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

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Date:	Sat, 19 Nov 2011 01:16:46 +0200
From:	Denis Kuzmenko <linux@...onet.org.ua>
To:	Stephen Warren <swarren@...dia.com>
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

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.

> 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);
?

> Presumably the pull is pretty weak, so that if/when the pin is actively
> driven later, the drive completely overrides this pull?
>

As far as know pull-resistor is _designed_ to be weak enough so driver
can always override it.

> Is this pull strong enough to light the LED? Judging by what
> s3c24xx_led_set() does, when the LED is off, the pin is tri-stated, and
> when the LED is on, it's driven (high/low depending on the active high/low
> flag). When tri-stated, if the pull happens to pull the same direction as
> the on state would drive it, won't the LED light? It seems like you need
> to pick a suitable pull direction based on flags & S3C24XX_LEDF_ACTLOW,
> and if that can't be set, it's an error.

I couldn't find any information about pull strength in s3c2440's
datasheet but usually pull is about 10-100k. So even in worst case
maximum current through it on 3.3V VCC is 0.33 mA (not taking into
account own LED's resistance and voltage-fall) which is definitely not
enough to light a LED.

> (BTW, when posting a new version, starting a new thread with just the
> patch, rather than pasting it into a reply and prefixing it with other
> text will make is easiest for people to take the patch and apply it)

Thank you, I'll do so in future.


-- 
Best regards, Denis Kuzmenko.
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