Hi Michael, Ben, On Fri, 17 Dec 2010 13:09:54 +0100, Michael Lawnick wrote: > 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. Well well, no matter how little you and Ben seem to like it, it appears that w1 is going more or less the route I had proposed for i2c-algo-bit: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=3fd306c85adcde7209281cb663dd8ea247e97cc3 So I am again considering something similar for i2c (at least as an option.) Presumably their local_irq_save() is cheaper than my spin_lock_irqsave(), so I'd use that, not sure why I didn't do it originally. -- Jean Delvare -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
