[PATCH 2/4] usb: gadget: uvc: fix interval_duration calculation

[Xu Yang <xu.yang_2@nxp.com>]

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@vger.kernel.org
Signed-off-by: Xu Yang <xu.yang_2@nxp.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);
 	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