linux-kernel - Re: [PATCH 1/2] gpio: add pin biasing and drive mode to gpiolib

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Date:	Thu, 21 Apr 2011 09:32:26 +1000
From:	Ben Nizette <bn@...sdigital.com>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-kernel@...r.kernel.org,
	Grant Likely <grant.likely@...retlab.ca>,
	Lee Jones <lee.jones@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/2] gpio: add pin biasing and drive mode to gpiolib

On 20/04/2011, at 10:21 PM, Linus Walleij wrote:

> 2011/4/19 Ben Nizette <bn@...sdigital.com>:
>> On 18/04/2011, at 10:26 PM, Alan Cox wrote:
>> 
>>> The logical extension to that is to delete the gpio layer because the
>>> board code can do it ?
>> 
>> Well anything that is done once at startup, yes,
> 
> This patch is about biasing and drive modes. We need to alter
> these at runtime (from board code, indeed) due to the fact that
> when you go to sleep e.g. floating a pin yeilds better power
> characteristics.

This is actually an interesting case because floating pins yeild
/worse/ power characteristics (each transistor of the push-pull
is on a little bit and you get a path straight through) [1].  To
get good power performance you want to pull an input pin high or
low but which of those two directions depends on external
constraints, i.e. the board.  This is a case where the driver
should /not/ go playing with things it can't fully understand.

Perhaps one of the properties that a board can set in a gpio chip
driver is the suspend state and have suspend/resume hooks in the
gpio chip take care of setting things up on each side.

	--Ben.

[1] http://processors.wiki.ti.com/index.php/Optimizing_IO_Power_Consumption

> 
> Linus Walleij

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