linux-kernel - Re: [PATCH] pinctrl: establish pull-up/pull-down terminology

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Date:	Fri, 21 Jun 2013 13:06:47 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	James Hogan <james.hogan@...tec.com>
CC:	Linus Walleij <linus.walleij@...ricsson.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Stephen Warren <swarren@...dia.com>,
	� <heiko@...ech.de>,
	Linus Walleij <linus.walleij@...aro.org>,
	Anmar Oueja <anmar.oueja@...aro.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH] pinctrl: establish pull-up/pull-down terminology

On 06/20/2013 06:38 AM, James Hogan wrote:
> On 19/06/13 23:03, Stephen Warren wrote:
>> On 06/16/2013 04:45 AM, Linus Walleij wrote:
>>> From: Linus Walleij <linus.walleij@...aro.org>
>>>
>>> It is counter-intuitive to have "0" mean disable in a boolean
>>> manner for electronic properties of pins such as pull-up and
>>> pull-down. Therefore, define that a pull-up/pull-down argument
>>> of 0 to such a generic option means that the pin is
>>> short-circuited to VDD or GROUND. Pull disablement shall be
>>> done using PIN_CONFIG_BIAS_DISABLE.
>>>
>>> Cc: Heiko St�bner <heiko@...ech.de>
>>> Cc: James Hogan <james.hogan@...tec.com>
>>> Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>
>>> Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
>>> ---
>>>  include/linux/pinctrl/pinconf-generic.h | 13 +++++++------
>>>  1 file changed, 7 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/linux/pinctrl/pinconf-generic.h b/include/linux/pinctrl/pinconf-generic.h
>>> index d414a77..67780f5 100644
>>> --- a/include/linux/pinctrl/pinconf-generic.h
>>> +++ b/include/linux/pinctrl/pinconf-generic.h
>>> @@ -36,14 +36,15 @@
>>>   *	tristate. The argument is ignored.
>>>   * @PIN_CONFIG_BIAS_PULL_UP: the pin will be pulled up (usually with high
>>>   *	impedance to VDD). If the argument is != 0 pull-up is enabled,
>>> - *	if it is 0, pull-up is disabled.
>>> + *	if it is 0, pull-up it total, i.e. the pin is connected to VDD.
>>>   * @PIN_CONFIG_BIAS_PULL_DOWN: the pin will be pulled down (usually with high
>>>   *	impedance to GROUND). If the argument is != 0 pull-down is enabled,
>>> - *	if it is 0, pull-down is disabled.
>>> + *	if it is 0, pull-down is total, i.e. the pin is connected to GROUND.
>>>   * @PIN_CONFIG_BIAS_PULL_PIN_DEFAULT: the pin will be pulled up or down based
>>>   *	on embedded knowledge of the controller, like current mux function.
>>> - *	If the argument is != 0 pull up/down is enabled, if it is 0,
>>> - *	the pull is disabled.
>>> + *	If the argument is != 0 pull up/down is enabled, if it is 0, the
>>> + *	configuration is ignored. The proper way to disable it is to use
>>> + *	@PIN_CONFIG_BIAS_DISABLE.
>>
>> Why treat PULL_UP/PULL_DOWN differently from PULL_PIN_DEFAULT?
>> PULL_PIN_DEFAULT is logically simply a macro that selects PULL_UP/DOWN
>> based on what's "normal" for the pin's expected usage, so surely the
>> value associated with that option should behave identically?
> 
> I'm not familiar with hardware that does this so I could be way wrong
> here, but presumably if there's a default up/down, there's probably a
> default resistance too. Does it really make sense to say
> "pull up or down depending on whatever the pin is intended for... but
> whichever it is must be XXX Ohm"?
> If you know the resistance you want, you surely already know whether you
> want it pull up or down with that resistence.

IIRC the idea of the PULL_PIN_DEFAULT was to avoid having to specify
UP/DOWN for each pin/group, but could just say "default", which would
then reduce the number of pinctrl mapping table entries or pinctrl DT
property entries. I don't think this would have any interaction with
setting the resistance; it's entirely plausible that you'd want the same
explicit, or same default, resistance, for all pins, irrespective of
pull-up vs. down.
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