linux-kernel - Re: [PATCH 2/4] usb: gadget: uvc: fix interval

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Message-ID: <aV/Vg4BBAa+kl+0k@lizhi-Precision-Tower-5810>
Date: Thu, 8 Jan 2026 11:04:19 -0500
From: Frank Li <Frank.li@....com>
To: Xu Yang <xu.yang_2@....com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Michael Grzeschik <m.grzeschik@...gutronix.de>, jun.li@....com,
	imx@...ts.linux.dev, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 2/4] usb: gadget: uvc: fix interval_duration calculation

On Thu, Jan 08, 2026 at 03:43:03PM +0800, Xu Yang wrote:
> According to USB specification:
>
>   For full-/high-speed isochronous endpoints, the bInterval value is
>   used as the exponent for a 2^(bInterval-1) value.
>
> To correctly convert bInterval as interval_duration:
>   interval_duration = 2^(bInterval-1) * frame_interval
>
> Because the unit of video->interval is 100ns, add a comment info to
> make it clear.
>
> Fixes: 48dbe731171e ("usb: gadget: uvc: set req_size and n_requests based on the frame interval")
> Cc: stable@...r.kernel.org
> Signed-off-by: Xu Yang <xu.yang_2@....com>
> ---
>  drivers/usb/gadget/function/uvc.h       | 2 +-
>  drivers/usb/gadget/function/uvc_video.c | 7 +++++--
>  2 files changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/usb/gadget/function/uvc.h b/drivers/usb/gadget/function/uvc.h
> index b3f88670bff801a43d084646974602e5995bb192..676419a049762f9eb59e1ac68b19fa34f153b793 100644
> --- a/drivers/usb/gadget/function/uvc.h
> +++ b/drivers/usb/gadget/function/uvc.h
> @@ -107,7 +107,7 @@ struct uvc_video {
>  	unsigned int width;
>  	unsigned int height;
>  	unsigned int imagesize;
> -	unsigned int interval;
> +	unsigned int interval;	/* in 100ns units */
>  	struct mutex mutex;	/* protects frame parameters */
>
>  	unsigned int uvc_num_requests;
> diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
> index 1c0672f707e4e5f29c937a1868f0400aad62e5cb..b1c5c1d3e9390c82cc84e736a7f288626ee69d51 100644
> --- a/drivers/usb/gadget/function/uvc_video.c
> +++ b/drivers/usb/gadget/function/uvc_video.c
> @@ -499,7 +499,7 @@ uvc_video_prep_requests(struct uvc_video *video)
>  {
>  	struct uvc_device *uvc = container_of(video, struct uvc_device, video);
>  	struct usb_composite_dev *cdev = uvc->func.config->cdev;
> -	unsigned int interval_duration = video->ep->desc->bInterval * 1250;
> +	unsigned int interval_duration;
>  	unsigned int max_req_size, req_size, header_size;
>  	unsigned int nreq;
>
> @@ -513,8 +513,11 @@ uvc_video_prep_requests(struct uvc_video *video)
>  		return;
>  	}
>
> +	interval_duration = int_pow(2, video->ep->desc->bInterval - 1);

2 << (video->ep->desc->bInterval - 1) or BIT(video->ep->desc->bInterval - 1);

int_pow() use while loop. slice better.

Reviewed-by: Frank Li <Frank.Li@....com>

Frank

>  	if (cdev->gadget->speed < USB_SPEED_HIGH)
> -		interval_duration = video->ep->desc->bInterval * 10000;
> +		interval_duration *= 10000;
> +	else
> +		interval_duration *= 1250;
>
>  	nreq = DIV_ROUND_UP(video->interval, interval_duration);
>
>
> --
> 2.34.1
>