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
