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

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Date:   Wed, 4 May 2022 16:40:31 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Felipe Balbi <balbi@...nel.org>,
        USB mailing list <linux-usb@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
Subject: Re: [PATCH 4/4] USB: gadget: Add a new bus for gadgets

On Tue, May 03, 2022 at 11:48:33AM -0400, Alan Stern wrote:
> On Tue, May 03, 2022 at 05:27:08PM +0200, Geert Uytterhoeven wrote:
> > Hi Alan,
> > 
> > On Tue, May 3, 2022 at 5:14 PM Alan Stern <stern@...land.harvard.edu> wrote:
> > > On Tue, May 03, 2022 at 12:14:30PM +0200, Geert Uytterhoeven wrote:
> > > > 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.
> > 
> > I'm not so sure where these names are coming from.
> > `git grep '"gadget"'` points to the following likely targets:
> > 
> > drivers/usb/gadget/udc/core.c:  dev_set_name(&gadget->dev, "gadget");
> > drivers/usb/renesas_usbhs/mod_gadget.c: gpriv->mod.name         = "gadget";
> > 
> > Changing both names reveals the problem is actually caused by
> > the former ;-)
> 
> Ah, good.
> 
> One way to attack this would be to keep a static counter and dynamically 
> set the name to "gadget.%d" using the counter's value.  Or keep a bitmap 
> of allocated gadget numbers and use the first available number.
> 
> Felipe, Greg, any opinions?

Just use an idr structure for the number, that's the simplest way to
track that.

thanks,

greg k-h