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

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Message-ID: <20111116163812.GM29986@opensource.wolfsonmicro.com>
Date:	Wed, 16 Nov 2011 16:38:12 +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 05:34:33PM +0100, Lars-Peter Clausen wrote:
> On 11/16/2011 05:16 PM, Mark Brown wrote:

> > This logic doesn't entirely follow - one can have registers which are
> > volatile but could be read once at startup.  Plus...

> 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.

> >> @@ -206,7 +206,7 @@ int regcache_read(struct regmap *map,

> >>  	BUG_ON(!map->cache_ops);

> >> -	if (!regmap_readable(map, reg))
> >> +	if (!regmap_writeable(map, reg))
> >>  		return -EIO;

> > ...the code winds up just looking like an obvious bug.

> Why? If a register is not writable we won't have anything in the cache for
> it. So reading from the cache for a register which is not writable doesn't
> make any sense.

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.
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