linux-kernel - Re: [PATCH RFC v2 8/8] ARM: zynq: DT: Add pinctrl information

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Date:	Fri, 31 Oct 2014 18:36:10 +0100
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Sören Brinkmann <soren.brinkmann@...inx.com>
Cc:	Michal Simek <michal.simek@...inx.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Steffen Trumtrar <s.trumtrar@...gutronix.de>
Subject: Re: [PATCH RFC v2 8/8] ARM: zynq: DT: Add pinctrl information

On Fri, Oct 31, 2014 at 5:57 PM, Sören Brinkmann
<soren.brinkmann@...inx.com> wrote:
> On Fri, 2014-10-31 at 09:17AM +0100, Linus Walleij wrote:

>> Again it seems to be a sequencing problem. And device tree is
>> not good at sequences, therefore all states should be self-contained.
>
> I agree, but how would I define a pin with pull-up enabled and
> tri-state disabled - assume the pin is currently in a random state that
> can have those things set/not set arbitrarily.

I was more thinking as everything you don't enable explicitly
in a state is per definition disabled.

So if you are in state A and tri-state is enabled there and you
move to state B where pull-up is enabled, then tri-state should
be disabled, since it is not explicitly enabled.

> I can't put bias-disable in DT since it would potentially disable both
> and the pull-up enable would have only a transient effect.

Well look at the callback from the core:

        int (*pin_config_set) (struct pinctrl_dev *pctldev,
                               unsigned pin,
                               unsigned long *configs,
                               unsigned num_configs);

You get all configs in an array. The driver can walk over the list and
make informed decisions on what to do *BEFORE* poking any registers.

Avoiding transients as you describe is part of why the callback
looks as it does. This is why every driver has its own for-loop.

> I can't do the sequencing in the driver either. If I see pull-up enable,
> I can't imply an effect on tri-state since I can't know whether there
> was/will be a tri-state property that sets it as well.

If you define that each state is self-contained you can.

Yours,
Linus Walleij
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