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

From: Bartosz Golaszewski <bgolaszewski@baylibre.com>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>,
	Linus Walleij <linus.walleij@linaro.org>,
	Alexander Graf <agraf@suse.de>,
	Peter Maydell <peter.maydell@linaro.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Magnus Damm <magnus.damm@gmail.com>,
	linux-gpio <linux-gpio@vger.kernel.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC] gpio: Add Virtual Aggregator GPIO Driver
Date: Tue, 9 Jul 2019 16:58:48 +0200	[thread overview]
Message-ID: <CAMpxmJUF1s1zyXVtoUGfbV7Yk+heua4rNjY=DrX=jr-v8UfNxA@mail.gmail.com> (raw)
In-Reply-To: <CAMuHMdWdb0dcS8Nvk-Poz2dT7nuHjFhqpsRPZZnSKsc3VffcRA@mail.gmail.com>

pon., 8 lip 2019 o 12:24 Geert Uytterhoeven <geert@linux-m68k.org> napisał(a):
>
> Hi Bartosz,
>
> On Mon, Jul 8, 2019 at 11:45 AM Bartosz Golaszewski
> <bgolaszewski@baylibre.com> wrote:
> > pt., 5 lip 2019 o 18:05 Geert Uytterhoeven <geert+renesas@glider.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
> > >
> > > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
>
> > I like the general idea and the interface looks mostly fine. Since
> > this is new ABI I think it needs to be documented as well.
>
> Sure.
>
> > I'm having trouble building this module:
> >
> >   CALL    scripts/atomic/check-atomics.sh
> >   CALL    scripts/checksyscalls.sh
> >   CHK     include/generated/compile.h
> >   Kernel: arch/arm/boot/Image is ready
> >   Building modules, stage 2.
> >   MODPOST 235 modules
> > ERROR: "gpiod_request" [drivers/gpio/gpio-virt-agg.ko] undefined!
> > ERROR: "gpiochip_get_desc" [drivers/gpio/gpio-virt-agg.ko] undefined!
> > ERROR: "gpiod_free" [drivers/gpio/gpio-virt-agg.ko] undefined!
> > scripts/Makefile.modpost:91: recipe for target '__modpost' failed
> > make[1]: *** [__modpost] Error 1
> > Makefile:1287: recipe for target 'modules' failed
> > make: *** [modules] Error 2
> > make: *** Waiting for unfinished jobs....
> >
> > I'm not sure what the problem is.
>
> Oops. As this is an RFC, I didn't bother trying to build this driver as
> a module, only builtin.  Apparently the 3 symbols above are not yet
> exported using EXPORT_SYMBOL_GPL().
>

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.

> > > --- /dev/null
> > > +++ b/drivers/gpio/gpio-virt-agg.c
> > > @@ -0,0 +1,390 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +//
> > > +// GPIO Virtual Aggregator
> > > +//
> > > +// Copyright (C) 2019 Glider bvba
> > > +
> > > +#include <linux/gpio/driver.h>
> > > +#include <linux/idr.h>
> > > +#include <linux/kernel.h>
> > > +#include <linux/module.h>
> > > +#include <linux/mutex.h>
> > > +#include <linux/platform_device.h>
> > > +
> > > +#include "gpiolib.h"
> > > +
> > > +#define DRV_NAME       "gpio-virt-agg"
> > > +#define MAX_GPIOS      32
> >
> > Do we really need this limit? I see it simplifies the code, but maybe
> > we can allocate the relevant arrays dynamically and not limit users?
>
> Sure. That limit can be lifted.
>
> > > +static int gpio_virt_agg_set_config(struct gpio_chip *chip,
> > > +                                   unsigned int offset, unsigned long config)
> > > +{
> > > +       struct gpio_virt_agg_priv *priv = gpiochip_get_data(chip);
> > > +
> > > +       chip = priv->desc[offset]->gdev->chip;
> > > +       if (chip->set_config)
> > > +               return chip->set_config(chip, offset, config);
> > > +
> > > +       // FIXME gpiod_set_transitory() expects success if not implemented
>
> BTW, do you have a comment about this FIXME?
>

Ha! Interesting. I'll give it a thought and respond elsewhere as it's
a different subject.

> > > +       return -ENOTSUPP;
> > > +}
>
> > > +static int gpio_virt_agg_probe(struct platform_device *pdev)
> > > +{
> > > +       struct device *dev = &pdev->dev;
> > > +       const char *param = dev_get_platdata(dev);
> > > +       struct gpio_virt_agg_priv *priv;
> > > +       const char *label = NULL;
> > > +       struct gpio_chip *chip;
> > > +       struct gpio_desc *desc;
> > > +       unsigned int offset;
> > > +       int error, i;
> >
> > This 'i' here is reported as possibly not initialized:
> >
> > drivers/gpio/gpio-virt-agg.c: In function ‘gpio_virt_agg_probe’:
> > drivers/gpio/gpio-virt-agg.c:230:13: warning: ‘i’ may be used
> > uninitialized in this function [-Wmaybe-uninitialized]
> >   int error, i;
> >              ^
>
> Oops, should be preinitialized to zero. WIll fix.
>
> > > +static int gpio_virt_agg_remove(struct platform_device *pdev)
> > > +{
> > > +       struct gpio_virt_agg_priv *priv = platform_get_drvdata(pdev);
> > > +       unsigned int i;
> > > +
> > > +       gpiochip_remove(&priv->chip);
> > > +
> > > +       for (i = 0; i < priv->chip.ngpio; i++)
> > > +               gpiod_free(priv->desc[i]);
>
> Perhaps I should use gpiod_put() instead, which is exported to modules?
>
> > > +
> > > +       return 0;
> > > +}
> >
> > You shouldn't need this function at all. It's up to users to free descriptors.
>
> This frees the upstream descriptors, not the descriptors used by users
> of the virtual gpiochip. Shouldn't they be freed, as they are no longer
> in use?
>
> Note that .probe() doesn't use devm_gpiochip_add_data(), as the upstream
> descriptors need to be freed after the call to gpiochip_remove().
>
> Thanks!

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.

Bart

>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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