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

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Message-ID: <51E98D89.9050407@wwwdotorg.org>
Date:	Fri, 19 Jul 2013 13:03:37 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Grygorii Strashko <grygorii.strashko@...com>
CC:	Tony Lindgren <tony@...mide.com>, 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 07/19/2013 04:29 AM, Grygorii Strashko wrote:
> Hi Tony, Stephen
> 
> On 07/19/2013 10:39 AM, Tony Lindgren wrote:
>> * Stephen Warren <swarren@...dotorg.org> [130718 12:33]:
>>> 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"...
>>
>> I think the only sane way to deal with this is to make the I2C controller
>> to show up as two separate I2C controller instances. Then use runtime
>> PM to save and restore the state for each instance, and locking between
>> the two driver instances.
>>
>> For the pin muxing part, I'd do this:
>>
>>             i2c1 instance    i2c2 instance    notes
>> default_state        0 pins        0 pins        (or dedicated pins only)
>> active_state        all pins    alls pins
>> idle_state        safe mode    safe mode
>>
>> Then when i2c1 instance is done, it's runtime PM suspend muxes the pins
>> to safe mode, or some nop state. Then when i2c2 instance is woken, it's
>> runtime PM resume muxes pins to i2c2.
> 
> First of all, I'd like to mention that these patches do *not* connect
> pinctrl to PM runtime, so until driver will call pinctrl_select_state()
> or pinctrl_pm_select_*() there will be no pins state changes.

Isn't the whole point of the pinctrl_pm_select*() APIs to eventually be
called automatically by the runtime PM core, so that we don't need to
write code to do this in every single driver, just like we moved the
call to pinctrl_select_state(default) into the device core so that we
didn't have to make every device do that manually?

> (As result, i2c-mux is not good example, seems:))

As such, I think all situations are good examples, because a generic
feature has to work in all cases.

The description you gave of the behavioural changes this patch creates
seems accurate at a quick glance.
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