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

["Heiko Stübner" <heiko@sntech.de>]

Am Samstag, 22. Juni 2013, 01:30:18 schrieb Laurent Pinchart:
> Hi Stephen,
> 
> On Friday 21 June 2013 13:06:47 Stephen Warren wrote:
> > 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@linaro.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@sntech.de>
> > >>> Cc: James Hogan <james.hogan@imgtec.com>
> > >>> Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > >>> Signed-off-by: Linus Walleij <linus.walleij@linaro.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 may be mistaken, but I've understood the PULL_PIN_DEFAULT option as a way
> to select the default bias configuration when the default value is
> hardware- controlled, not as a pure software option.

Yep that was the original intent ... to handle hardware that hides the 
complete logic from the software and you only get to turn it on or off, while 
the direction and other params are determined internally for example by the 
pin function.


> 
> > 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.