[PATCH 3/7] drm/i915: clear up the fdi dotclock semantics for M/N computation

[Daniel Vetter <daniel.vetter@ffwll.ch>]

We currently mutliply the link_bw of the fdi link with the pixel
multiplier, which is wrong: The FDI link doesn't suddenly grow more
bandwidth. In reality the pixel mutliplication only happens in the PCH,
before the pixels are fed into the port.

But since we our code treats the uses the target clock after pixels
are doubled (tripled, ...) already, we need to correct this.

Semantically it's clearer to divide the target clock to get the fdi
dotclock instead of multiplying the bw, so do that instead.

Note that the target clock is already multiplied by the same factor,
so the division will never loose accuracy for the M/N computation.

The lane computation otoh used the wrong value, we also need to feed
the fdi dotclock to that.

Split out on a request from Paulo Zanoni.

v2: Also fix the lane computation.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
 drivers/gpu/drm/i915/intel_display.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index a29295e..761254d 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -3992,7 +3992,7 @@ static int ironlake_fdi_compute_config(struct intel_crtc *intel_crtc,
 {
 	struct drm_device *dev = intel_crtc->base.dev;
 	struct drm_display_mode *adjusted_mode = &pipe_config->adjusted_mode;
-	int target_clock, lane, link_bw;
+	int target_clock, lane, link_bw, fdi_dotclock;
 	bool setup_ok, needs_recompute = false;
 
 retry:
@@ -4010,14 +4010,16 @@ retry:
 	else
 		target_clock = adjusted_mode->clock;
 
-	lane = ironlake_get_lanes_required(target_clock, link_bw,
+	fdi_dotclock = target_clock;
+	if (pipe_config->pixel_multiplier > 1)
+		fdi_dotclock /= pipe_config->pixel_multiplier;
+
+	lane = ironlake_get_lanes_required(fdi_dotclock, link_bw,
 					   pipe_config->pipe_bpp);
 
 	pipe_config->fdi_lanes = lane;
 
-	if (pipe_config->pixel_multiplier > 1)
-		link_bw *= pipe_config->pixel_multiplier;
-	intel_link_compute_m_n(pipe_config->pipe_bpp, lane, target_clock,
+	intel_link_compute_m_n(pipe_config->pipe_bpp, lane, fdi_dotclock,
 			       link_bw, &pipe_config->fdi_m_n);
 
 	setup_ok = ironlake_check_fdi_lanes(intel_crtc->base.dev,
-- 
1.7.11.7