Re: [PATCH v4 3/3] Input: gpio_keys.c: Enable use with non-local GPIO chips.

[Grant Likely <grant.likely@secretlab.ca>]

On Sat, Jun 18, 2011 at 07:51:54AM -0700, Dmitry Torokhov wrote:
> On Sat, Jun 18, 2011 at 07:18:28AM -0600, Grant Likely wrote:
> > On Sat, Jun 18, 2011 at 4:17 AM, Dmitry Torokhov
> > <dmitry.torokhov@gmail.com> wrote:
> > > On Thu, Jun 16, 2011 at 01:27:32PM -0600, Grant Likely wrote:
> > >> On Tue, Jun 14, 2011 at 11:08:11AM +0200, David Jander wrote:
> > >> > Use a threaded interrupt handler in order to permit the handler to use
> > >> > a GPIO driver that causes things like I2C transactions being done inside
> > >> > the handler context.
> > >> > Also, gpio_keys_init needs to be declared as a late_initcall, to make sure
> > >> > all needed GPIO drivers have been loaded if the drivers are built into the
> > >> > kernel.
> > >>
> > >> ...which is a horrid hack, but until device dependencies can be
> > >> described, it isn't one that can be solved easily.
> > >>
> > >
> > > I really do not want to apply this... Currently the order of
> > > initialization does not matter since nothing actually happens until
> > > corresponding device appears on the bus. Does the OF code creates
> > > devices before all resources are ready?
> > 
> > It's not an OF problem.  The problem is that all the platform_devices
> > typically get registered all at once at machine_init time (on arm),
> > and if the gpio expander isn't a platform_device, (like an i2c gpio
> > expander which would end up being a child of a platform_device), then
> > it won't be ready.
> 
> Ah, I see. But that can be handled in board code that should ensure that
> it registers devices in correct order.

Unfortunately, handling it in board code doesn't really work either.
It just shuffles the complexity to the board code to implement some
kind of deferred mechanism for registering devices, and it has to take
into account that it may be a long time before the device actually
appears, such as when the driver is configured as a module.

I completely agree that shuffling initcall order isn't maintainable
though.

A related concern is that changing the device registration order, or
the initcall order, does absolutely nothing to tell runtime PM about
the dependencies between devices.  For instance, how does runtime PM
know when it is safe to PM a gpio controller, when it has no reference
to devices depending on it, like gpio-keys?  (although gpio-keys isn't
a great example because it doesn't really have any runtime PM states).

I think part of the solution is to give drivers the option of
returning a 'defer' code at probe time if it cannot obtain all it's
resources, and have the driver core re-probe it when more devices
become available, but I haven't had time to prototype it yet.

g.

> 
> >  The real problem is that we have no mechanism for
> > holding off or deferring a driver probe if it depends on an
> > asynchronous resource.
> 
> The mechanism we do have - we should not be creating the device for the
> driver to bind to unless all resources that are needed by that device
> are ready.
> 
> Just shuffling the initcall order is not maintanable. Next there will be
> GPIO expander that is for some reason registered as late_initcall and
> we'll be back to square one. I am going to take the threaded IRQ bit but
> will drop the initcall bit from the patch.
> 
> Thanks.
> 
> -- 
> Dmitry