linux-kernel - Re: [NEW DRIVER V1 5/7] DA9058 GPIO driver

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Message-ID: <20120807171437.GC16861@opensource.wolfsonmicro.com>
Date:	Tue, 7 Aug 2012 18:14:38 +0100
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	"Opensource [Anthony Olech]" <anthony.olech.opensource@...semi.com>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [NEW DRIVER V1 5/7] DA9058 GPIO driver

On Mon, Aug 06, 2012 at 03:15:17PM +0000, Opensource [Anthony Olech] wrote:

> I do realize that REGMAP does locking on individual register accesses,
> however, the each GPIO line is controlled by 4-bits in a register, with
> the meaning of the most significant bit depending on the GPIO direction,
> so it is essential that the register be read first before do an update, thus
> two sequential register accesses must be protected by a mutex to
> prevent another process changing the register (and hence the meaning
> of the most-significant bit) in the middle of the two accesses.

> I hope this explains to your satisfaction why a driver mutex is required
> in addition to the regmap's register access mutex

This seems a bit excessive and complicated - I'd be inclined to either
just say that the caller is responsible for avoiding confusion here
(obviously if you're changing the direction there's a race anyway) or
store the data in a variable locally rather than having to do I/O on the
device under lock every time it's interacted with.
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