[Bug 93956] Display Port BYT-M [N2807] - Data link training fails sporadically

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93956

--- Comment #6 from Zoran Stojsavljevic <zoran.stojsavljevic at intel.com> ---
Hello OTC team,

Today, as I have promised, Werner Zeh from Siemens MC came to IMU Feldkirchen,
to work with me together on the DDI Port C BYT-M (N2807) issue/bug. 

The following we did, in order to close to the potential FW problem and to find
the Root Cause of this problem.
[1] I did use my BBAY Fab. 3 CRB, swapping E3826 (two core BYT-I) with issue
infested N2807 BYT-M. The reballed "bad" N2807 worked immediately with BBAY
Fab. 3.
[2] Once I had problematic N2807, I did verify all the parameters from CCG
internal BIOS X64.A093.R42 I have build, to check the validity of this BIOS.
[3] The BIOS shows correct 0x30678 N2807 BYT-M CPUID, as well as the latest
used MCU 833.
[4] The BIOS implemented (assembled by me) appears to be UEFI compliant BIOS,
64 bit one, visually checked with version, as well as with .efi 32/64 size
checker.
[5] With this BIOS (X64.A093.R42) it is IMPOSSIBLE to experience/show this
issue with EMGD vBIOS 3909, as well as with GOP 7.2.1013 (used Fedora 23,
kernel 4.3.5-300.fc23.x86_64)!
[6] Then we switch gears to FSP MR4/MR5 (irrelevant, both work the same way),
and single channel N2807 does expose/show this issue very clearly.
[7] There was investigation going on, so we concluded that something in FSP is
not either initialized accordingly with UEFI BIOS, or there is time
de-synchronization.
[8] I set this use case according to INTEL rules to prove this issue on BBAY
Fab.3 CRB, with FSP used.
[9] I have BBAY Fab.3 CRB with N2807 and FSP as real prove that we, INTEL, have
the problem!

Now... I am just wondering if anything can/could be done from OTC/booting
kernel levels, so some additional registers can be initialized by i915 driver
to solve this issue (it is clear that issue comes from legacy BIOS/FSP levels,
which does not necessarily mean that it could not be fixed by Linux kernel
using i915 GFX driver). 

Thank you for understanding,
Zoran

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