On Tue, Nov 20, 2012 at 04:57:16PM +0800, Daniel Kurtz wrote: > On Tue, Nov 20, 2012 at 12:49 PM, Mark Brown > > On the hardware I was using when I wrote the original code here we were > > hitting 1-2 spins often enough to be interesting - starting off with a > > direct busy wait was definitely useful when doing large batches of I/O, > > especially compared to sleeps which might cause us to schedule. > We check the status first to avoid any sleep()/schedule() in the case, > that the CPU is slower than I2C transaction. Right, but this only works if we hit this on the very first spin. > Remember, this loop only happens after the event_wait loop has been > woken up by the i2c irq. Duh. > Since you are talking about hitting a tiny window of time at some > arbitrary point after an irq, the CPU time to this point & I2C > finishing would have to be very precisely aligned for the 1-2 loops > (at CPU clock rate) to matter. On some systems that can happen enormously reliably, finger in the air it's your fast case on the A15s you're playing with scaled down to a much slower CPU. The 20 spins I was setting the loop to was a massive overestimate for conservativism but more than 1 was common enough, IIRC spinning 5 times would have covered essentially everything.
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