Message-ID: <23e3a116-7ba6-4e1a-8b02-4eafc22d7e40@kernel.org>
Date: Mon, 8 Sep 2025 13:17:13 +0200
From: Hans Verkuil <hverkuil+cisco@...nel.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 Ricardo Ribalda <ribalda@...omium.org>
Cc: Hans de Goede <hdegoede@...hat.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>,
 Sakari Ailus <sakari.ailus@...ux.intel.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Linus Walleij
 <linus.walleij@...aro.org>, Bartosz Golaszewski <brgl@...ev.pl>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Len Brown <lenb@...nel.org>,
 linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-usb@...r.kernel.org, devicetree@...r.kernel.org,
 linux-gpio@...r.kernel.org, linux-acpi@...r.kernel.org
Subject: Re: [PATCH v2 10/12] media: uvcvideo: Add get_* functions to
 uvc_entity

On 08/09/2025 12:13, Laurent Pinchart wrote:
> A question for Hans Verkuil below.
> 
> On Thu, Aug 07, 2025 at 09:35:14AM +0200, Ricardo Ribalda wrote:
>> On Wed, 16 Jul 2025 at 12:32, Ricardo Ribalda wrote:
>>> On Tue, 15 Jul 2025 at 21:35, Laurent Pinchart wrote:
>>>> On Mon, Jul 14, 2025 at 05:46:40PM +0200, Ricardo Ribalda wrote:
>>>>> On Mon, 14 Jul 2025 at 16:30, Laurent Pinchart wrote:
>>>>>> On Tue, Jul 01, 2025 at 01:13:10PM +0200, Ricardo Ribalda wrote:
>>>>>>> On Sun, 29 Jun 2025 at 20:13, Laurent Pinchart wrote:
>>>>>>>> On Thu, Jun 05, 2025 at 05:53:03PM +0000, Ricardo Ribalda wrote:
>>>>>>>>> Virtual entities need to provide more values than get_cur and get_cur
>>>>>>>>
>>>>>>>> I think you meant "get_info and get_cur".
>>>>>>>>
>>>>>>>>> for their controls. Add support for get_def, get_min, get_max and
>>>>>>>>> get_res.
>>>>>>>>
>>>>>>>> Do they ? The UVC specification defines controls that don't list
>>>>>>>> GET_DEF, GET_MIN, GET_MAX and GET_RES as mandatory requests. Can't we do
>>>>>>>> the same for the software controls ? This patch is meant to support the
>>>>>>>> UVC_SWENTITY_ORIENTATION and UVC_SWENTITY_ROTATION control in the next
>>>>>>>> patch, and those are read-only controls. Aren't GET_INFO and GET_CUR
>>>>>>>> enough ?
>>>>>>>
>>>>>>> V4L2_CID_CAMERA_ROTATION has the type UVC_CTRL_DATA_TYPE_UNSIGNED,
>>>>>>> that time requires get_min and get_max.
>>>>>>
>>>>>> Where does that requirement come from ? Is it because how the
>>>>>> corresponding V4L2 type (V4L2_CTRL_TYPE_INTEGER) is handled in
>>>>>> uvc_ctrl_clamp() ? uvc_ctrl_clamp() is only called when setting a
>>>>>> control, from uvc_ctrl_set(), and V4L2_CID_CAMERA_ROTATION should be
>>>>>> read-only.
>>>>>
>>>>> It its for VIDIOC_QUERY_EXT_CTRL
>>>>>
>>>>> uvc_query_v4l2_ctrl -> __uvc_query_v4l2_ctrl -> __uvc_queryctrl_boundaries
>>>>>
>>>>> We need to list the min, max, def and step for every control. They are
>>>>> fetched with uvc_ctrl_populate_cache()
>>>>
>>>> Ah, I see, thanks.
>>>>
>>>> For GET_RES, I think we can leave it unimplemented.
>>>> __uvc_queryctrl_boundaries() will set v4l2_ctrl->step = 0 which seems to
>>>> be the right behaviour for a read-only control whose value never
>>>> changes.
>>>
>>> That will break v4l2-compatiblity. Step needs to be != 0
>>> https://git.linuxtv.org/v4l-utils.git/tree/utils/v4l2-compliance/v4l2-test-controls.cpp#n77
>>>
>>> Control ioctls (Input 0):
>>>                 fail: v4l2-test-controls.cpp(77): step == 0
>>>                 fail: v4l2-test-controls.cpp(201): invalid control 009a0923
> 
> Is that an issue in v4l2-compliance ? For integer controls,
> https://docs.kernel.org/userspace-api/media/v4l/vidioc-queryctrl.html#c.V4L.v4l2_ctrl_type
> documents the step value as "any". For a read-only control whose value
> is constant, do we want to enforce a non-zero value ? If so we should
> update the specification.
> 
> Hans, what's your opinion ?

