linux-kernel - Re: [PATCH] pinctrl: document the pinctrl PM states

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Message-ID: <51C88B54.9000902@wwwdotorg.org>
Date:	Mon, 24 Jun 2013 12:09:24 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Tony Lindgren <tony@...mide.com>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	Stephen Warren <swarren@...dia.com>,
	Kevin Hilman <khilman@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH] pinctrl: document the pinctrl PM states

On 06/24/2013 04:10 AM, Tony Lindgren wrote:
> * Stephen Warren <swarren@...dotorg.org> [130621 12:18]:
>> On 06/21/2013 12:25 AM, Tony Lindgren wrote:
>>> * Stephen Warren <swarren@...dotorg.org> [130620 12:32]:
>>>>
>>>> I assume you mean there shouldn't be any issue *modifying* the pinctrl
>>>> API to allow multiple states to be active at once? And where you're
>>>> talking about having multiple sets active at once already, you're
>>>> talking about some other API?
>>>
>>> Nope, the standard pinctrl API. At least I have not seen issues with
>>> having multiple states active the same time in a single driver.
>>
>> Please take a look at the implementation of pinctrl_select_state(). It
>> very explicitly performs the following steps:
>>
>> 1) Find all pins(groups) that are used in the current state but not the
>> new state, and execute pinctrl_disable_setting() on them. (For mux
>> settings only, not pin config, since the core doesn't have any idea how
>> to reverse config settings).
>>
>> 2) For all settings in the new state, apply those settings.
>>
>> So, it very explicitly only allows a single state to be set at a time.
>> Equally, p->state (the field which stores the currently selected state)
>> is a single item, not a set/list/array.
> 
> OK thanks I get now what you're saying. I did not see the p->state
> issue as the disable function won't do anything for the SoCs that I
> mostly deal with.
> 
>> So, this code needs rework if you want the core to support the concept
>> of having multiple states active at once, since it needs separate
>> pinctrl_activate_state() and pinctrl_deactivate_state() APIs, in order
>> to avoid step (1) above. And of course, p->state would need to be a
>> set/list/array.
> 
> I'll think about it a bit and do a patch to fix this. It seems that
> that we need just two entries in the p->state array: static (default),
> and dynamic. Then the dynamic would be typically one of: active, idle,
> rx, tx.

I'm not entirely convinced that "2" is the right number. If we start
allowing drivers to "piece together" multiple different state names, why
wouldn't you allow 3 (or n) different state names to be active at once?
Off-hand, I don't have specific use-cases in mind for more than 2 state
(or even 1 in my case I suspect) - it just seems like expecting to
arbitrarily restrict the number of co-active states is unlikely to last
for long, and it'll end up being a slippery slope.
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