linux-kernel - Re: [PATCH 4/4] USB: gadget: Add a new bus for gadgets

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Message-ID: <YnFCEn45XwDWM/9Y@rowland.harvard.edu>
Date:   Tue, 3 May 2022 10:54:10 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        USB mailing list <linux-usb@...r.kernel.org>,
        linux-renesas-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] USB: gadget: Add a new bus for gadgets

On Tue, May 03, 2022 at 12:14:30PM +0200, Geert Uytterhoeven wrote:
> 	Hi Alan,
> 
> On Sat, 23 Apr 2022, Alan Stern wrote:
> > This patch adds a "gadget" bus and uses it for registering gadgets and
> > their drivers.  From now on, bindings will be managed by the driver
> > core rather than through ad-hoc manipulations in the UDC core.
> > 
> > As part of this change, the driver_pending_list is removed.  The UDC
> > core won't need to keep track of unbound drivers for later binding,
> > because the driver core handles all of that for us.
> > 
> > However, we do need one new feature: a way to prevent gadget drivers
> > from being bound to more than one gadget at a time.  The existing code
> > does this automatically, but the driver core doesn't -- it's perfectly
> > happy to bind a single driver to all the matching devices on the bus.
> > The patch adds a new bitflag to the usb_gadget_driver structure for
> > this purpose.
> > 
> > A nice side effect of this change is a reduction in the total lines of
> > code, since now the driver core will do part of the work that the UDC
> > used to do.
> > 
> > A possible future patch could add udc devices to the gadget bus, say
> > as a separate device type.
> > 
> > Signed-off-by: Alan Stern <stern@...land.harvard.edu>
> 
> Thanks for your patch, which is now commit fc274c1e997314bf ("USB:
> gadget: Add a new bus for gadgets") in usb-next.
> 
> This patch cause a regression on the Renesas Salvator-XS development
> board, as R-Car H3 has multiple USB gadget devices:

Then these gadgets ought to have distinct names in order to avoid the 
conflict below:

>     sysfs: cannot create duplicate filename '/bus/gadget/devices/gadget'
>     CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.18.0-rc1-arm64-renesas-00074-gfc274c1e9973 #1587
>     Hardware name: Renesas Salvator-X 2nd version board based on r8a77951 (DT)
>     Call trace:
>      dump_backtrace+0xcc/0xd8
>      show_stack+0x14/0x30
>      dump_stack_lvl+0x88/0xb0
>      dump_stack+0x14/0x2c
>      sysfs_warn_dup+0x60/0x78
>      sysfs_do_create_link_sd.isra.0+0xe4/0xf0
>      sysfs_create_link+0x20/0x40
>      bus_add_device+0x64/0x110
>      device_add+0x31c/0x850
>      usb_add_gadget+0x124/0x1a0
>      usb_add_gadget_udc_release+0x1c/0x50
>      usb_add_gadget_udc+0x10/0x18
>      renesas_usb3_probe+0x450/0x728
...

Having three gadget devices, all named "gadget", doesn't seem like a 
good idea.

> After boot-up, only one gadget device is visible:

Naturally, since the first registration succeeds and the later ones fail 
because they can't reuse the same name.

>     root@...salvator-xs:~# ls -l /sys/bus/gadget/devices/
>     total 0
>     lrwxrwxrwx 1 root root 0 Feb 14  2019 gadget -> ../../../devices/platform/soc/e659c000.usb/gadget
>     root@...salvator-xs:~#
> 
> Reverting this patch fixes the issue.

This doesn't seem like it should be too hard to fix, although I'm not 
at all familiar with the renesas-usb3 driver.  Do you know who maintains 
that driver?  Is it you?

Alan Stern