linux-kernel - Re: [PATCH 1/5] max44000: Initial commit

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Message-ID: <57153733.1070605@kernel.org>
Date:	Mon, 18 Apr 2016 20:36:19 +0100
From:	Jonathan Cameron <jic23@...nel.org>
To:	Mark Brown <broonie@...nel.org>,
	Crestez Dan Leonard <leonard.crestez@...el.com>
Cc:	Peter Meerwald-Stadler <pmeerw@...erw.net>,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Daniel Baluta <daniel.baluta@...el.com>
Subject: Re: [PATCH 1/5] max44000: Initial commit

On 18/04/16 13:34, Mark Brown wrote:
> On Mon, Apr 18, 2016 at 03:15:54PM +0300, Crestez Dan Leonard wrote:
> 
>> As a further clarification: regmap_write will write to hardware even if
>> the cache is known to be up-to-date and no matter the regcache_type. Did
>> I understand this correctly?
> 
>> I'm basing this on reading the code, it seems to me that map->reg_write
>> is only avoided on error paths or if map->cache_only is set to true.
> 
>> This always-write guarantee is not obvious and if it's OK for drivers to
>> rely on it perhaps it should be explicitly documented on regmap_write.
> 
> Yes.  I have to say that you are the first person I've encountered who
> has been confused by this, I'm not sure why you'd expect writes to be
> discarded.
> 
It confused me too :)  To my mind a classic cache optimization would be
to not write to the hardware if the value is already known to be as
desired.

Still, I guess it would add another check to identify which
registers you really wanted to hammer whatever vs which can be assumed
not to read the write is not worth the effort for this case that 
inherently won't be hit that often.

Jonathan