linux-kernel - Re: Using regmap_update_bits to update a write only register

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Message-ID: <20150305175356.GB21293@sirena.org.uk>
Date:	Thu, 5 Mar 2015 17:53:57 +0000
From:	Mark Brown <broonie@...nel.org>
To:	Daniel Baluta <daniel.baluta@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Using regmap_update_bits to update a write only register

On Thu, Mar 05, 2015 at 07:35:32PM +0200, Daniel Baluta wrote:

> Is it possible to use regmap_update_bits to update the values
> of some bits in a write only register?

That should work.

> I was hoping that by filling the .reg_defaults field of regmap_config
> the regmap_update_bits function will not try to read the register
> from hardware.

> Instead I think first call of regmap_update_bits will try to read the register
> from hardware, but this fails with -EIO because the register is marked as
> write only.

> Am I doing something wrong?

Probably, or there's a bug.  What should happen is that if the register
default appeared successfully then the read will get statisfied from the
cache in the manner you describe - presumably that's gone wrong somehow.
Have you set num_reg_defaults?  That's the obvious thing...

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