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

On Sat, 18 Jun 2011 09:16:45 -0600
Grant Likely [off-list ref] wrote:

On Sat, Jun 18, 2011 at 07:51:54AM -0700, Dmitry Torokhov wrote:

quoted

On Sat, Jun 18, 2011 at 07:18:28AM -0600, Grant Likely wrote:

quoted

On Sat, Jun 18, 2011 at 4:17 AM, Dmitry Torokhov
[off-list ref] wrote:

quoted

On Thu, Jun 16, 2011 at 01:27:32PM -0600, Grant Likely wrote:

quoted

On Tue, Jun 14, 2011 at 11:08:11AM +0200, David Jander wrote:

quoted

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.

Besides... we don't want anymore board-code, do we? I mean, if a board can use
a generic board configuration and specify all it needs in the device-tree, why
should something as trivial as connecting a gpio_keys device to a I2C GPIO
expander force us to do special board setup all of a sudden?
IMHO specifying I2C-gpios to be used for gpio_keys should "just work", even if
declared in a device-tree.

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

I also agree, and if there is a better solution to make this work without
additional board-support code, please tell me.
I just think that this patch makes the already cool gpio_keys driver quite a
bit more awesome. IMO, being able to just hook it all up in the device-tree is
just fantastic, and we should make it possible.

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.

Sounds interesting. So the probe function could return some sort of -ENOTYET
or -EAGAIN and have it called again later?

But, does that mean that we really need to miss this use-case until something
like this gets approved and merged? Can't we just declare this late_initcall
for now and fix it later? Please!

quoted

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 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.

How would we do that in a device-tree?

quoted

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.

That would destroy the whole purpose of this patch. Do you mean to say, what I
want to do has no acceptable implementation? That would be a pity, since IMHO
it is a very cool feature, and quite trivial to implement this way.
Our boards do not need any board setup code. Actually just adding one
line of code in arch/powerpc/platforms/512x/mpc5121_generic.c or
arch/powerpc/platforms/52xx/mpc5200_simple.c is enough to support any of our
boards that need this driver... the rest is done in the device-tree. Don't you
think this is worth that little bit of (temporary) ugliness?

Best regards,

-- 
David Jander
Protonic Holland.