linux-kernel - Re: [PATCH 3/4] media: uvcvideo: Run uvc_ctrl_init

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Message-ID: <20250913134216.GA10328@pendragon.ideasonboard.com>
Date: Sat, 13 Sep 2025 16:42:16 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Hans de Goede <hansg@...nel.org>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Hans Verkuil <hverkuil@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: [PATCH 3/4] media: uvcvideo: Run uvc_ctrl_init_ctrl for all
 controls

Hi Ricardo,

Thank you for the patch.

On Mon, Aug 18, 2025 at 08:15:38PM +0000, Ricardo Ribalda wrote:
> The function uvc_ctrl_init_ctrl() is called for every control for every
> entity, but it exits early if the entity is a extension unit. The comment
> claims that this is done to avoid querying XU controls during probe.
> 
> We only query a control if its entity GUIDs and index matches the
> uvc_ctrls list. There are only controls for the following GUIDs:
> UVC_GUID_UVC_PROCESSING, UVC_GUID_UVC_CAMERA and
> UVC_GUID_EXT_GPIO_CONTROLLER.
> 
> In other words, XU controls will not be queried even without this
> condition.

Looking at the commit that introduced this code

commit 52c58ad6f95ff60343bf0c517182d5f649ca0403
Author: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Date:   Wed Sep 29 16:03:03 2010 -0300

    [media] uvcvideo: Delay initialization of XU controls

we see it contains the following change

-       /* Query XU controls for control information */
-       if (UVC_ENTITY_TYPE(ctrl->entity) == UVC_VC_EXTENSION_UNIT) {
-               struct uvc_control_info info;
-               int ret;
-
-               ret = uvc_ctrl_fill_xu_info(dev, ctrl, &info);
-               if (ret < 0)
-                       return;
-
-               ret = uvc_ctrl_add_info(dev, ctrl, &info);
-               if (ret < 0) {
-                       /* Skip the control */
-                       uvc_trace(UVC_TRACE_CONTROL, "Failed to initialize "
-                               "control %pUl/%u on device %s entity %u\n",
-                               info.entity, info.selector, dev->udev->devpath,
-                               ctrl->entity->id);
-                       memset(ctrl, 0, sizeof(*ctrl));
-               }
+       /* XU controls initialization requires querying the device for control
+        * information. As some buggy UVC devices will crash when queried
+        * repeatedly in a tight loop, delay XU controls initialization until
+        * first use.
+        */
+       if (UVC_ENTITY_TYPE(ctrl->entity) == UVC_VC_EXTENSION_UNIT)
                return;
-       }


This is followed by

        for (; info < iend; ++info) {
                if (uvc_entity_match_guid(ctrl->entity, info->entity) &&
                    ctrl->index == info->index) {
                        uvc_ctrl_add_info(dev, ctrl, info);
                        break;
                 }
        }

        if (!ctrl->initialized)
                return;

(this loops over uvc_ctrls). As uvc_ctrls doesn't contain any non-XU
control, you're right that the function never calls uvc_ctrl_add_info()
for XU controls. The ctrl->initialized check then causes the function to
return, as uvc_ctrl_add_info() wasn't called and initialized is false.
The entity type check was therefore not needed, and can be removed.

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

> In future patches we want to add ChromeOS XU controls that need to the
> initialized. We will make sure that all cameras with ChromeOS XU can
> be queried at probe time.
> 
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
>  drivers/media/usb/uvc/uvc_ctrl.c | 9 ---------
>  1 file changed, 9 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_ctrl.c b/drivers/media/usb/uvc/uvc_ctrl.c
> index efe609d7087752cb2ef516eef0fce12acd13e747..ff975f96e1325532e2299047c07de5d1b9cf09db 100644
> --- a/drivers/media/usb/uvc/uvc_ctrl.c
> +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> @@ -3181,15 +3181,6 @@ static void uvc_ctrl_init_ctrl(struct uvc_video_chain *chain,
>  {
>  	unsigned int i;
>  
> -	/*
> -	 * XU controls initialization requires querying the device for control
> -	 * information. As some buggy UVC devices will crash when queried
> -	 * repeatedly in a tight loop, delay XU controls initialization until
> -	 * first use.
> -	 */
> -	if (UVC_ENTITY_TYPE(ctrl->entity) == UVC_VC_EXTENSION_UNIT)
> -		return;
> -
>  	for (i = 0; i < ARRAY_SIZE(uvc_ctrls); ++i) {
>  		const struct uvc_control_info *info = &uvc_ctrls[i];
>  

-- 
Regards,

Laurent Pinchart