Re: i915: RC6 causes sudden shutdowns

[Andrew Lutomirski]

On Thu, Dec 1, 2011 at 9:00 PM, Patrik Kullman <patrik.kullman@xxxxxxxxx> wrote:
> 2011/12/1 Andy Lutomirski <luto@xxxxxxx>:
>> On 11/23/2011 03:43 PM, Patrik Kullman wrote:
>>> Hi,
>>>
>>> I recently acquired the Asus Zenbook UX31E but have been looking for
>>> ways to extend the battery life.
>>>
>>> I read that the i915_enable_rc6 parameter could decrease power usage
>>> significantly and tried it with the 3.1.1 and 3.1.2 kernels, and
>>> indeed it does.
>>> However, I (and the other Zenbook-owners from the Ubuntu forums)
>>> experience sudden, unexpected shutdowns (for me, 1-4 times per day)
>>> with this parameter.
>>> It's as sudden as a power loss.
>>
>> Do you have DMAR enabled?  If so, it's a known problem and it's being
>> worked on (and the default will not change for you AFAICT).  If not, you
>> should send a bug report to the intel-gfx -- several people there care
>> about this.
>>
>> --Andy
>
> Well, it's not something I've explicitly enabled.
>
> Is it on by default?

Depends on config options.  You can probably get a good idea by
looking for "DMAR" in your kernel log.

>
> If so, any reason why I wouldn't want to disable it?

There are three reasons to enable it:

1. If you're paranoid and want to (partially) protect your system
against malicious PCI/PCIe cards.
2. If you assign devices in your system to VM guests.
3. If you have more than 4GB of RAM and you have DMA-capable devices
that can only address 32 bits, then you might get a performance boost.

In other words, for normal desktop use, you don't need it.  Try
booting with intel_iommu=off.

--Andy
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