linux-kernel - Re: [PATCH V1 2/3] Documentation: gpio: Add details of open-drain configuration

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Date:	Tue, 14 Feb 2012 14:29:32 +0530
From:	Laxman Dewangan <ldewangan@...dia.com>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
CC:	Grant Likely <grant.likely@...retlab.ca>,
	"linus.walleij@...ricsson.com" <linus.walleij@...ricsson.com>,
	"dunlap@...otime.net" <dunlap@...otime.net>,
	"lrg@...com" <lrg@...com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>
Subject: Re: [PATCH V1 2/3] Documentation: gpio: Add details of open-drain
 configuration

On Tuesday 14 February 2012 03:36 AM, Mark Brown wrote:
> * PGP Signed by an unknown key
>
>> Implicitly, the gpio api already supports open-drain and several drivers
>> make use of it.  Instead of being a separate flag; users who need open
>> drain will set the pin to input.  For example, see the i2c-gpio driver.
>> I'm not convinced this is needed; but my opinion could be swayed.
> The actual idea is to provide support for doing the switch to input to
> drivers that just want to set a logic level and don't (at their level)
> care one way or another if it's a CMOS or open drain output but instead
> leaves it up to board configuration which mode is used.  Laxman posted a
> patch for doing this to a regulator driver but looking at it the code
> for emulating open drain while also maintaining support for regular CMOS
> is fiddly enough that it seemed like it should be factored out of the
> individual drivers.
>

Implementing open-drain handling in every gpio client (like 
i2c-gpio/regulator/fixed.c) is just duplicating the code everywhere. 
Also it leaves to have such implementation buggy which is in following 
piece of code (taken from i2c-gpio.c)

         if (pdata->sda_is_open_drain) {
                 gpio_direction_output(pdata->sda_pin, 1);
                 bit_data->setsda = i2c_gpio_setsda_val;
         } else {
                 gpio_direction_input(pdata->sda_pin);
                 bit_data->setsda = i2c_gpio_setsda_dir;
         }

Header says
  * @sda_is_open_drain: SDA is configured as open drain, i.e. the pin
  *      isn't actively driven high when setting the output value high.
  *      gpio_get_value() must return the actual pin state even if the
  *      pin is configured as an output.

And hence the check should be
if (!pdata->sda_is_open_drain) {
:::::
}


All these can be avoided by supporting open-drain flag and handling in 
gpio library.

>> Also, you should include a patch to make i2c-gpio.c use this new
>> functionality as a proof-of-concept.  You may not be able to test it,
>> but it will make it a lot easier to justify merging by showing how it
>> cleans up a gpio user.

I can do the cleanup in i2c-gpio also which will reduce lots of code if 
this change in gpiolib is acceptable and fine with everyone.
Let me know if patch regulator/fixed.c V1 is sufficient or not to 
justify. I will create the patch for i2c-gpio  also.


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