From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755342Ab1KUTfy (ORCPT ); Mon, 21 Nov 2011 14:35:54 -0500 Received: from mail-bw0-f46.google.com ([209.85.214.46]:53973 "EHLO mail-bw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751035Ab1KUTfx (ORCPT ); Mon, 21 Nov 2011 14:35:53 -0500 Message-ID: <4ECAA894.4050500@solonet.org.ua> Date: Mon, 21 Nov 2011 21:37:56 +0200 From: Denis Kuzmenko User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.16) Gecko/20111110 Icedove/3.0.11 MIME-Version: 1.0 To: Stephen Warren CC: "linux-kernel@vger.kernel.org" , Grant Likely , Linus Walleij , Richard Purdie , Wolfram Sang Subject: Re: [PATCH] s3c/s3c24xx: arm: leds: Make s3c24xx LEDS driver use gpiolib References: <4EC572E1.1020209@solonet.org.ua> <74CDBE0F657A3D45AFBB94109FB122FF1740D74E87@HQMAIL01.nvidia.com> <4EC6C785.5010801@solonet.org.ua> <4EC6D1D5.9060808@solonet.org.ua> <74CDBE0F657A3D45AFBB94109FB122FF1740D74FAD@HQMAIL01.nvidia.com> <4EC6DD8F.9070904@solonet.org.ua> <74CDBE0F657A3D45AFBB94109FB122FF1740D74FE6@HQMAIL01.nvidia.com> <4EC6E75E.2080200@solonet.org.ua> <74CDBE0F657A3D45AFBB94109FB122FF174F08C135@HQMAIL01.nvidia.com> In-Reply-To: <74CDBE0F657A3D45AFBB94109FB122FF174F08C135@HQMAIL01.nvidia.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >>>>> >>>>>> 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.