[personal profile] mjg59
(Edit to add: this issue is restricted to the mobile SKUs. Desktop parts have very different power management behaviour)

Linux 4.5 seems to have got Intel's Skylake platform (ie, 6th-generation Core CPUs) to the point where graphics work pretty reliably, which is great progress (4.4 tended to lose all my windows every so often, especially over suspend/resume). I'm even running Wayland happily. Unfortunately one of the reasons I have a laptop is that I want to be able to do things like use it on battery, and power consumption's an important part of that. Skylake continues the trend from Haswell of moving to an SoC-type model where clock and power domains are shared between components that were previously entirely independent, and so you can't enter deep power saving states unless multiple components all have the correct power management configuration. On Haswell/Broadwell this manifested in the form of Serial ATA link power management being involved in preventing the package from going into deep power saving states - setting that up correctly resulted in a reduction in full-system power consumption of about 40%[1].

I've now got a Skylake platform with a nice shiny NVMe device, so Serial ATA policy isn't relevant (the platform doesn't even expose a SATA controller). The deepest power saving state I can get into is PC3, despite Skylake supporting PC8 - so I'm probably consuming about 40% more power than I should be. And nobody seems to know what needs to be done to fix this. I've found no public documentation on the power management dependencies on Skylake. Turning on everything in Powertop doesn't improve anything. My battery life is pretty poor and the system is pretty warm.

The best thing about this is the following statement from page 64 of the 6th Generation Intel ® Processor Datasheet for U-Platforms:

Caution: Long term reliability cannot be assured unless all the Low-Power Idle States are enabled.

which is pretty concerning. Without support for states deeper than PC3, Linux is running in a configuration that Intel imply may trigger premature failure. That's obviously not good. Until this situation is improved, you probably shouldn't buy any Skylake systems if you're planning on running Linux.

[1] These patches never went upstream. Someone reported that they resulted in their SSD throwing errors and I couldn't find anybody with deeper levels of SATA experience who was interested in working on the problem. Intel's AHCI drivers for Windows do the right thing, but I couldn't find anybody at Intel who could get any information from their Windows driver team.

Re: SATA PM Patches

Date: 2016-04-16 01:53 am (UTC)
From: [identity profile] hobbs.cleverdomain.org
Everyone else has "quirks", but libata has "horkages"? I love it.

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Matthew Garrett

About Matthew

Power management, mobile and firmware developer on Linux. Security developer at Google. Member of the Free Software Foundation board of directors. Ex-biologist. @mjg59 on Twitter. Content here should not be interpreted as the opinion of my employer.

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