From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Fri, 3 Dec 2010 21:13:16 +0000 (+0000)
Subject: drm/i915: Explain why we need to write DPLL twice
X-Git-Url: http://drtracing.org/?a=commitdiff_plain;h=a589b9f429ac0e5bcdebda0f74ee313d39d69b7f;p=deliverable%2Flinux.git

drm/i915: Explain why we need to write DPLL twice

... it's because setting the Pixel Multiply bits only takes effect once
the PLL is enabled and stable.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index f7962b741c94..e3b8d0dc7a7d 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -4089,13 +4089,13 @@ static int intel_crtc_mode_set(struct drm_crtc *crtc,
 			}
 			I915_WRITE(DPLL_MD(pipe), temp);
 		} else {
-			/* write it again -- the BIOS does, after all */
+			/* The pixel multiplier can only be updated once the
+			 * DPLL is enabled and the clocks are stable.
+			 *
+			 * So write it again.
+			 */
 			I915_WRITE(dpll_reg, dpll);
 		}
-
-		/* Wait for the clocks to stabilize. */
-		POSTING_READ(dpll_reg);
-		udelay(150);
 	}
 
 	intel_crtc->lowfreq_avail = false;