linux-kernel - RE: [PATCH] s3c/s3c24xx: arm: leds: Make s3c24xx LEDS driver use gpiolib

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Date:	Mon, 21 Nov 2011 10:07:50 -0800
From:	Stephen Warren <swarren@...dia.com>
To:	Denis Kuzmenko <linux@...onet.org.ua>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	Linus Walleij <linus.walleij@...aro.org>,
	Richard Purdie <rpurdie@...ys.net>,
	Wolfram Sang <w.sang@...gutronix.de>
Subject: RE: [PATCH] s3c/s3c24xx: arm: leds: Make s3c24xx LEDS driver use
 gpiolib

Denis Kuzmenko wrote at Friday, November 18, 2011 4:17 PM:
> On 11/19/2011 12:44 AM, Stephen Warren wrote:
> > Denis Kuzmenko wrote at Friday, November 18, 2011 3:35 PM:
> >> On 11/18/2011 11:59 PM, Stephen Warren wrote:
> >>> Denis Kuzmenko wrote at Friday, November 18, 2011 2:45 PM:
> >>>> Make s3c24xx LEDS driver use gpiolib. Disable using pull-resistor when not
> >>>> using S3C24XX_LEDF_TRISTATE and enble it when in opposite case.
> >>>>
> >>>> Signed-off-by: Denis Kuzmenko <linux@...onet.org.ua>
> >>>
> >>>>  	if (pdata->flags & S3C24XX_LEDF_TRISTATE) {
> >>>> -		s3c2410_gpio_setpin(pdata->gpio, 0);
> >>>> -		s3c2410_gpio_cfgpin(pdata->gpio, S3C2410_GPIO_INPUT);
> >>>> +		/*
> >>>> +		 * pull is needed here to protect pin from being left
> >>>> +		 * floating
> >>>> +		 */
> >>>> +		ret = s3c_gpio_setpull(pdata->gpio, S3C_GPIO_PULL_UP);
> >>>> +		if (ret)
> >>>> +			s3c_gpio_setpull(pdata->gpio, S3C_GPIO_PULL_DOWN);
> >>>
> >>> Sorry, could you explain why it's appropriate to configure a pull here
> >>> at all, let alone why it's OK to have a random pull on the line?
> >>
> >> Of course I'll explain.
> >> Imagine you are working with generic GPIO lines on your board connecting
> >> and disconnecting LEDs and other stuff. In this case there can be
> >> situation where GPIO line is configured as TRISTATE LED but have nothing
> >> connected physically to pin. This configuration is dangerous because
> >> input pin without _any_ pull-resistor is _much_ more sensitive to
> >> statical electricity (ESD) so you can *burn* (unsure this is correct
> >> word) your pin much easily (especially is you are using soldering iron
> >> as much as I do). Most of GPIO modules I worked with have "input with
> >> pull-up" as default and most safe initial state (and s3c2440's one is
> >> not an exception).
> >> Maybe, I need to write more wide exlanation in comment above?
> >
> > OK, I see the need for a pull of some kind (although aren't there meant
> > to be ESD protection diodes for this purpose; relying on what are probably
> > pretty weak pullup/down resistors doesn't seem like it will provide much
> > protection at all).
> 
> I don't mean pull as any kind of good protection. But it's much better
> to have it than not.

Hmm. I'm not entirely convinced. If the board already has a pull-up/down,
it seems like it won't really make much difference to ESD, and you can't
make any assumptions in the core driver about whether such an external
resistor is already present. In fact, adding another pull resistor inside
the SoC in parallel will reduce the overall resistance, and increase wasted
power.

> > I have a slight feeling this detail should be hidden inside the gpiolib
> > driver.
> 
> Do you mean to add to a function that makes pin act as input some kind
> of logic like:
> 	if(!(flags & FLAG_PULL_NONE))
> 		try_to_enable_pull(PULL_ANY);

I meant that /if/ the GPIO HW or SoC really requires this for safety, then
the implementation behind gpio_direction_input() should be doing this.
That said, it seems pretty magic to do this.

Can you get the SoC vendor and gpiolib implementor for this SoC to weigh in
on this and answer if "magically" enabling a tri-state is a good thing to
do?

-- 
nvpublic

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