linux-kernel - Re: [PATCH v2] pinctrl: reserve pins when states are activated

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Date:	Mon, 22 Oct 2012 10:21:19 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Tony Lindgren <tony@...mide.com>
Cc:	Linus Walleij <linus.walleij@...ricsson.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Stephen Warren <swarren@...dia.com>,
	Anmar Oueja <anmar.oueja@...aro.org>,
	Patrice Chotard <patrice.chotard@...com>,
	Jean Nicolas Graux <jean-nicolas.graux@...ricsson.com>,
	Loic Pallardy <loic.pallardy@...com>
Subject: Re: [PATCH v2] pinctrl: reserve pins when states are activated

On Fri, Oct 19, 2012 at 8:10 PM, Tony Lindgren <tony@...mide.com> wrote:
> [Me]
>> Instead: let use reserve the pins when the state is activated
>> and drop them when the state is disabled, i.e. when we move to
>> another state. This way different devices/functions can use the
>> same pins at different times.
>
> Hmm doesn't this mean that we are now doing lots of extra
> reserving and dropping of pins? Performance is important from
> latency point of view for cases where we need to remux pins
> constantly runtime PM.

It is only done in case the pinmux state is switched in runtime
suspend/resume, so it's e.g. possible to just alter the pin config.

But in general what you say is true.

We used to to the same thing by having drivers call
pinctrl_get()/pinctrl_put() in this case instead, but that went
away with the introduction of states, so we cannot encode
different pin sets with say
pinctrl_get(dev, "foo")/pinctrl_get(dev, "bar")
anymore since there is only one pinctrl handle per device,
but multiple states.

If this turns out to be a severe performance bottleneck, I
suggest to add some additional constraint API, like
pinctrl_set_pinmux_homegeneous_pinsets(true) that will
at runtime select whether the pin allocation is done when
getting the pinctrl handle instead.

Yours,
Linus Walleij
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