapi/linux/v4l2-controls.h
>>> +++ b/include/uapi/linux/v4l2-controls.h
>>> @@ -162,6 +162,7 @@ enum v4l2_colorfx {
>>>  /* The base for the imx driver controls.
>>>   * We reserve 16 controls for this driver. */
>>>  #define V4L2_CID_USER_IMX_BASE                       (V4L2_CID_USER_BASE + 0x10b0)
>>> +#define V4L2_CID_USER_IMX_ASRC_RATIO_MOD     (V4L2_CID_USER_IMX_BASE + 0)
>>>
>>>  /*
>>>   * The base for the atmel isc driver controls.
>>
>> Regards,
>>
>>         Hans