The spec is not quite precise enough w.r.t. the step value. Whenever that table
says 'any' for the step value, it really should read '>= 1'. It does that already
for TYPE_STRING, but it is equally true for INTEGER(64) and U8/16/32.

If you create a control using the control framework, then that's actually checked.
It's verified by the check_range() function in v4l2-ctrls-core.c.

Regards,

	Hans

> 
> In any case, if GET_RES isn't implemented, we could update
> __uvc_queryctrl_boundaries() to set step to 1 instead of 0. That would
> fix v4l2-compliance for real controls that don't implement GET_RES.
> 
>>>> As for the minimum and maximum, they are currently set to 0 if the
>>>> corresponding operations are not supported. I wonder if we should set
>>>> them to the current value instead for read-only controls (as in controls
>>>> whose flags report support for GET_CUR only)..
>>>
>>> I am not sure that I like that approach IMO the code looks worse...
>>> but if you prefer that, we can go that way
>>
>> I am almost ready to send a new version.
>>
>> What approach do you prefer?
> 
> I particularly like the change in __uvc_queryctrl_boundaries(). That
> could probably be done without the rest of the changes though, as
> ctrl->uvc_data is already allocated with kzalloc().
> 
> I also like the fact that the driver can rely on the min/max values to
> always be populated in the control data. This could be useful for real
> read-only UVC controls.
> 
> Thinking a bit more about this, for read-only controls whose value never
> changes, min, max and step are meaningless. V4L2 requires their value to
> be set, that's a decision we made in the V4L2 API, but I think a model
> where min, max and step would be undefined (or 0) wouldn't be worse. So
> maybe it makes sense to handle this in __uvc_queryctrl_boundaries(),
> which is where the adaptation between UVC and V4L2 is handled, instead
> of storing CUR in the ctrl->uvc_data DEF/MIN/MAX in
> uvc_ctrl_populate_cache() ? I think the code could look cleaner.
> 
>>> diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> u> > index ec472e111248..47224437018b 100644
>>> --- a/drivers/media/usb/uvc/uvc_ctrl.c
>>> +++ b/drivers/media/usb/uvc/uvc_ctrl.c
>>> @@ -35,6 +35,8 @@
>>>  /* ------------------------------------------------------------------------
>>>   * Controls
>>>   */
>>> +static int __uvc_ctrl_load_cur(struct uvc_video_chain *chain,
>>> +                              struct uvc_control *ctrl);
> 
> I think you can move the function up instead of adding a forward
> declaration.
> 
>>>
>>>  static const struct uvc_control_info uvc_ctrls[] = {
>>>         {
>>> @@ -1272,6 +1274,13 @@ static int uvc_ctrl_populate_cache(struct uvc_video_chain *chain,
>>>                                         uvc_ctrl_data(ctrl, UVC_CTRL_DATA_DEF));
>>>                 if (ret < 0)
>>>                         return ret;
>>> +       } else if (!(ctrl->info.flags & UVC_CTRL_FLAG_SET_CUR)) {
> 
> A comment (probably at the top of the function) to explain the fallback
> would be useful.
> 
>>> +               ret = __uvc_ctrl_load_cur(chain, ctrl);
>>> +               if (!ret) {
>>> +                       memcpy(uvc_ctrl_data(ctrl, UVC_CTRL_DATA_DEF),
>>> +                              uvc_ctrl_data(ctrl, UVC_CTRL_DATA_CURRENT),
>>> +                              ctrl->info.size);
>>> +               }
>>>         }
>>>
>>>         if (ctrl->info.flags & UVC_CTRL_FLAG_GET_MIN) {
>>> @@ -1279,14 +1288,31 @@ static int uvc_ctrl_populate_cache(struct uvc_video_chain *chain,
>>>                                         uvc_ctrl_data(ctrl, UVC_CTRL_DATA_MIN));
>>>                 if (ret < 0)
>>>                         return ret;
>>> +       } else if (!(ctrl->info.flags & UVC_CTRL_FLAG_SET_CUR)) {
