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

[Linus Walleij <linus.walleij@linaro.org>]

On Fri, Jul 5, 2019 at 5:05 PM Geert Uytterhoeven
<geert+renesas@glider.be> 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
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> ---
> Aggregating GPIOs and exposing them as a new gpiochip was suggested in
> response to my proof-of-concept for GPIO virtualization with QEMU[1][2].
>
> Sample session on r8a7791/koelsch:
>
>   - Disable the leds node in arch/arm/boot/dts/r8a7791-koelsch.dts
>
>   - Create virtual aggregators:
>
>     $ echo "e6052000.gpio 19 20" \
>             > /sys/bus/platform/drivers/gpio-virt-agg/new_device
>
>     gpio-virt-agg gpio-virt-agg.0: GPIO 0 => e6052000.gpio/19
>     gpio-virt-agg gpio-virt-agg.0: GPIO 1 => e6052000.gpio/20
>     gpiochip_find_base: found new base at 778
>     gpio gpiochip8: (gpio-virt-agg.0): added GPIO chardev (254:8)
>     gpiochip_setup_dev: registered GPIOs 778 to 779 on device: gpiochip8 (gpio-virt-agg.0)
>
>     $ echo "e6052000.gpio 21, e6050000.gpio 20 21 22" \
>             > /sys/bus/platform/drivers/gpio-virt-agg/new_device
>
>     gpio-virt-agg gpio-virt-agg.1: GPIO 0 => e6052000.gpio/21
>     gpio-virt-agg gpio-virt-agg.1: GPIO 1 => e6050000.gpio/20
>     gpio-virt-agg gpio-virt-agg.1: GPIO 2 => e6050000.gpio/21
>     gpio-virt-agg gpio-virt-agg.1: GPIO 3 => e6050000.gpio/22
>     gpiochip_find_base: found new base at 774
>     gpio gpiochip9: (gpio-virt-agg.1): added GPIO chardev (254:9)
>     gpiochip_setup_dev: registered GPIOs 774 to 777 on device: gpiochip9 (gpio-virt-agg.1)
>
>   - Adjust permissions on /dev/gpiochip[89] (optional)
>
>   - Control LEDs:
>
>     $ gpioset gpiochip8 0=0 1=1 # LED6 OFF, LED7 ON
>     $ gpioset gpiochip8 0=1 1=0 # LED6 ON, LED7 OFF
>     $ gpioset gpiochip9 0=0     # LED8 OFF
>     $ gpioset gpiochip9 0=1     # LED8 ON
>
>   - Destroy virtual aggregators:
>
>     $ echo gpio-virt-agg.0 \
>             > /sys/bus/platform/drivers/gpio-virt-agg/delete_device
>     $ echo gpio-virt-agg.1 \
>             > /sys/bus/platform/drivers/gpio-virt-agg/delete_device
>
> Thanks for your comments!
>
> References:
>   - [1] "[PATCH QEMU POC] Add a GPIO backend"
>         (https://lore.kernel.org/linux-renesas-soc/20181003152521.23144-1-geert+renesas@glider.be/)
>   - [2] "Getting To Blinky: Virt Edition / Making device pass-through
>          work on embedded ARM"
>         (https://fosdem.org/2019/schedule/event/vai_getting_to_blinky/)

I'm looping in my friends at Google for this discussion.

They need a virtualized gpio_chip for their Android emulator,
and their current approach for other devices has been around
using virtio in most cases and an emulated AC97 for the
audio case as far as I remember.

It would be great to have their input on this so we can create a
virtualization/aggregate that works for all.

Please include adelva@google.com on future postings of this!

Yours,
Linus Walleij