linux-kernel - Re: Driver lis3lv02d_i2c not working on Nokia RX-51

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Date:	Sun, 24 Mar 2013 23:04:37 +0000
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Eric Piel <eric.piel@...mplin-utc.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Driver lis3lv02d_i2c not working on Nokia RX-51

On Sun, Mar 24, 2013 at 11:44:59PM +0100, Pali Rohár wrote:
> On Sunday 24 March 2013 23:14:46 Mark Brown wrote:

> > Well, you should seek support from the board vendor then. 

> Not possible. Nokia is already using Windows Phones and life 
> cycle for Nokia N900 phone is at the end. And Nokia never 
> released any HW documentations to community...

There may well still be people with the required information, though in
general this sort of thing is always going to be a hazard when working
on undocumented hardware.

> Another question: what was reason for that commit 
> ec400c9fab99d16a491cea17d27d0c6a5780b97c
> "lis3lv02d: make regulator API usage unconditional" ?

Failing to provide power to a device is typically a very serious
problem for device operation but the code that was there handles such
errors by ignoring them.  This isn't a robust way forwards and such code
should never have been merged in the first place, as the changelog says
the regulator core provides a number of facilities for stubbing itself
out when it is not required which boards should use.

Handling the possibility that supplies may not be there not only creates
needless repetitive complexity in device drivers but also decreases the
robustness of the system since error handling for access to powered down
devices often isn't very pretty and other drivers or the core may
disrupt the operation of the device by for example powering it down due
to not thinking it's in use.

> I think that for N900 support is reverting above commit needed, I 
> do not see other solution...

If you're convinced that the regulator is kept on for some reason you
could always just provide a fake supply, though obviously it would be
better to hook up the real regulator since this may break if at some
point the kernel decides that whatever is actually providing the supply
is unused and can be turned off.

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