>>> +               ret = __uvc_ctrl_load_cur(chain, ctrl);
>>> +               if (!ret) {
>>> +                       memcpy(uvc_ctrl_data(ctrl, UVC_CTRL_DATA_MIN),
>>> +                              uvc_ctrl_data(ctrl, UVC_CTRL_DATA_CURRENT),
>>> +                              ctrl->info.size);
>>> +               }
>>>         }
>>> +
>>>         if (ctrl->info.flags & UVC_CTRL_FLAG_GET_MAX) {
>>>                 ret = uvc_ctrl_query_entity(chain->dev, ctrl, UVC_GET_MAX,
>>>                                         uvc_ctrl_data(ctrl, UVC_CTRL_DATA_MAX));
>>>                 if (ret < 0)
>>>                         return ret;
>>> +       } else if (!(ctrl->info.flags & UVC_CTRL_FLAG_SET_CUR)) {
>>> +               ret = __uvc_ctrl_load_cur(chain, ctrl);
>>> +               if (!ret) {
>>> +                       memcpy(uvc_ctrl_data(ctrl, UVC_CTRL_DATA_MAX),
>>> +                              uvc_ctrl_data(ctrl, UVC_CTRL_DATA_CURRENT),
>>> +                              ctrl->info.size);
>>> +               }
>>>         }
>>> +
>>>         if (ctrl->info.flags & UVC_CTRL_FLAG_GET_RES) {
>>> +               u8 *res;
>>>                 ret = uvc_ctrl_query_entity(chain->dev, ctrl, UVC_GET_RES,
>>>                                         uvc_ctrl_data(ctrl, UVC_CTRL_DATA_RES));
>>>                 if (ret < 0) {
>>> @@ -1304,7 +1330,13 @@ static int uvc_ctrl_populate_cache(struct uvc_video_chain *chain,
>>>                                       "an XU control. Enabling workaround.\n");
>>>                         memset(uvc_ctrl_data(ctrl, UVC_CTRL_DATA_RES), 0,
>>>                                ctrl->info.size);
>>> +                       res = uvc_ctrl_data(ctrl, UVC_CTRL_DATA_RES);
>>> +                       *res = 1
>>>                 }
>>> +       } else {
>>> +               memset(uvc_ctrl_data(ctrl, UVC_CTRL_DATA_RES), 0, ctrl->info.size);
>>> +               res = uvc_ctrl_data(ctrl, UVC_CTRL_DATA_RES);
>>> +               *res = 1;
>>>         }
>>>
>>>         ctrl->cached = 1;
>>> @@ -1541,11 +1573,8 @@ static int __uvc_queryctrl_boundaries(struct uvc_video_chain *chain,
>>>                         return ret;
>>>         }
>>>
>>> -       if (ctrl->info.flags & UVC_CTRL_FLAG_GET_DEF)
>>>                 v4l2_ctrl->default_value = uvc_mapping_get_s32(mapping,
>>>                                 UVC_GET_DEF, uvc_ctrl_data(ctrl, UVC_CTRL_DATA_DEF));
> 
> You forgot to reduce the indentation here.
> 
>>> -       else
>>> -               v4l2_ctrl->default_value = 0;
>>>
>>>         switch (mapping->v4l2_type) {
>>>         case V4L2_CTRL_TYPE_MENU:
>>> @@ -1576,23 +1605,14 @@ static int __uvc_queryctrl_boundaries(struct uvc_video_chain *chain,
>>>                 break;
>>>         }
>>>
>>> -       if (ctrl->info.flags & UVC_CTRL_FLAG_GET_MIN)
>>> -               v4l2_ctrl->minimum = uvc_mapping_get_s32(mapping, UVC_GET_MIN,
>>> -                               uvc_ctrl_data(ctrl, UVC_CTRL_DATA_MIN));
>>> -       else
>>> -               v4l2_ctrl->minimum = 0;
>>> +       v4l2_ctrl->minimum = uvc_mapping_get_s32(mapping, UVC_GET_MIN,
>>> +                                       uvc_ctrl_data(ctrl, UVC_CTRL_DATA_MIN));
>>>
>>> -       if (ctrl->info.flags & UVC_CTRL_FLAG_GET_MAX)
>>> -               v4l2_ctrl->maximum = uvc_mapping_get_s32(mapping, UVC_GET_MAX,
>>> -                               uvc_ctrl_data(ctrl, UVC_CTRL_DATA_MAX));
>>> -       else
>>> -               v4l2_ctrl->maximum = 0;
>>> +       v4l2_ctrl->maximum = uvc_mapping_get_s32(mapping, UVC_GET_MAX,
>>> +                                       uvc_ctrl_data(ctrl, UVC_CTRL_DATA_MAX));
>>>
>>> -       if (ctrl->info.flags & UVC_CTRL_FLAG_GET_RES)
