[Intel-gfx] [PATCH] drm/i915: clarify why we need to enable fdi plls so early

[Daniel Vetter]

For reference, see "Graphics BSpec: vol4g North Display Engine
Registers [IVB], Display Mode Set Sequence", step 4 of the enabling
sequence:

a. "Enable PCH FDI Receiver PLL, wait for warmup plus DMI latency
b. "Switch from Rawclk to PCDclk in FDI Receiver
c. "Enable CPU FDI Transmitter PLL, wait for warmup"

Cc: Paulo Zanoni <przanoni at gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/intel_display.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 90617cf..64f654c 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -3256,6 +3256,9 @@ static void ironlake_crtc_enable(struct drm_crtc *crtc)
 	is_pch_port = intel_crtc_driving_pch(crtc);
 
 	if (is_pch_port) {
+		/* Note: FDI PLL enabling _must_ be done before we enable the
+		 * cpu pipes, hence this is separate from all the other fdi/pch
+		 * enabling. */
 		ironlake_fdi_pll_enable(intel_crtc);
 	} else {
 		assert_fdi_tx_disabled(dev_priv, pipe);
-- 
1.7.11.4