On Wed, Sep 25, 2013 at 04:35:33PM -0700, Ben Widawsky wrote: > The policy's max frequency is not equal to the CPU's max frequency. The > ring frequency is derived from the CPU frequency, and not the policy > frequency. > > One example of how this may differ through sysfs. If the sysfs max > frequency is modified, that will be used for the max ring frequency > calculation. > (/sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq). As far as I > know, no current governor uses anything but max as the default, but in > theory, they could. Similarly distributions might set policy as part of > their init process.
Actually this seems to differ based on policy as well. The pstate may disable turbo mode, in which case policy->max <= policy->cpuinfo.max. But excluding user intervention, it seems that max is what we want, and it is possibly arguable that we do not want to push the core to turbo for a GPU bound workload - though I'm not sure if the ring frequency continues to scale with turbo cpu frequencies. > It's ideal to use the real frequency because when we're currently scaled > up on the GPU. In this case we likely want to race to idle, and using a > less than max ring frequency is non-optimal for this situation. Also note that scaling the cpu frequency cuts into our thermal headroom, so for ULT devices it may not be clearly beneficial. > AFAIK, this patch should have no impact on a majority of people. And also obsolete since HSW. > This behavior hasn't been changed since it was first introduced: > commit 23b2f8bb92feb83127679c53633def32d3108e70 > Author: Jesse Barnes <[email protected]> > Date: Tue Jun 28 13:04:16 2011 -0700 > > drm/i915: load a ring frequency scaling table v3 > > CC: Jesse Barnes <[email protected]> > Signed-off-by: Ben Widawsky <[email protected]> FWIW, Reviewed-by: Chris Wilson <[email protected]> I think using the real range of the CPU frequency rather than the adjusted policy maximum is sensible. Just the hw implementation is just a bit silly and it is not clear exactly what the best policy should be. -Chris -- Chris Wilson, Intel Open Source Technology Centre _______________________________________________ Intel-gfx mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/intel-gfx
