Re: [PATCH v3 6/7] docs: gpio: Add GPIO Aggregator/Repeater documentation

[Geert Uytterhoeven]

Hi Linus,

On Sat, Jan 4, 2020 at 1:21 AM Linus Walleij <address@hidden> wrote:
> On Thu, Dec 12, 2019 at 3:48 PM Geert Uytterhoeven <address@hidden> wrote:
> > On Thu, Dec 12, 2019 at 3:42 PM Linus Walleij <address@hidden> wrote:
> > > On Wed, Nov 27, 2019 at 9:43 AM Geert Uytterhoeven
> > > <address@hidden> wrote:
> > > > +The GPIO Aggregator allows access control for individual GPIOs, by 
> > > > aggregating
> > > > +them into a new gpio_chip, which can be assigned to a group or user 
> > > > using
> > > > +standard UNIX file ownership and permissions.  Furthermore, this 
> > > > simplifies and
> > > > +hardens exporting GPIOs to a virtual machine, as the VM can just grab 
> > > > the full
> > > > +GPIO controller, and no longer needs to care about which GPIOs to grab 
> > > > and
> > > > +which not, reducing the attack surface.
> > > > +
> > > > +Aggregated GPIO controllers are instantiated and destroyed by writing 
> > > > to
> > > > +write-only attribute files in sysfs.
> > >
> > > I suppose virtual machines will have a lengthy config file where
> > > they specify which GPIO lines to pick and use for their GPIO
> > > aggregator, and that will all be fine, the VM starts and the aggregator
> > > is there and we can start executing.
> > >
> > > I would perhaps point out a weakness as with all sysfs and with the 
> > > current
> > > gpio sysfs: if a process creates an aggregator device, and then that
> > > process crashes, what happens when you try to restart the process and
> > > run e.g. your VM again?
> > >
> > > Time for a hard reboot? Or should we add some design guidelines for
> > > these machines so that they can cleanly tear down aggregators
> > > previously created by the crashed VM?
> >
> > No, the VM does not create the aggregator.
> >
> > The idea is for the user to create one or more aggregators, set up
> > permissions on /dev/gpiochipX, and launch the VM, passing the aggregated
> > /dev/gpiochipX as parameters.
> > If the VM crashes, just launch it again.
> >
> > Destroying the aggregators is a manual and independent process, after
> > the VM has exited.
>
> I'm thinking about someone making some industrial application for some
> control of a machinery say a robotic arm.
>
> And do make sure this VM is only controlling these GPIOs related to
> this robotic arm, they create a GPIO aggregator. And we care about
> cases like that since we provide this security argument.
>
> Surely that machine will be rebooted.
>
> Surely they don't have a printed paper with all the commands lying
> at the console, and asking whoever powers it back on to manually
> type it all in again. That feels a bit 1981.
>
> So they will have a script for this I suppose. Possibly in some
> initscript so it is set up on boot. And this script echos stuff
> all over the place to set up the aggregator.
>
> Is this the use case you're thinking of?

Exactly.

And they can configure that by echoing the GPIO specifiers to
/sys/bus/platform/drivers/gpio-aggregator/new_device.

If their system has DT, another option is to describe the device in DT,
and add its compatible value to gpio_aggregator_dt_ids[], cfr. the
frobnicator example.

> I just like to have the whole picture here.

Sure. If anything is still unclear, please let me know!
Thanks!

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