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

[Geert Uytterhoeven]

Hi Phil,

On Thu, Jul 11, 2019 at 11:24 AM Phil Reid <address@hidden> wrote:
> On 10/07/2019 18:21, Geert Uytterhoeven wrote:
> > 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
> >>>

> >> 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?
>
> They can be given line names via the dt property gpio-line-names.
> Which can be used by user space to find a gpio. Not sure if there's an 
> equivalent api inkerenl.
> But it looks like we can find the info via struct gpiochip_info / 
> gpioline_info linfo and work
> out the chip name and line offsets. So probably not required.
>
> Find the right gpio always seems tricky.
> We have systems with multiple i2c gpio behind muxes that may or may not be 
> present.
> So i2c bus numbers are never consistent. And then different board revisions 
> move the
> same gpio line to a different pin (or cahnge the gpio chip type completely) 
> to make routing easier etc.

OK, so extending lookup to line names makes sense.
This requires making gpio_name_to_desc() public.

Gr{oetje,eeting}s,

                        Geert

-- 
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