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 07:24:52 -0800
From:	Greg KH <gregkh@...e.de>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Jani Nikula <ext-jani.1.nikula@...ia.com>,
	"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 04:17:00PM +0100, Kay Sievers wrote:
> On Thu, Dec 10, 2009 at 15:49, Greg KH <gregkh@...e.de> wrote:
> > On Thu, Dec 10, 2009 at 04:32:17PM +0200, Jani Nikula wrote:
> >> 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?
> 
> We can not do anything like that. The symlink name must always match
> the name in /sys/devices/, and it must be a 1:1 relation. We can not
> allow anything else, it confuses tools which rightfully expect this.
> 
> You could convert the class to a "bus", the /sys/class layout is
> pretty much broken by design, and should better be avoided. It's not
> extensible, flat in its layout, and today just a too dumb version of
> the "bus" subsystem which does not have all these limitations.
> 
> All the device links would show up in /sys/bus/gpio/devices/, here you
> have the same rules as in /sys/class/: no additional symlinks, the
> same names as in /sys/devices -- but you could add a custom folder to
> the  /sys/bus/gpio/ directory where you could do whatever fits your
> needs. :)

Ah, yeah, good point, the pci "slots" directory does something like this
today, with symlinks pointing out of that directory to other devices.

It's messy, but it somehow works...

thanks,

greg k-h
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