Re: [PATCH 3/4] i2c-s3c2410: use exponential back off while polling for bus idle

[Mark Brown <broonie-yzvPICuk2AATkU/dhu1WVueM+bqZidxxQQ4Iyu8u01E@public.gmane.org>]

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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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