Messages in this thread | | | Date | Mon, 21 Nov 2011 21:37:56 +0200 | From | Denis Kuzmenko <> | Subject | Re: [PATCH] s3c/s3c24xx: arm: leds: Make s3c24xx LEDS driver use gpiolib |
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On 11/21/2011 08:07 PM, Stephen Warren wrote: > 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@solonet.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 don't think it's a real protection. It's rather "mistake-proofing" (Poka-Yoke). You are right, I didn't considered additional pulls (however I can't imagine tristate LED usage with additional external pull) and power consumptions. I was just wondering, why was pull needed in previous implementation. Additional ESD protection was the only thing I could imagine. I don't think it's needed there and I'm OK to remove pull-related code. So I'll remove it, test and send patch V3?
> 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?
I don't like magic things neither believe that it's possible to get those people to this conversation. So lets just remove that code?
-- Best regards, Denis Kuzmenko.
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