Subject: Re: A better way to sequence driver initialization?

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sun, 11 Apr 2010 17:26:57 +1000

On Sat, 2010-04-10 at 20:31 -0500, Bill Gatliff wrote:
> Right now I'm thinking mostly about GPIO devices which need to register
> before things like gpio-leds can use them.  Sometimes the GPIO expansion
> chip of interest is on i2c, other times it's spi, and still others it's
> a platform or USB device.  Linking the gpio-led driver late helps that
> specific situation.

What it boils down to is that driver A depends on a service provided by
driver B. At some stage, your dependency -has- to be expressed. Either
using the device-tree, or by hard wiring a GPIO number, maybe by having
the platform code figure out the GPIO number and shoving it into a
platform_data structure, etc... but at -some- point, driver "A" needs
some kind of "identifier" to the service provided by driver "B". In our
case, let's say it's a GPIO number.

That's when you can start kicking in a simple mechanism for exporting
services. Let's say the GPIO layer does

	register_service("gpio", gpio_service_query);

With something like

	int gpio_service_query(const char *query_string);

Which can parse whatever arguments are passed after the service name and
return success or failure (or even return a struct device *, whatever,
this is just a rough sketch).

>>From there, you can have for the drivers something like:

	request_service(const char *name, struct device *myself);

Which takes a service name of the form "service:args", for example
"gpio:5" for GPIO #5.

That returns 0 if found, -EAGAIN if not found yet, -EINVAL if rejected
(the service "gpio" can decide that there cannot be a GPIO 5 on the
system, whatever) for example. In addition, it would attach "myself" (if
non-NULL) to an internal list so that later, it's probe() routine can be
called again.

We then add a way for probe() to return -EAGAIN to put the driver "on
hold" silently.

And finally, maybe a way for services to re-run their query (new GPIOs
were added for example) for all "pending" drivers.

I haven't looked at the flip side here which is service -removal- of
course. We may get away without dealing with it... tbd.

> But what about when I want to use a pin on a GPIO expansion device as
> the card-detect for an MMC slot?  Or, as I've just recently noticed in
> one hardware platform I'm working on, using a pin on a SPI GPIO expander
> as a chip-select for another SPI device, but through gpiolib? 
> Eventually, the dependencies don't become circular but changing the link
> order will break some and fix others.  Ugh.

As I said above, at some stage, -somebody- can identify the dependency,
which ends up being translated into a GPIO number, an OF device path, or
something .... That should be enough for the above to be made to fit
no ?

> Definitely, a free-for-all isn't going to work.  But I don't think that
> having every driver do its own kthread_run() is a solution, either. 
> I'll keep tinkering.  :) 

I'd rather keep threads out of the picture for now. IE. Leave the
decision as to do threaded probing or not at a different level.

Cheers,
Ben.