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

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Date:   Mon, 9 May 2022 12:47:19 -0400
From:   Alan Stern <stern@...land.harvard.edu>
To:     Greg KH <gregkh@...uxfoundation.org>
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 Mon, May 09, 2022 at 06:23:31PM +0200, Greg KH wrote:
> On Mon, May 09, 2022 at 11:05:06AM -0400, Alan Stern wrote:
> > On Mon, May 09, 2022 at 04:42:01PM +0200, Geert Uytterhoeven wrote:
> > > Hi Alan,
> > > 
> > > On Mon, May 9, 2022 at 4:15 PM Alan Stern <stern@...land.harvard.edu> wrote:
> > > > On Mon, May 09, 2022 at 09:46:25AM +0200, Geert Uytterhoeven wrote:
> > > > > > Geert:
> > > > > >
> > > > > > Can you test the patch below?  It ought to fix the problem (although it
> > > > >
> > > > > Thanks!
> > > > >
> > > > > root@...salvator-xs:~# ls -l /sys/bus/gadget/devices/
> > > > > total 0
> > > > > lrwxrwxrwx 1 root root 0 Feb 14  2019 gadget.0 ->
> > > > > ../../../devices/platform/soc/e659c000.usb/gadget.0
> > > > > lrwxrwxrwx 1 root root 0 Feb 14  2019 gadget.1 ->
> > > > > ../../../devices/platform/soc/ee020000.usb/gadget.1
> > > > > lrwxrwxrwx 1 root root 0 Feb 14  2019 gadget.2 ->
> > > > > ../../../devices/platform/soc/e6590000.usb/gadget.2
> > > > >
> > > > > Tested-by: Geert Uytterhoeven <geert+renesas@...der.be>
> > > > >
> > > > > LGTM, so
> > > > > Reviewed-by: Geert Uytterhoeven <geert+renesas@...der.be>
> > > >
> > > > Thanks!
> > > >
> > > > > > might end up causing other problems down the line...)
> > > > >
> > > > > Can you please elaborate? I'm not too familiar with UBS gadgets.
> > > >
> > > > I was concerned about the fact that changing the name of a file,
> > > > directory, or symbolic link in sysfs means changing a user API, and so
> > > > it might cause some existing programs to fail.  That would be a
> > > > regression.
> > > >
> > > > Perhaps the best way to work around the problem is to leave the name set
> > > > to "gadget" if the ID number is 0, while adding the ID number on to the
> > > > name if the value is > 0.  What do you think?
> > > 
> > > Oh, you mean the "gadget.N" subdirs, which are the targets of the
> > > symlinks above? These were indeed named "gadget" before.
> > > 
> > > Would it be possible to append the ".N" suffixes only to the actual
> > > symlinks, while keeping the target directory names unchanged?
> > > E.g. /sys/bus/gadget/devices/gadget.0 ->
> > > ../../../devices/platform/soc/e659c000.usb/gadget
> > 
> > No, it's not possible.  Or at least, not without significant changes to 
> > the driver core.  Besides, people expect these names to be the same.
> 
> It should always be ok to change the names of devices as those are not
> going to be persistent / determinisitic.  It's the attributes of devices
> that are supposed to be used to determine those types of things.
> 
> So I think let's start out with the .N suffix for everything for now.
> I'll be glad to submit the fixed patch to the Android kernel build
> system to see what their testing comes back with to see if they happened
> to make any name assumptions as that's the largest user of this
> codebase.

Okay.  Do you need me to send it as a proper patch before you try it 
out?

Alan Stern