[Bug 95741] Broadwell graphics on Dell XPS13 (early 2015) loses horizontal sync on shift to tty console

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=95741

--- Comment #29 from Jani Nikula <jani.nikula at intel.com> ---
(In reply to Ben Widawsky from comment #28)
> (In reply to nils.alex from comment #26)
> > i915.enable_ips=0 seems to fix the problem for me (using 4.1-rc6+). What do
> > I miss without ips?
> 
> If you're able to get into the higher package C-states, probably not a lot.
> IPS is an intermediate buffer which allows longer delays between DRAM
> fetches for the scanout, enabling DDR to sleep when it otherwise may not
> have been able to.
> 
> For higher resolution scanouts, it can be quite important.

At the higher resolutions, you may hit bug [1]. We shouldn't enable IPS when
the pipe pixel rate is greater than 95% of the CDCLK frequency.

I've presumed all or most bugs that get fixed by i915.enable_ips=0 would be
that. Maybe we have something else there too. But it seems clear to me there
are several different issues mixed up in this report.

[1] https://bugs.freedesktop.org/show_bug.cgi?id=83497

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