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

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Date:	Fri, 17 Dec 2010 13:09:54 +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 Ben,
> 
> On Thu, 16 Dec 2010 16:00:46 +0000, Ben Dooks wrote:
>> On Thu, Dec 16, 2010 at 03:06:38PM +0100, Jean Delvare wrote:
>> > Add a spinlock to every user of i2c-algo-bit, which is taken before
>> > raising SCL and released after lowering SCL. We don't really need
>> > the exclusion functionality, but we have to disable local interrupts.
>> > This is needed to comply with SMBus requirements that SCL shouldn't
>> > be high for longer than 50 us.
>> > 
>> > SMBus slaves can consider SCL being high for 50 us as a timeout
>> > condition. This has been observed to happen reproducibly with the
>> > Melexis MLX90614.
>> > 
>> > The drawback of this approach is that spin_lock_irqsave() and
>> > spin_unlock_irqrestore() will be called once for each bit going on the
>> > I2C bus in either direction. This can mean up to 100 kHz for standard
>> > I2C and SMBus and up to 250 kHz for fast I2C. The good thing is that
>> > this limits the latency to reasonable values (2us at 250 kHz, 5 us at
>> > 100 kHz and 50 us at 10 kHz).
>> 
>> Hmm, this is going to be a drain on interrupt latency... disabling
>> interrupts in a system for that long could cause other things to
>> jitter.
> 
> So you consider that even disabling interrupts for 5 us is too long? Or
> are you only worried by the 50 us case?

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.

JM2C

-- 
KR
Michael
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