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

From: Bill Gatliff <[email-masked]>
Date: Sat, 10 Apr 2010 08:35:41 -0500

Benjamin Herrenschmidt wrote:
> On Fri, 2010-04-09 at 14:23 -0500, Bill Gatliff wrote:
>   
>> My recent post, "Requesting a GPIO that hasn't been registered yet", and
>> Anton's reply thereto (thanks, Anton!) on linuxppc-dev got me thinking
>> about the problem of dependencies between devices in different  classes,
>> and/or between drivers/devices in general.  I'd like to float an idea to
>> see if it's worth pursuing. 
>>     
>
> I'd rather do things a bit more explicitely and less prone to break
> existing stuff... something along the lines of, first defining a variant
> of the initcalls to enable that "multithreaded" stuff, along with an
> explicit wait_for_service("subsys:hid"); for example.
>
> One could also expose service deps via the module info, thus delaying
> module init, or things like that (in fact, initcalls could even come
> with a list of dependencies).
>   

The general problem with your approach is that the module in question
might not know what services it needs to wait for.

Specific to my situation, the gpio-led code doesn't have any way of
knowing that it needs to wait until my pca953x i2c devices have all been
installed so that the gpio pin I have specified even exists.  And short
of setting up some kind of table in the board-specific code (or device
tree, actually), I don't know how to communicate such a dependency
without touching the generic gpio-led code--- which I'm trying to avoid.

I just want gpio-led to try again if the gpio pin it has been provided
can't be requested yet.  And I can see the need for similar behavior in
several other of my drivers, hence the desire to generalize things a
bit.  I'm pretty sure my approach is only half-baked, but it does seem
to avoid the need to touch a bunch of existing code--- especially to add
board-specific dependencies.  It just give the system a tool to sort
things out on its own.

I do think your overall criticism is valid, though.

b.g.

-- 
Bill Gatliff
[email-masked]