linux-kernel - Re: [RFC] i2c-algo-bit: Disable interrupts while SCL is high

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Message-ID: <4D0F0497.7090306@gmx.de>
Date:	Mon, 20 Dec 2010 08:24:07 +0100
From:	Michael Lawnick <ml.lawnick@....de>
To:	Jean Delvare <khali@...ux-fr.org>
CC:	Ben Dooks <ben-i2c@...ff.org>,
	Linux I2C <linux-i2c@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Matthias Zacharias <Matthias.Zacharias@...-solutions.de>
Subject: Re: [RFC] i2c-algo-bit: Disable interrupts while SCL is high

Jean Delvare said the following:
> Hi Michael,
> 
> On Fri, 17 Dec 2010 13:09:54 +0100, Michael Lawnick wrote:
>> Sorry to disturb, but
>> <MANTRA>
>> 	Disabling interrupts may be done only for a few instructions.</MANTRA>
>> 
>> Even 1 us is an eternity on modern systems.
> 
> Don't be sorry, this is exactly the kind of input I was asking for. I'm
> a little surprised, I thought disabling interrupts for a couple
> microseconds was happening all the time, but I'll trust your
> experience. 

I can't tell whether this is happening all the time, but I can imagine
and I highly discourage this. This is IMHO one of the lessons many LINUX
developers have still to learn. Maybe it's a history reason.

> Given your point and Ben's, it seems clear that my patch is
> not acceptable as is, and at the very least I should make the spinlock
> usage optional.

At last you might not come around your solution, but a H/W-S/W
combination driving you in such a direction should be considered broken.
Using it in professional environment needs heavy discussions about pros
and cons, best would be to beat the H/W designers to provide a real
controller.
Of course it may be used in a case, where you simply need a (temporary)
hack to get something working.

-- 
KR
Michael
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