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

From: Geert Uytterhoeven
Subject: Re: [Qemu-devel] [PATCH RFC] gpio: Add Virtual Aggregator GPIO Driver
Date: Wed, 10 Jul 2019 12:21:35 +0200

Hi Phil,

On Wed, Jul 10, 2019 at 4:00 AM Phil Reid <address@hidden> wrote:
> On 6/07/2019 00:05, Geert Uytterhoeven wrote:
> > 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
> >
> Nice.
> This provides similar functionality to the "gpio inverter" driver currently 
> on the list.
> Other than being just a buffer.

Indeed, both drivers forward GPIO calls, but the gpio inverter modifies
some parameters passed.

The way the drivers obtain references to GPIOs is different, though: the
inverter driver obtains a fixed description from DT, while the virtual
aggregator receives the description at runtime, from sysfs.

But perhaps both drivers could share some code?

> Would it be possible to do the lookup via line names?

Doesn't the fact that a GPIO has a line name means that it is in use, and
thus cannot be aggregated and exported to another user?




Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- address@hidden

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

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