[PATCH v8] reset: Add driver for gpio-controlled reset pins

[swarren@wwwdotorg.org (Stephen Warren)]

On 07/15/2013 07:50 PM, Shawn Guo wrote:
> Hi Philipp,
> 
> On Thu, May 30, 2013 at 11:09:00AM +0200, Philipp Zabel wrote:
>> This driver implements a reset controller device that toggle a gpio
>> connected to a reset pin of a peripheral IC. The delay between assertion
>> and de-assertion of the reset signal can be configured via device tree.
>>
>> Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
>> Reviewed-by: Stephen Warren <swarren@nvidia.com>
> 
> I see this patch is very useful, as GPIOs are widely used to reset
> components/devices on board.  But I do not find the patch in v3.11-rc1.
> What's your plan about it?
> 
> Also, I'm wondering if we should register the driver a little bit
> early.  Please see the following patch.  If it makes sense to you,
> I can send the patch to you, or you can just quash it into yours.
> 
> Shawn
> 
> ---8<--------
> 
> From 2f8ce9e5d6525b98f3828b707458e83fabb39d50 Mon Sep 17 00:00:00 2001
> From: Shawn Guo <shawn.guo@linaro.org>
> Date: Sun, 14 Jul 2013 20:41:00 +0800
> Subject: [PATCH] ENGR00269945: reset: register gpio-reset driver in
>  arch_initcall
> 
> It's a little bit late to register gpio-reset driver at module_init
> time, because gpio-reset provides reset control via gpio for other
> devices which are mostly probed at module_init time too.  And it
> becomes even worse, when the gpio comes from IO expander on I2C bus,
> e.g. pca953x.  In that case, gpio-reset needs to be ready before I2C
> bus driver which is generally ready at subsys_initcall time.  Let's
> register gpio-reset driver in arch_initcall() to have it ready early
> enough.

There's no need for the reset driver to be registered before its users;
the users of the reset GPIO will simply have their probe deferred until
the reset controller is available, and then everything will work out
just fine.

> The defer probe mechanism is not used here, because a reset controller
> driver should be reasonably registered early than other devices.  More
> importantly, defer probe doe not help in some nasty cases, e.g. the
> gpio-pca953x device itself needs a reset from gpio-reset driver to start
> working.

That should work fine with deferred probe.