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

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Date:	Wed, 16 Nov 2011 18:09:06 +0100
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
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 11/16/2011 05:56 PM, Mark Brown wrote:
> 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).
> 

Ah ok, now I get it, you are talking about that this will hypothetical break
a future patch ;)

>>> 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.

Doesn't make much sense. We call regmap_readable from regcache_read, which
is only called if we use a cache. So if we let regmap_readable return true
in case we use a cache it will always be true in regcache_read and we can
drop the check entirely.

I'll update the patch to just drop the check.

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