linux-kernel - Re: [PATCH] i2c: Let users disable "Probe an I2C bus for certain devices"

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Date:	Fri, 7 Jun 2013 13:10:38 +0300
From:	Grygorii Strashko <grygorii.strashko@...com>
To:	Wolfram Sang <wsa@...-dreams.de>
CC:	<linux-i2c@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-omap@...r.kernel.org>
Subject: Re: [PATCH] i2c: Let users disable "Probe an I2C bus for certain
 devices"

Hi Wolfram,
On 06/07/2013 12:06 PM, Wolfram Sang wrote:
>> 3) Thinking about Mainline: To reach the same target - no I2C
>> detection - and taking
>> into account above assumption "No changes in default behavior"
>> the following will need to be done:
>> - change i2c-omap/i2c-gpio DT bindings and add parameter which will
>> allow to change
>>    .class value for adapter. Not sure, it's possible because this parameter
>>    will be Linux and not HW specific (smth. like "i2c_disable_detection")
>> - update drivers i2c-omap/i2c-gpio to use "i2c_disable_detection"
>> - update OMAP4/5 DTS files
>>
>> So, It seemed a good solution for me to add 6 lines of code in i2c-core.c
>> instead of doing all that stuff.
> Well... I understand the "default behaviour" issue, yet I still think
> that setting class to 0 is the right thing to do. OMAP is an embedded
> SoC which always had i2c_board_info or devictree which are the preferred
> ways of instantiating. Given that, I would accept a patch setting it to
> 0. The more user friendly way might be to introduce a new class which
> makes users aware of the issue. Proof of concept follows, only compile
> tested.
>
That sounds good to me - I can prepare patch for i2c-omap.c.
But, there is still an open question regarding *i2c-gpio.c* which,
actually, a source of biggest part of delay.

Potentially, it can be tunned using "timeout" value. But, again, this
is additional work/patches to workaround unneeded system feature:
- "timeout" should not break interaction with I2C devices on i2c-gpio
bus and, at same time, speed up boot;
- DTS/board files need to be updated.

May be, just may be), we can continue with 
CONFIG_I2C_DISABLE_DEVICE_DETECTION,
and print deprecation warning for each registered adapter
if this config option is defined.

- grygorii
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