linux-kernel - Re: [PATCH V2 4/6] pinctrl: API changes to support multiple states per device

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Message-ID: <20120301030931.GA27020@shlinux2.ap.freescale.net>
Date:	Thu, 1 Mar 2012 11:09:32 +0800
From:	Dong Aisheng <aisheng.dong@...escale.com>
To:	Stephen Warren <swarren@...dia.com>
CC:	Linus Walleij <linus.walleij@...ricsson.com>,
	Linus Walleij <linus.walleij@...aro.org>,
	"B29396@...escale.com" <B29396@...escale.com>,
	"s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
	"dongas86@...il.com" <dongas86@...il.com>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"thomas.abraham@...aro.org" <thomas.abraham@...aro.org>,
	"tony@...mide.com" <tony@...mide.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 4/6] pinctrl: API changes to support multiple states
 per device

On Wed, Feb 29, 2012 at 09:22:42AM -0800, Stephen Warren wrote:
> Dong Aisheng wrote at Tuesday, February 28, 2012 11:47 PM:
.....
> > > +	if (p->state) {
> > > +		/*
> > > +		 * The set of groups with a mux configuration in the old state
> > > +		 * may not be identical to the set of groups with a mux setting
> > > +		 * in the new state. While this might be unusual, it's entirely
> > > +		 * possible for the "user"-supplied mapping table to be written
> > > +		 * that way. For each group that was configured in the old state
> > > +		 * but not in the new state, this code puts that group into a
> > > +		 * safe/disabled state.
> >
> > It means the old state function of some groups may not have been disabled before
> > enabling the new function mode.
> > May it be a little error-prone?
> > Maybe we can not depend on user to disable a function before enable another for
> > a group when doing pinmux_enable_setting considering they may be different
> > registers.
> 
> The idea here is that there are two cases:
> 
> 1) (hopefully the typical case)
> 
> Group X HAS a mux function set in OLD state
> Group X HAS a mux function set in NEW state
> 
> In this case, we simply call pinmux_enable_setting(new state).
> 
> We do this so that we can transition directly from the old state to the
> new state without going through any artificial intermediate states, to
> make the mux switching as glitchless as possible.
> 
> If the HW has some restriction where it has to operate like:
> 
> Old state -> idle -> new state
> 
> Rather than:
> 
> Old state -> new state
> 
> Then I think that should be taken care of within the individual driver;
> given most mux selection is just a register field that feeds into a simple
> signal mux in HW, such a requirement seems pretty unlikely to me, but you
> never know.
> 
I thought before that the mux function for GROUP X in OLD state may be set
in register A while mux function in NEW state may be set in register B.
So before setting new state function, user should disable old state function
(clear register A)first. (Maybe i thought too far)

But now i realize that you're correct, this can be handled in individual driver
for different HW requirement.

> 2)
> 
> Group X HAS a mux function set in OLD state
> Group X DOES NOT have a mux function set in NEW state
> 
> In this case, we call pinmux_disable_setting(old state) to ensure that
> the pin is put into some idle state that can't interfere with any other
> pin.
> 
Thanks for the detailed clarification.

Regards
Dong Aisheng

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