Re: [PATCH]PM QOS refresh against next-20100430

[Jonathan Corbet <corbet@lwn.net>]

On Mon, 03 May 2010 10:01:50 -0700
Kevin Hilman <khilman@deeprootsystems.com> wrote:

> > Well, shallow could mean that the state lacks the CPUIDLE_FLAG_DEEP
> > flag; that should be relatively portable.  In any case, it seems
> > more so than "if I put in a 55us latency requirement, I'll stay out
> > of C3".  
> 
> I guess it depends on your goal.  Do you just want to stay out of C3
> on your current platform?  or do you want to stay out of any low-power
> state (on any platform) where you'll have a latency of > 55 usecs?

My case is a camera driver; as long as latencies are on the scale of
the frame rate, things won't go too wrong.  1ms latencies would not be
a huge problem.  What happens is that the hardware corrupts the data if
it goes into C3 while video capture is happening.

Comments in drivers/net/e1000e/netdev.c suggest that they were dealing
with similar issues.

> The former is not portable (as C-states don't have the same meanings
> across arches/SoCs) where as using a real-world number in usecs will
> have meaning on any platform.

Understood.  But the end result is that I've ended up specifying a
latency requirement that is far tighter than the situation really needs
in order to achieve the real goal.

> My main concern is that drivers not be written with latency
> constraints that assume an Intel-centric set of power states.  There
> are other SoCs out there with different sets of states and
> corresponding latencies, so keeping things in real-world numbers
> (latency, throughput, etc.) seems to me to be the only portable way.

Understood.  Perhaps the pm_qos interface isn't the best way to achieve
the real goal here, but it's all we have at the moment.

Thanks,

jon