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

[Paulo Zanoni]

Hi

2012/12/8 Damien Lespiau <damien.lespiau at intel.com>:
> 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.

They're different problems and require different patches. This one
fixes the "FDI RX polarity" problem, not "lane reversal".

>
> 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



-- 
Paulo Zanoni