[Bug 95736] [IVB bisected] *ERROR* uncleared fifo underrun on pipe A

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=95736

--- Comment #20 from Tomeu Vizoso <tomeu at tomeuvizoso.net> ---
(In reply to Fabian Henze from comment #19)
> (In reply to Tomeu Vizoso from comment #18)
> > Hi, I think 10 would be a bit too little. I would expect the value to be
> > between 12 and 20, but maybe more could be needed.
>  
> I tried a few values between 13 and 100 (13, 20, 50 and 100 iirc) and all
> had the fifo underrun. What now ...?

So, your last log showed this:

[    1.034165] [drm:gen6_check_mch_setup] Wrong MCH_SSKPD value: 0x16040307
This can cause underruns.

This usually means that the BIOS programmed a too low priority (ox7 instead of
0xc) for memory requests from the display, but that register is locked by when
the kernel starts so we cannot fix it.

I see your machine currently has fw 1.42 and there's a 1.43 available from
Lenovo's support site, but the BIOS changelog doesn't mention such a fix. It
may be worth it anyway to upgrade the BIOS to 1.43.

In the kernel sources it's mentioned that when the BIOS has such a bug and it
cannot be updated to a fixed version, that the issue can only be worked around
by increasing the latencies. It may still be worth it though to check that the
watermarks are programmed at the right values.

But you report that fixing up the latencies above 12 didn't help, so we might
not be able to do anything about it other than hiding the message.

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