linux-kernel - Re: [PATCH 3/4] pinctrl: Add support for additional dynamic states

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Message-ID: <51E84180.6080902@wwwdotorg.org>
Date:	Thu, 18 Jul 2013 13:26:56 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Tony Lindgren <tony@...mide.com>
CC:	linus.walleij@...aro.org, linux-omap@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/4] pinctrl: Add support for additional dynamic states

On 07/18/2013 01:36 AM, Tony Lindgren wrote:
> * Stephen Warren <swarren@...dotorg.org> [130717 14:30]:
>> On 07/16/2013 03:05 AM, Tony Lindgren wrote:
...
>> Why shouldn't e.g. a pinctrl-based I2C mux also be able to do runtime
>> PM? Does the mux setting select which states are used for runtime PM, or
>> does runtime PM override the basic mux setting, or must the pincrl-I2C
>> mux manually implement custom runtime-PM/pinctrl interaction since
>> there's no generic answer to those questions? How many more custom
>> exceptions will there be?
> 
> The idea is that runtime PM will never touch the basic mux settings
> at all. The "default" state should be considered a static state
> that is claimed during driver probe, and released when the driver
> is unloaded. This is typically let's say 90% of the pins for any
> device.
> 
> For runtime PM, we can just toggle the PM related pinctrl states as
> they have been verified to match the active state during init.
> 
> So I don't see why pinctrl-I2C would have to do anything specific.
> All that is required is that the pins are grouped for the consumer
> driver, and we can provide some automated checks on the states for
> runtime PM.

So, consider a pinctrl-based I2C mux. It has 2 states to cover two I2C
buses:

a) bus 1: I2C controller is muxed out onto one set of pins.

b) bus 2: I2C controller is muxed out onto another set of pins.

Now, the system could go idle in either of those 2 states, and then
later need to return to one of those states. I just don't see how that
would work, since the runtime PM code in this series switches to *an*
active state when the device becomes non-idle. If the definition of the
idle state switches the mux function for both sets of pins to some
idle/quiescent value, then you'd need to do different reprogramming when
going back to "the" active state; I guess the system would need to
remember which state was active before switching to idle, then switch
back to that same state rather than hard-coding the active state name as
"active"...
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