Re: transition latency

[Dominik Brodowski <linux@dominikbrodowski.net>]

On Thu, Sep 28, 2006 at 07:32:47PM -0500, Ryan Underwood wrote:
> 
> What's the purpose of the transition_latency field with respect to the
> cpufreq infrastructure?  Should I be setting this to:
> * the time it takes for my set_cpu_state function to execute and return
> * the latency between the call to set_cpu_state and the actual CPU speed
> change
> * the time it takes for the hardware to respond to MY request (inside
> set_cpu_state function) to change speeds
> 
> Just a little confused.  Here the problem is that my set_cpu_state
> function could take up to 35ms to execute, but the actual frequency
> transition ( a component of that function) happens within 7ms.  10ms is
> the ondemand/conservative governors' upper limit for transition latency,
> so if I take the whole function into account, those governors reject my
> driver.

My original intention was this:

<timer starts>
- "system freeze" / IRQs off / ... (if needed)
- actual setting of the new frequency
- "system unfreeze" / IRQs on
- waiting until it is safe to return, and until
  it would also be safe to set the frequency anew.
<timer ends>

Note that 3 and 4 might be ordered differently in "real life". Also I'm not
totally sure that this "latency" is really what we need for ondemand et al.
So if anyone has a better input on what "latency" should mean here, speak up
please.

Thanks,
	Dominik