Re: [PATCH] i2c-isch: Decrease delay in the loop checking the BUSY state of the bus

[Jean Delvare <khali-PUYAD+kWke1g9hUCZPvPmw@public.gmane.org>]

On Tue, 24 Jan 2012 15:07:50 +0100, Olivier Sobrie wrote:
> I performed the same test you did on my system and I observed this:
> 
> * msleep(1)
>    real    0m 51.20s
>    user    0m 0.29s
>    sys     0m 0.00s

I assume your kernel has HZ=100, that means 20 ms per transaction i.e.
2 jiffies, as expected from the msleep() implementation.

> 
> * usleep_range(100, 200)
>    real    0m 1.46s
>    user    0m 0.10s
>    sys     0m 0.10s
> 
> * usleep_range(250, 500)
>    real    0m 2.01s
>    user    0m 0.05s
>    sys     0m 0.25s

It's really curious that this can take more CPU time than
usleep_range(100, 200). I can't explain it.

> 
> * usleep_range(50, 150)
>    real    0m 1.43s
>    user    0m 0.07s
>    sys     0m 0.23s
> 
> I think usleep_range(100, 200) is the best compromise.

I agree.

> (...)
> I agree udelay() is not a good solution!
> I did the test without hrtimers using usleep_range(100, 200) and got:
>     real    0m 25.60s
>     user    0m 0.30s
>     sys     0m 0.00s
> So that's not slower than msleep(1) in the case of no hrtimers.

In fact that's exactly twice as fast as msleep(2), i.e. 1 jiffy per
transaction instead of 2. That's pretty good that there is a benefit
even without hrtimers support. Thanks for testing and reporting,
that's one less thing on my to-do list :)

I'll send a patch similar to yours for the i2c-i801 driver in a minute.
Thanks for showing us the way!

-- 
Jean Delvare