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

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Date:   Mon, 9 May 2022 18:23:31 +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 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.

thanks,

greg k-h