[Intel-gfx] [PATCH 3/3] drm/i915: set the LPT FDI RX polarity reversal bit when needed

[Damien Lespiau]

On Sat, Dec 1, 2012 at 2:04 PM, Paulo Zanoni <przanoni at gmail.com> wrote:
> From: Paulo Zanoni <paulo.r.zanoni at intel.com>
>
> If we fail to set the bit when needed we get some nice FDI link
> training failures (AKA "black screen on VGA output").
>
> While we don't really know how to properly choose whether we need to
> set the bit or not (VBT?), just read the initial value set by the BIOS
> and store it for later usage.
>
> Signed-off-by: Paulo Zanoni <paulo.r.zanoni at intel.com>

Sad that we did not catch that before. It's not just the polarity
(support for exchanging the - and + wires on a differential pair), but
also the FDI lane reversal (0 becomes 3, 1 becomes 2) setting that we
need to preserve. There's a fused bit for the DMI lane reversal and
FDI defaults to following that DMI lane reversal configuration, but we
also have a bit to override that behaviour in the FDI RX register, we
need to preserve this bit  as well.

I think this is the same story for DDI_BUF_CTL, DDI ports support lane reversal.

I could not find either where these settings are stored, preserving
what the BIOS does for those seems the best option so far.

-- 
Damien