linux-kernel - Re: [PATCH 2/3] gpiolib: add support for having symlinks under gpio class directory

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Date:	Thu, 10 Dec 2009 06:49:20 -0800
From:	Greg KH <gregkh@...e.de>
To:	Jani Nikula <ext-jani.1.nikula@...ia.com>
Cc:	"dbrownell@...rs.sourceforge.net" <dbrownell@...rs.sourceforge.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dsilvers@...tec.co.uk" <dsilvers@...tec.co.uk>,
	"ben@...tec.co.uk" <ben@...tec.co.uk>,
	"Bityutskiy Artem (Nokia-D/Helsinki)" <Artem.Bityutskiy@...ia.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] gpiolib: add support for having symlinks under
	gpio class directory

On Thu, Dec 10, 2009 at 04:32:17PM +0200, Jani Nikula wrote:
> On Thu, 2009-12-10 at 03:48 +0100, ext Greg KH wrote:
> > On Wed, Dec 09, 2009 at 03:49:03PM +0200, Jani Nikula wrote:
> > > Extend the functionality of gpio_export_link() to allow exported GPIOs
> > > to have names using sysfs links under /sys/class/gpio.
> > 
> > No, please don't create symlinks under a class, that is not something
> > that any userspace tool is expecting.
> > 
> > I don't understand what you are trying to do here, why do you need a
> > symlink?  What is wrong with the original device names?
> 
> The problem
> ~~~~~~~~~~~
> 
> GPIOs can be exported to gpiolib sysfs at /sys/class/gpio/ like this:
> 
> # ls -l /sys/class/gpio/
> --w-------    1 root     0            4096 Jan  1 00:00 export
> lrwxrwxrwx    1 root     0               0 Jan  1 00:00 gpio25 -> ../../devices/virtual/gpio/gpio25
> lrwxrwxrwx    1 root     0               0 Jan  1 00:00 gpio38 -> ../../devices/virtual/gpio/gpio38
> ...
> 
> The GPIO lines may and do change from board revision to another. For
> example a power button's GPIO number might be 25 in board rev 1.0 but 66
> in board rev 1.1.
> 
> We want to assign symbolic names to GPIO lines and hide the numbering
> changes from userspace, because it is very painful to amend userspace
> for every board revision.

True, just like it is hard to change the kernel for every type of
configuration as well :)

This is the problem that udev solves, from usersapce, but you don't have
device nodes for it to manage, right?

> Existing solution
> ~~~~~~~~~~~~~~~~~
> 
> gpio_export_link() can be used to create symlinks from drivers' sysfs
> to gpiolib, but this obviously requires a driver. For example:
> 
> # ls -l /sys/devices/platform/foo/
> lrwxrwxrwx    1 root     0               0 Jan  2 00:31 power_button -> ../../virtual/gpio/gpio25
> ...
> 
> This doesn't really work for GPIO lines not associated with any driver.

Understood

> How we addressed the problem
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> We simply decided to extend gpio_export_link(). When it is called
> with 'dev == NULL' (no driver), it creates an additional symlink
> in /sys/class/gpio/
> 
> # ls -l /sys/class/gpio/
> --w-------    1 root     0            4096 Jan  1 00:00 export
> lrwxrwxrwx    1 root     0               0 Jan  1 00:00 power_button -> ../../devices/virtual/gpio/gpio25
> lrwxrwxrwx    1 root     0               0 Jan  1 00:00 gpio25 -> ../../devices/virtual/gpio/gpio25
> lrwxrwxrwx    1 root     0               0 Jan  1 00:00 gpio38 -> ../../devices/virtual/gpio/gpio38
> ...
> 
> This is exactly what our patchset allows to do.
> 
> An alternative would be a dummy driver just to create a home in sysfs
> for the standalone GPIOs and use the gpio_export_link() to make the
> links there.
> 
> Any other suggestions?

This type of "policy" should be better off done in userspace whereever
possible.  Can't you just have a udev rule to create symlinks from
somewhere else, into /sys/class/gpio/ that show this type of information
that you are wanting to have?

thanks,

greg k-h
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