[v4] usb: gadget: uvc: increase worker prio to WQ_HIGHPRI

This patch is changing the simple workqueue in the gadget driver to be
allocated as async_wq with a higher priority. The pump worker, that is
filling the usb requests, will have a higher priority and will not be
scheduled away so often while the video stream is handled. This will
lead to fewer streaming underruns.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>

---
v3 -> v4: - improved commit message to be more clear
v2 -> v3: - renamed workqueue to "uvcgadget"
v1 -> v2: - added destroy_workqueue in uvc_function_unbind
          - reworded comment above allow_workqueue

 drivers/usb/gadget/function/f_uvc.c     | 4 ++++
 drivers/usb/gadget/function/uvc.h       | 1 +
 drivers/usb/gadget/function/uvc_v4l2.c  | 2 +-
 drivers/usb/gadget/function/uvc_video.c | 9 +++++++--
 4 files changed, 13 insertions(+), 3 deletions(-)

Hi Michael,

On Wed, Sep 07, 2022 at 11:58:18PM +0200, Michael Grzeschik wrote:
> This patch is changing the simple workqueue in the gadget driver to be
> allocated as async_wq with a higher priority. The pump worker, that is
> filling the usb requests, will have a higher priority and will not be
> scheduled away so often while the video stream is handled. This will
> lead to fewer streaming underruns.
> 
> Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>

...

> diff --git a/drivers/usb/gadget/function/uvc.h b/drivers/usb/gadget/function/uvc.h
> index 58e383afdd4406..1a31e6c6a5ffb8 100644
> --- a/drivers/usb/gadget/function/uvc.h
> +++ b/drivers/usb/gadget/function/uvc.h
> @@ -88,6 +88,7 @@ struct uvc_video {
>  	struct usb_ep *ep;
>  
>  	struct work_struct pump;
> +	struct workqueue_struct *async_wq;
>  
>  	/* Frame parameters */
>  	u8 bpp;

I am commenting here because this is the most recent change but after
this showed up in -next as commit 9b91a6523078 ("usb: gadget: uvc:
increase worker prio to WQ_HIGHPRI"), I see the following warning/error
when building s390 allmodconfig:

  In file included from ../include/linux/string.h:253,
                   from ../include/linux/bitmap.h:11,
                   from ../include/linux/cpumask.h:12,
                   from ../include/linux/smp.h:13,
                   from ../include/linux/lockdep.h:14,
                   from ../include/linux/rcupdate.h:29,
                   from ../include/linux/rculist.h:11,
                   from ../include/linux/pid.h:5,
                   from ../include/linux/sched.h:14,
                   from ../include/linux/ratelimit.h:6,
                   from ../include/linux/dev_printk.h:16,
                   from ../include/linux/device.h:15,
                   from ../drivers/usb/gadget/function/f_uvc.c:9:
  In function ‘fortify_memset_chk’,
      inlined from ‘uvc_register_video’ at ../drivers/usb/gadget/function/f_uvc.c:424:2:
  ../include/linux/fortify-string.h:301:25: error: call to ‘__write_overflow_field’ declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
    301 |                         __write_overflow_field(p_size_field, size);
        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This commit did not directly cause this, it just made the issue more
obvious. In commit e4ce9ed835bc ("usb: gadget: uvc: ensure the vdev is
unset"), also authored by you, the size parameter appears to be wrong?
It is using the size of 'struct uvc_video', instead of the size of
'struct video_device'. It appears to be pure luck that everything worked
up until this point, as those two types had the same size (1400 bytes)
before this change but now 'struct uvc_video' is 1408 bytes, meaning
there is now an overwrite. Any reason this is not the fix?

Cheers,
Nathan

diff --git a/drivers/usb/gadget/function/f_uvc.c b/drivers/usb/gadget/function/f_uvc.c
index e6948cf8def3..836601227155 100644
--- a/drivers/usb/gadget/function/f_uvc.c
+++ b/drivers/usb/gadget/function/f_uvc.c
@@ -421,7 +421,7 @@ uvc_register_video(struct uvc_device *uvc)
 	int ret;
 
 	/* TODO reference counting. */
-	memset(&uvc->vdev, 0, sizeof(uvc->video));
+	memset(&uvc->vdev, 0, sizeof(uvc->vdev));
 	uvc->vdev.v4l2_dev = &uvc->v4l2_dev;
 	uvc->vdev.v4l2_dev->dev = &cdev->gadget->dev;
 	uvc->vdev.fops = &uvc_v4l2_fops;