From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757930Ab1KRXPT (ORCPT ); Fri, 18 Nov 2011 18:15:19 -0500 Received: from mail-bw0-f46.google.com ([209.85.214.46]:56225 "EHLO mail-bw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757283Ab1KRXPR (ORCPT ); Fri, 18 Nov 2011 18:15:17 -0500 Message-ID: <4EC6E75E.2080200@solonet.org.ua> Date: Sat, 19 Nov 2011 01:16:46 +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> In-Reply-To: <74CDBE0F657A3D45AFBB94109FB122FF1740D74FE6@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 11/19/2011 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. > 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.