Re: [PATCH 0/6] GPIO character device skeleton

[Johan Hovold <johan@kernel.org>]

On Tue, Nov 03, 2015 at 06:18:42PM +0100, Linus Walleij wrote:
> On Tue, Nov 3, 2015 at 1:06 PM, Johan Hovold <johan@kernel.org> wrote:
> > [Pargmann]
> >> As an idea: We could use the complete path to create some sort of unique id for
> >> the device (perhaps hash or something different). This id can be exported as
> >> device attribute and would allow udev to create some links as known from
> >> /dev/disk/by-id for example. This would make identifying a single chip quite
> >> easy for any userspace application and we would avoid having this really long
> >> path somewhere.
> >
> > The unique ids are already there in sysfs, for example:
> >
> > $ for x in /sys/bus/gpio/devices/gpiochip*; do readlink $x; done
> > ../../../devices/platform/68000000.ocp/48310000.gpio/gpiochip0
> > ../../../devices/platform/68000000.ocp/49050000.gpio/gpiochip1
> > ../../../devices/platform/68000000.ocp/49052000.gpio/gpiochip2
> > ../../../devices/platform/68000000.ocp/49054000.gpio/gpiochip3
> > ../../../devices/platform/68000000.ocp/49056000.gpio/gpiochip4
> > ../../../devices/platform/68000000.ocp/49058000.gpio/gpiochip5
> > ../../../devices/platform/68000000.ocp/48070000.i2c/i2c-0/0-0048/twl4030-gpio/gpiochip6
> > ../../../devices/platform/68000000.ocp/48064000.usbhshost/48064800.ehci/usb1/1-2/1-2.3/1-2.3:1.0/gpiochip7
> >
> > And libudev can be used to lookup devices based on (parent) attributes
> > (such as USB VID/PID, serial numbers, etc).
> >
> > We could also export further attributes if that would help (e.g.
> > gpio-chip labels).
> 
> Yeah sysfs does provide this.
>
> This has the problem that everyone and their dog need to use udev
> or something like it. Some library or so that run around in sysfs like
> libudev+systemdlib does currently.
> 
> And since Android does not use udev, and Busybox does not use udev,
> there are quite a few million users there. Or at least two big projects that
> need to reimplement the same idea. It could be argued that they should,
> since sysfs is indeed an ABI.
>
> In the Busybox mdev case part of the goal is to minimize userspace code
> size too, and it does not support the complex rules of udev for example.
> 
> It'd be nice if devices could be uniquely identified in the chardev alone
> I think, then we don't need to much reliance on external assumptions
> and traversing sysfs somehow for more info.

Point is that this is not a gpio-specific problem. Userspace needs to
deal with this for any resource it wants to access (e.g. i2c or spi).

And you should be able to use libudev for tree-traversal without actually
using udevd, right?

Johan