Re: [PATCH 2/2 updated] Measure transition latency at driver initialization

[Eric Piel <Eric.Piel@tremplin-utc.net>]

01.12.2005 20:31, Mattia Dongili wrote/a écrit:
> ok, updated patch attached (with a shiny diffstat!!). Looks a lot
> nicer now :)
> 
> Oh, what do you think of the request to report out-of-range frequencies
> and set CPUFREQ_ETERNAL instead of setting an arbitrary high value? If
> ok, shall I use my email address instead of the list's?
> 
I was about to say this is fairly reasonable... until I tried it on my 
laptop and notice that _I_ got this message :-( Transition measured: 
4500 nSec. It's probably because the clock source on my laptop is the 
TSC, which on this old processor stops during transition. Therefore, 
after being in the shoe of the poor user still not having the ondemand 
governor working, I'd tend to prefer if the fallback transition latency 
could be a high and safe, but still usable, value. Anyway, we _know_ 
that those processors are not so slugish to do transition that it should 
be considered "eternal" :-)

So what about putting it to 500 uSec, knowing that on my 1GHz PIII it 
took something like 200uSec, it should be safe everywhere:


:
> +	/* sanytize the latency measurement if too high or too low
> +	 * and ask for reports of out-of-range values
> +	 */
> +	if (*transition_latency > 10000000 || *transition_latency < 50000) {
> +		printk (KERN_ERR "CPUFreq: measured latency of the frequency transition out "
> +				"of range, please report the following value to "
> +				"cpufreq@lists.linux.org.uk\n%u nSec\n", *transition_latency);
> +		*transition_latency = CPUFREQ_ETERNAL;
> +	}
It would be something like:
*transition_latency = 500000;
KERN_WARNING "speedstep: frequency transition measured seems out of "
              "range, falling back to a safe one of %u nSec.\n", 
*transition_latency


Eric