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

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Hi Nathan!

On Wed, Sep 28, 2022 at 09:29:55AM -0700, Nathan Chancellor wrote:
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@xxxxxxxxxxxxxx>

...

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;


This sounds right. Do you send a proper patch?

Michael

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