linux-kernel - Re: Simple MFD driver example

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Date:	Mon, 16 Dec 2013 14:00:56 +0000
From:	Laszlo Papp <lpapp@....org>
To:	Lee Jones <lee.jones@...aro.org>
Cc:	sameo@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: Simple MFD driver example

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

On Mon, Dec 16, 2013 at 8:25 AM, Lee Jones <lee.jones@...aro.org> wrote:
>> 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.
>
> Which device are you trying to write a driver for? Is there a
> datasheet you can point to?
>
> NB: When emailing subsystem maintainers for help, please don't forget
> to CC the relevant Mailing Lists. All lists can be found in the
> MAINTAINERS file, which is parsed by <kernel>/scripts/get_maintainer.pl.
>
> --
> Lee Jones
> Linaro STMicroelectronics Landing Team Lead
> Linaro.org │ Open source software for ARM SoCs
> Follow Linaro: Facebook | Twitter | Blog
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