linux-kernel - Re: Simple MFD driver example

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Date:	Mon, 16 Dec 2013 15:05:14 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Laszlo Papp <lpapp@....org>
Cc:	sameo@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: Simple MFD driver example

Please refrain from top posting.

Responses should appear under quoted lines, which they reference.

> >> I have been trying to write about 50-100 LOC for 1-2 weeks now. I have
> >> a fairly simple use case for MFD where I need to have gpio and hwmon
> >> functionality for a GPIO.
> >>
> >> I read the mfd drivers inside and out by now, but it looks a bit
> >> inconsistent to me. I am not sure what to follow etc. The
> >> documentation seems to be virtually missing which makes my life and
> >> involvement even more difficult. :(
> >>
> >> May I ask for some pointer how to do all this? It would be nice to see
> >> a minimal example where the parent does not do anything else than just
> >> enumerating the sub-device drivers. Is that possible with an empty
> >> probe callback?
> >>
> >> If I understand the regmap correctly (again, it is possible I do not),
> >> then I would not need this. But in general, it is really hard to deal
> >> with the linux kernel in this regard when basic documentations are
> >> missing.
> >>
> >> I hope this would change over time for the posterity. It is a pity
> >> that contributors do not join probably due to this. It is a bit
> >> chaotic, but I hope it can be changed in the future.
> >>
> >> Apologies for my frustration...
> >
> > Due to the nature of the MFD subsystem, there isn't a simple use-case
> > which you can use as a template. It's completely dependent on the
> > device you are trying to enable. There are lots of well written
> > drivers in the subsystem that you can use as good examples though.

> Well, if you ask me the MFD subsystem is underdocumented, and there is
> some inconsistency between the drivers. It is hard to take any of them
> as a starting point, at least for me.

Feel free to write some. I'm always happy to take helpful patches.

> Currently, I wrote a simple one from scratch which I will post soon.

Okay great.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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