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 -- 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
