Re: [PATCH v10 2/5] gpio: sim: new testing module

[Kent Gibson <warthog618@gmail.com>]

On Thu, Nov 25, 2021 at 02:14:20PM +0100, Bartosz Golaszewski wrote:
> On Wed, Nov 24, 2021 at 12:43 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> >
> > Implement a new, modern GPIO testing module controlled by configfs
> > attributes instead of module parameters. The goal of this driver is
> > to provide a replacement for gpio-mockup that will be easily extensible
> > with new features and doesn't require reloading the module to change
> > the setup.
> >
> > Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
> > ---
> >  Documentation/admin-guide/gpio/gpio-sim.rst |   80 ++
> >  drivers/gpio/Kconfig                        |    8 +
> >  drivers/gpio/Makefile                       |    1 +
> >  drivers/gpio/gpio-sim.c                     | 1370 +++++++++++++++++++
> >  4 files changed, 1459 insertions(+)
> >  create mode 100644 Documentation/admin-guide/gpio/gpio-sim.rst
> >  create mode 100644 drivers/gpio/gpio-sim.c
> >
> 
> Hi guys!
> 
> I'd like to get your opinion on some parts of the interface.
> 
> Should we allow creating multiple gpiochips per platform device like
> some drivers do? And if so - should the sysfs groups be created for
> each gpiochip device kobject and not the parent?
> 
> Currently we do this:
> 
> # Create the chip (platform device + single gpiochip):
> mkdir /sys/kernel/config/gpio-sim/my-chip
> # Configure it
> echo 8 > /sys/kernel/config/gpio-sim/my-chip/num_lines
> # Enable it
> echo 1 > /sys/kernel/config/gpio-sim/my-chip/live
> 
> What I mean above would make it look like this:
> 
> # Create the platform device
> mkdir /sys/kernel/config/gpio-sim/my-gpio-device
> 
> # what's inside?
> ls /sys/kernel/config/gpio-sim/my-gpio-device
> live
> 
> # Create GPIO chips
> mkdir /sys/kernel/config/gpio-sim/my-gpio-device/chip0
> mkdir /sys/kernel/config/gpio-sim/my-gpio-device/chip1
> 
> # Configure chips
> echo 8 > /sys/kernel/config/gpio-sim/my-gpio-device/chip0/num_lines
> echo 4 > /sys/kernel/config/gpio-sim/my-gpio-device/chip1/num_lines
> echo foobar > /sys/kernel/config/gpio-sim/my-gpio-device/chip1/label
> 
> # Enable both chips
> echo 1 > /sys/kernel/config/gpio-sim/my-gpio-device/live
> 
> And in sysfs instead of current:
> 
> echo pull-up > /sys/devices/platform/gpio-sim.0/sim_line0/pull
> 
> We'd have to do:
> 
> echo pull-up > /sys/devices/platform/gpio-sim.0/gpiochip1/sim_line0/pull
> 
> While I don't see any usefulness of that at this time, if we don't do
> it now, then it'll be hard to extend this module later. What are your
> thoughts?
> 

I might be missing something, but I don't see the platform abstraction
adding anything that can't be easily emulated in userspace using multiple
chips, and it complicates the minimal case as you now have to create a
platform as well as the chip.
So I'd keep it simple and stick with the chip level abstraction.

Cheers,
Kent.