drm/i915/dp: Fix 2.7 Gbps DP_LINK_BW value on g4x

On g4x we currently use the 96MHz non-SSC refclk, which can't actually
generate an exact 2.7 Gbps link rate. In practice we end up with 2.688
Gbps which seems to be close enough to actually work, but link training
is currently failing due to miscalculating the DP_LINK_BW value (we
calcualte it directly from port_clock which reflects the actual PLL
outpout frequency).

Ideas how to fix this:
- nudge port_clock back up to 270000 during PLL computation/readout
- track port_clock and the nominal link rate separately so they might
  differ a bit
- switch to the 100MHz refclk, but that one should be SSC so perhaps
  not something we want

While we ponder about a better solution apply some band aid to the
immediate issue of miscalculated DP_LINK_BW value. With this
I can again use 2.7 Gbps link rate on g4x.

Cc: [email protected]
Fixes: 665a7b0 ("drm/i915: Feed the DPLL output freq back into crtc_state")
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Imre Deak <[email protected]>
(cherry picked from commit a8b8746)
Signed-off-by: Rodrigo Vivi <[email protected]>

Author: vsyrjala

drivers/gpu/drm/i915/display/intel_dp.c

@@ -1604,6 +1604,12 @@ int intel_dp_rate_select(struct intel_dp *intel_dp, int rate)
 void intel_dp_compute_rate(struct intel_dp *intel_dp, int port_clock,
 			   u8 *link_bw, u8 *rate_select)
 {
+	struct intel_display *display = to_intel_display(intel_dp);
+
+	/* FIXME g4x can't generate an exact 2.7GHz with the 96MHz non-SSC refclk */
+	if (display->platform.g4x && port_clock == 268800)
+		port_clock = 270000;
+
 	/* eDP 1.4 rate select method. */
 	if (intel_dp->use_rate_select) {
 		*link_bw = 0;