linux-kernel - Re: [PATCH RFC] gpio: Add Virtual Aggregator GPIO Driver

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Date:   Fri, 6 Sep 2019 13:14:10 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Bartosz Golaszewski <bgolaszewski@...libre.com>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Linus Walleij <linus.walleij@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Magnus Damm <magnus.damm@...il.com>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC] gpio: Add Virtual Aggregator GPIO Driver

Hi Bartosz,

On Fri, Jul 12, 2019 at 11:27 AM Bartosz Golaszewski
<bgolaszewski@...libre.com> wrote:
> wt., 9 lip 2019 o 17:59 Geert Uytterhoeven <geert@...ux-m68k.org> napisał(a):
> > On Tue, Jul 9, 2019 at 4:59 PM Bartosz Golaszewski
> > <bgolaszewski@...libre.com> wrote:
> > > pon., 8 lip 2019 o 12:24 Geert Uytterhoeven <geert@...ux-m68k.org> napisał(a):
> > > > On Mon, Jul 8, 2019 at 11:45 AM Bartosz Golaszewski
> > > > <bgolaszewski@...libre.com> wrote:
> > > > > pt., 5 lip 2019 o 18:05 Geert Uytterhoeven <geert+renesas@...der.be> napisał(a):
> > > > > > GPIO controllers are exported to userspace using /dev/gpiochip*
> > > > > > character devices.  Access control to these devices is provided by
> > > > > > standard UNIX file system permissions, on an all-or-nothing basis:
> > > > > > either a GPIO controller is accessible for a user, or it is not.
> > > > > > Currently no mechanism exists to control access to individual GPIOs.
> > > > > >
> > > > > > Hence add a virtual GPIO driver to aggregate existing GPIOs (up to 32),
> > > > > > and expose them as a new gpiochip.  This is useful for implementing
> > > > > > access control, and assigning a set of GPIOs to a specific user.
> > > > > > Furthermore, it would simplify and harden exporting GPIOs to a virtual
> > > > > > machine, as the VM can just grab the full virtual GPIO controller, and
> > > > > > no longer needs to care about which GPIOs to grab and which not,
> > > > > > reducing the attack surface.
> > > > > >
> > > > > > Virtual GPIO controllers are instantiated by writing to the "new_device"
> > > > > > attribute file in sysfs:
> > > > > >
> > > > > >     $ echo "<gpiochipA> <gpioA1> [<gpioA2> ...]"
> > > > > >            "[, <gpiochipB> <gpioB1> [<gpioB2> ...]] ...]"
> > > > > >             > /sys/bus/platform/drivers/gpio-virt-agg/new_device
> > > > > >
> > > > > > Likewise, virtual GPIO controllers can be destroyed after use:
> > > > > >
> > > > > >     $ echo gpio-virt-agg.<N> \
> > > > > >             > /sys/bus/platform/drivers/gpio-virt-agg/delete_device
> >
> > > Am I doing it right? I'm trying to create a device and am only getting this:
> > >
> > > # echo gpiochip2 23 > new_device
> > > [  707.507039] gpio-virt-agg gpio-virt-agg.0: Cannot find gpiochip gpiochip2
> > >
> > > gpiochip2 *does* exist in the system.
> >
> > Please try the name of the platform device instead.
> > I.e. for my koelsch (R-Car M2-W), it needs "e6052000.gpio" instead
> > of "gpiochip2".
> >
> > Probably the driver should match on both.
> >
> > > I see. I'll try to review it more thoroughly once I get to play with
> > > it. So far I'm stuck on creating the virtual chip.
>
> This is not a show-stopper but one thing that's bothering me in this
> is that lines used by the aggregator are considered 'used' in regard
> to the original chip. I'm wondering how much effort would it take to
> have them be 'muxed' into two (real and virtual) chips at once.

Is that really what you want?
If a GPIO is aggregated with othrs, it's intended to be used only through
the aggregator, isn't it?

> Other than that - seems to works pretty nice other than the matching
> by chip name and by line names.

Thanks, working on that...

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds