linux-kernel - Re: pm_qos request for 0 cpu_dma_latency is ignored on Linus master

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Date:	Fri, 19 Oct 2012 18:34:39 +0100
From:	Simon Farnsworth <simon.farnsworth@...lan.co.uk>
To:	linux-kernel@...r.kernel.org
Cc:	len.brown@...el.com, linux-pm@...r.kernel.org, lenb@...nel.org
Subject: Re: pm_qos request for 0 cpu_dma_latency is ignored on Linus master

Sorry for the noise. I've just found it's my code at fault - we changed
to a 10,000 usec PM request for most platforms a while ago.

Simon

On Friday 19 October 2012 18:28:18 Simon Farnsworth wrote:
> (please cc me on replies - I'm not directly subscribed to linux-kernel or linux-pm)
> 
> I'm trying to track down why a Sandybridge system with a PCIe to PCI riser
> has problems after enabling GPU RC6, and I've reduced my problem to the menu
> cpuidle governor selecting high C-states from intel_idle, even though I've
> given a cpu_dma_latency requirement of 0 usecs.
> 
> I have a TV capture card in the riser, and it appears that I get data lost
> whenever the package enters a high C state. My userspace opens
> /dev/cpu_dma_latency and writes (uint32_t)0 to this fd whenever we have things
> on-screen, closing the fd when we put the screen into DPMS off states.
> 
> However, even with a userspace provided request of 0 usec, I'm seeing the CPU
> cores (and hence the entire package when GPU RC6 is enabled) enter C6 state.
> 
> I've confirmed that forcing the governor to limit itself to C0 fixes my
> problem with:
> 
> # for state in /sys/devices/system/cpu/cpu*/cpuidle/state[1-9]*/disable ; do echo 1 > $state ; done
> 
> What do I need to do to get the menu cpuidle governor and the intel_idle
> cpuidle driver to respect the /dev/cpu_dma_latency pm_qos request of 0 usec
> and keep the Sandybridge CPU package out of low C states? Note that a CPU
> core in low C state is fine, as long as the DMA latency stays low.
> 
> My goal is to keep the system in high-performance state when we're rendering
> digital signage on screen (as any glitching may upset viewers, and compared
> to the screen's full power use, we're irrelevant), but to go into lower power
> states when the screen is off (when we're only drawing power to keep the
> administrator interface available).
> 
> My kernel is currently 3.7-rc1+, based on Linus git of Tuesday 16th October
> 2012. I'm happy to try patches, even if all they get you is better debug
> output to work out why the userspace request is ignored.
> 
Simon Farnsworth
Software Engineer
ONELAN Ltd
http://www.onelan.com

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