linux-kernel - Re: [PATCH 5/7] regmap: Check if a register is writable instead of readable in regcache

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Message-ID: <20111116165622.GP29986@opensource.wolfsonmicro.com>
Date:	Wed, 16 Nov 2011 16:56:22 +0000
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Dimitris Papastamos <dp@...nsource.wolfsonmicro.com>,
	Jonathan Cameron <jic23@...nel.org>,
	Michael Hennerich <michael.hennerich@...log.com>,
	linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
	device-drivers-devel@...ckfin.uclinux.org, drivers@...log.com
Subject: Re: [PATCH 5/7] regmap: Check if a register is writable instead of
 readable in regcache_read

On Wed, Nov 16, 2011 at 05:52:49PM +0100, Lars-Peter Clausen wrote:
> On 11/16/2011 05:38 PM, Mark Brown wrote:

> >> Hm? The use case here is chips which do not support readback. So we never
> >> want to fallback to a hardware read but still want to be able to do a cached
> >> read.

> > This code will be run on every chip, including chips with read/write
> > access.  Caches are useful for all chips.

> Of course. And it still works for chips with read/write support with this
> patch, but it doesn't work for chips without read support without this patch.

No, it'll fail if we ever cache volatile registers at startup (which
is a perfectly sensible thing to do for things like chip revisions -
they're not something we can hard code the default for but they're not
going to change at runtime).

> > If you're looking at the read function and it's checking to see if the
> > register is writeable the first thought would be that this is a
> > cut'n'paste error.  The above code is at best *way* too cute.

> We can of course add a comment explaining why it is regmap_writable instead
> of regmap_readable.

No, really - just do something legible and robust.  For example, teach
regmap_readable() about the cache.
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