linux-kernel - Re: [RFC] regmap: allow volatile register writes with cached only read maps

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Message-ID: <20180513022202.GA17611@sirena.org.uk>
Date:   Sun, 13 May 2018 11:22:02 +0900
From:   Mark Brown <broonie@...nel.org>
To:     Jorge Ramirez-Ortiz <jramirez@...libre.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [RFC] regmap: allow volatile register writes with cached only
 read maps

On Fri, May 11, 2018 at 12:29:42PM +0200, Jorge Ramirez-Ortiz wrote:
> On 05/11/2018 04:00 AM, Mark Brown wrote:

> > We don't currently suppress writes except when regmap_update_bits()
> > notices that the modification was a noop.  You probably want to be using
> > regmap_write_bits() here instead of regmap_update_bits(), that will
> > always do the write.

> but isnt that interface at a different level?

It's at the level where we suppress writes - the write suppression isn't
a feature of the caching, it's something that regmap_update_bits() does
if it notices that it won't change anything.  It'll happen even if
there's no cache at all.

> I am not sure if you are asking me to review my patch or just discarding the
> RFC and highlighting that I have a configuration problem.

I don't understand your patch as-is.

> In my use case and what triggered this RFC (config below), an 'amixer set'
> might never reach the driver's .reg_write interface even though the register
> is configured as volatile (to me this is not consistent since volatile_reg
> is being silently ignored).

I'm not seeing any inconsistency there.  Volatility means the register
can't be cached as it might change underneath us, it doesn't have any
impact on writes and it's happening at a lower level.  Like I say if you
absolutely need a write to happen you should be explicitly doing a
write, though if you need a write to happen for a noop control change it
sounds like there's something weird with that control that's possibly a
problem anyway.

> So I dont see where/how your recommendation fits; maybe you could clarify a
> bit more please?

As I've been saying if you explicitly need a write to happen don't use
regmap_update_bits(), do something that guarantees you'll get a write
like regmap_write() or regmap_write_bits().

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