linux-kernel - Re: [PATCH] pinctrl: document the "GPIO mode" pitfall

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Message-ID: <CACRpkdag=YTeQH=VjW4fQ+G94REWeURhC=ErJxjmSUuP=pKYbA@mail.gmail.com>
Date:	Thu, 25 Apr 2013 23:39:18 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	Linus Walleij <linus.walleij@...ricsson.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Stephen Warren <swarren@...dia.com>,
	Anmar Oueja <anmar.oueja@...aro.org>,
	Pankaj Dev <pankaj.dev@...com>
Subject: Re: [PATCH] pinctrl: document the "GPIO mode" pitfall

On Tue, Apr 23, 2013 at 3:33 PM, Laurent Pinchart
<laurent.pinchart@...asonboard.com> wrote:

>> +And your machine configuration may look like this:
>> +--------------------------------------------------
>> +
>> +static unsigned long uart_default_mode[] = {
>> +    PIN_CONF_PACKED(PIN_CONFIG_DRIVE_PUSH_PULL, 0),
>> +};
>> +
>> +static unsigned long uart_sleep_mode[] = {
>> +    PIN_CONF_PACKED(PIN_CONFIG_OUTPUT, 0),
>> +};
>
> I'm having a bit of trouble with PIN_CONFIG_DRIVE_PUSH_PULL and
> PIN_CONFIG_OUTPUT. Strictly speaking, when configured in output mode, the pin
> will be in a push-pull configuration.

For your system or for any system? Open drain, open source are also
output modes, and none of them are push-pull.

> Could you clarify the exact scope of the
> two configuration parameters ?

PIN_CONFIG_OUTPUT is left a bit unspecified, but here the
idea was a passive drive, like just connecting the pin to
VDD or GND without any driver stage at all. Maybe I should
patch the documentation since we seem to be the only
user?

In the above case (which is derived from the ABx500) I think what is
happening is that the pin is connected to ground during sleep,
without any enabled driver stages, which saves a lot of power,
since you do not need to bias the totempole during
sleep in that way.

Yours,
Linus Walleij
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