Hi Paul,
(CC'ing Felipe Balbi and Roger Quadros)
Thank you for the patch.
Have you used scripts/get_maintainer.pl ? It should point you to Felipe Balbi,
the maintainer of the USB gadget subsystem, who I recommend you CC, at least
on patch 2/4. The script also points to Greg, who I don't think needs to be
CC'ed as he doesn't deal with USB gadget as much as with USB in general, and
who is fairly busy as usual.
While the get_maintainer script doesn't point to Roger, his name can be found
as the author of the USB_GADGET_DELAYED_STATUS mechanism. He authored commit
1b9ba000177e ("usb: gadget: composite: Allow function drivers to pause control
transfers") with his nokia.com address back then, but git log --author 'Roger
Quadros' shows that he's still active on USB and working for TI now. I've thus
CC'ed him on this reply.
On Wednesday, 18 April 2018 06:18:14 EEST Paul Elder wrote:
> The completion of the usb status phase from uvc_function_set_alt needs
> to be delayed until uvc_v4l2_streamon/off. This is currently done by
> uvc_function_set_alt returning USB_GADGET_DELAYED_STATUS and
> composite_setup detecting this to increment cdev->delayed_status.
> However, if uvc_v4l2_streamon/off is called in between this return and
> increment, uvc_function_setup_continue within uvc_v4l2_streamon/off will
> WARN that cdev->delayed_status is zero.
While this is correct, I wouldn't mention UVC here as the patch is for the USB
composite gadget framework and isn't specific to UVC. You should write a more
generic explanation of the problem to explain why the race between returning
USB_GADGET_DELAYED_STATUS (and processing it in the caller) and calling
usb_composite_setup_continue() can't be properly solved in gadget drivers,
thus requiring a new function.
> To fix situations like this, add a function to increment
> cdev->delayed_status.
>
> Signed-off-by: Paul Elder <paul.elder@xxxxxxxx>
> ---
> drivers/usb/gadget/composite.c | 6 ++++++
> include/linux/usb/composite.h | 2 ++
> 2 files changed, 8 insertions(+)
>
> diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
> index 77c7ecca816a..c02ab640a7ae 100644
> --- a/drivers/usb/gadget/composite.c
> +++ b/drivers/usb/gadget/composite.c
> @@ -1548,6 +1548,12 @@ static int fill_ext_prop(struct usb_configuration *c,
> int interface, u8 *buf) return 0;
> }
>
> +void usb_composite_setup_delay(struct usb_composite_dev *cdev)
> +{
> + cdev->delayed_status++;
According to include/linux/usb/composite.h, the delayed_status field should be
protected by cdev->lock, which you should use here.
I've read through the code and found out that, while all callers of
reset_config(), as well as usb_composite_setup_continue(), correctly take the
lock, it isn't taken around f->set_alt() in composite_setup(). This causes the
race condition. I wonder if a simpler fix wouldn't be to take the lock before
calling f->set_alt() and releasing it after incrementing delayed_status. I am
however worried that this could lead to deadlocks if one of the existing
set_alt() handlers calls a function that takes the same lock. Another worry
is that some of the .set_alt() handlers might not expect to be called with
interrupts disabled. This should be analyzed, and I hope that Roger and/or
Felipe will have some insight on this.
If usb_composite_setup_delay() turns out to be the preferred solution, it
would be nice to document the function with kerneldoc.
Finally, I just came to wonder whether the UVC gadget driver really needs to
defer the status phase of the SET_INTERFACE request, or if it couldn't just
proceed normally.
> +}
> +EXPORT_SYMBOL(usb_composite_setup_delay);
> +
> /*
> * The setup() callback implements all the ep0 functionality that's
> * not handled lower down, in hardware or the hardware driver(like
> diff --git a/include/linux/usb/composite.h b/include/linux/usb/composite.h
> index cef0e44601f8..049f77a4d42b 100644
> --- a/include/linux/usb/composite.h
> +++ b/include/linux/usb/composite.h
> @@ -524,6 +524,8 @@ extern int composite_setup(struct usb_gadget *gadget,
> extern void composite_suspend(struct usb_gadget *gadget);
> extern void composite_resume(struct usb_gadget *gadget);
>
> +extern void usb_composite_setup_delay(struct usb_composite_dev *c);
> +
> /*
> * Some systems will need runtime overrides for the product identifiers
> * published in the device descriptor, either numbers or strings or both.
--
Regards,
Laurent Pinchart
--
To unsubscribe from this list: send the line "unsubscribe linux-usb" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
