Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760887AbZLJO60 (ORCPT ); Thu, 10 Dec 2009 09:58:26 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760870AbZLJO6Z (ORCPT ); Thu, 10 Dec 2009 09:58:25 -0500 Received: from cantor.suse.de ([195.135.220.2]:45441 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760845AbZLJO6Y (ORCPT ); Thu, 10 Dec 2009 09:58:24 -0500 Date: Thu, 10 Dec 2009 06:49:20 -0800 From: Greg KH To: Jani Nikula Cc: "dbrownell@users.sourceforge.net" , "linux-kernel@vger.kernel.org" , "dsilvers@simtec.co.uk" , "ben@simtec.co.uk" , "Bityutskiy Artem (Nokia-D/Helsinki)" , "akpm@linux-foundation.org" Subject: Re: [PATCH 2/3] gpiolib: add support for having symlinks under gpio class directory Message-ID: <20091210144920.GA27237@suse.de> References: <71eff9c88d3fec3daa218341c83c19a106ee1f62.1260364108.git.ext-jani.1.nikula@nokia.com> <20091210024834.GB20410@suse.de> <1260455537.25352.1762.camel@jani-desktop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1260455537.25352.1762.camel@jani-desktop> User-Agent: Mutt/1.5.17 (2007-11-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3337 Lines: 86 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/