>>> -               v4l2_ctrl->step = uvc_mapping_get_s32(mapping, UVC_GET_RES,
>>> -                               uvc_ctrl_data(ctrl, UVC_CTRL_DATA_RES));
>>> -       else
>>> -               v4l2_ctrl->step = 0;
>>> +       v4l2_ctrl->step = uvc_mapping_get_s32(mapping, UVC_GET_RES,
>>> +                                       uvc_ctrl_data(ctrl, UVC_CTRL_DATA_RES));
>>>
>>>         return 0;
>>>  }
>>>
>>>>>>> We can create a new type UVC_CTRL_DATA_TYPE_UNSIGNED_READ_ONLY that
>>>>>>> fakes min, max and res, but I think that it is cleaner this approach.
>>>>>>>
>>>>>>>>> This is a preparation patch.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
>>>>>>>>> ---
>>>>>>>>>  drivers/media/usb/uvc/uvc_ctrl.c | 12 ++++++++++++
>>>>>>>>>  drivers/media/usb/uvc/uvcvideo.h |  8 ++++++++
>>>>>>>>>  2 files changed, 20 insertions(+)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
>>>>>>>>> index 21ec7b978bc7aca21db7cb8fd5d135d876f3330c..59be62ae24a4219fa9d7aacf2ae7382c95362178 100644
>>>>>>>>> --- a/drivers/media/usb/uvc/uvc_ctrl.c
>>>>>>>>> +++ b/drivers/media/usb/uvc/uvc_ctrl.c
>>>>>>>>> @@ -596,6 +596,18 @@ static int uvc_ctrl_query_entity(struct uvc_device *dev,
>>>>>>>>>       if (query == UVC_GET_CUR && ctrl->entity->get_cur)
>>>>>>>>>               return ctrl->entity->get_cur(dev, ctrl->entity,
>>>>>>>>>                                            ctrl->info.selector, data, len);
>>>>>>>>> +     if (query == UVC_GET_DEF && ctrl->entity->get_def)
>>>>>>>>> +             return ctrl->entity->get_def(dev, ctrl->entity,
>>>>>>>>> +                                          ctrl->info.selector, data, len);
>>>>>>>>> +     if (query == UVC_GET_MIN && ctrl->entity->get_min)
>>>>>>>>> +             return ctrl->entity->get_min(dev, ctrl->entity,
>>>>>>>>> +                                          ctrl->info.selector, data, len);
>>>>>>>>> +     if (query == UVC_GET_MAX && ctrl->entity->get_max)
>>>>>>>>> +             return ctrl->entity->get_max(dev, ctrl->entity,
>>>>>>>>> +                                          ctrl->info.selector, data, len);
>>>>>>>>> +     if (query == UVC_GET_RES && ctrl->entity->get_res)
>>>>>>>>> +             return ctrl->entity->get_res(dev, ctrl->entity,
>>>>>>>>> +                                          ctrl->info.selector, data, len);
>>>>>>>>>       if (query == UVC_GET_INFO && ctrl->entity->get_info)
>>>>>>>>>               return ctrl->entity->get_info(dev, ctrl->entity,
>>>>>>>>>                                             ctrl->info.selector, data);
>>>>>>>>> diff --git a/drivers/media/usb/uvc/uvcvideo.h b/drivers/media/usb/uvc/uvcvideo.h
>>>>>>>>> index a931750bdea25b9062dcc7644bf5f2ed89c1cb4c..d6da8ed3ad4cf3377df49923e051fe04d83d2e38 100644
>>>>>>>>> --- a/drivers/media/usb/uvc/uvcvideo.h
>>>>>>>>> +++ b/drivers/media/usb/uvc/uvcvideo.h
>>>>>>>>> @@ -261,6 +261,14 @@ struct uvc_entity {
>>>>>>>>>                       u8 cs, u8 *caps);
>>>>>>>>>       int (*get_cur)(struct uvc_device *dev, struct uvc_entity *entity,
>>>>>>>>>                      u8 cs, void *data, u16 size);
>>>>>>>>> +     int (*get_def)(struct uvc_device *dev, struct uvc_entity *entity,
>>>>>>>>> +                    u8 cs, void *data, u16 size);
>>>>>>>>> +     int (*get_min)(struct uvc_device *dev, struct uvc_entity *entity,
>>>>>>>>> +                    u8 cs, void *data, u16 size);
>>>>>>>>> +     int (*get_max)(struct uvc_device *dev, struct uvc_entity *entity,
>>>>>>>>> +                    u8 cs, void *data, u16 size);
>>>>>>>>> +     int (*get_res)(struct uvc_device *dev, struct uvc_entity *entity,
>>>>>>>>> +                    u8 cs, void *data, u16 size);
>>>>>>>>>
>>>>>>>>>       unsigned int ncontrols;
>>>>>>>>>       struct uvc_control *controls;
> 

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