linux-kernel - Re: [PATCH v2 2/4] gpio: add viperboard gpio driver

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Date:	Tue, 16 Oct 2012 12:00:13 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Lars Poeschel <poeschel@...onage.de>
Cc:	Lars Poeschel <larsi@....tu-dresden.de>, sameo@...ux.intel.com,
	linux-kernel@...r.kernel.org, jic23@....ac.uk, khali@...ux-fr.org,
	ben-linux@...ff.org, w.sang@...gutronix.de,
	grant.likely@...retlab.ca
Subject: Re: [PATCH v2 2/4] gpio: add viperboard gpio driver

On Tue, Oct 16, 2012 at 8:51 AM, Lars Poeschel <poeschel@...onage.de> wrote:
> On Monday 15 October 2012 at 15:00:12, Linus Walleij wrote:
>> > +       /* if io is set to output, just return the saved value */
>> > +       if (gpio->gpioa_out & (1 << offset))
>> > +               return gpio->gpioa_val & (1 << offset);
>>
>> That's not going to work if the hardware changes state
>> behind the back of the driver right? Oh well, maybe
>> it doesn't matter.
>
> I thought about that and then did this "cache" only in case the gpio is a
> output to save to usb ping-pong that is needed otherwise. I thought that
> nothing can change to state of the output but the driver itself.

On a second note there is already a standard mechanism for caching
registers in the kernel, and that is regmap. Sadly it's a bit
undocumented but there are several examples and the code
lives in drivers/base/regmap and include/linux/regmap.h

Yours,
Linus Walleij
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