[Intel-gfx] [PATCH] drm/i915: Allow PCH DPLL sharing regardless of DPLL_SDVO_HIGH_SPEED

[ville.syrjala at linux.intel.com]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

DPLL_SDVO_HIGH_SPEED must be set for SDVO/HDMI/DP, but nowhere is it
forbidden to set it for LVDS/CRT as well. So let's also set it on
CRT to make it possible to share the DPLL between HDMI and CRT.

What that bit apparently does is enable the x5 clock to the port,
which then pumps out the bits on both edges of the clock. The DAC
doesn't need that clock since it's not pumping out bits, but I don't
think it hurts to have the DPLL output that clock anyway.

This is fairly important on IVB since it has only two DPLLs with three
pipes. So trying to drive three or more PCH ports with three pipes
is only possible when at least one of the DPLLs gets shared between
two of the pipes.

SNB doesn't really need to do this since it has only two pipes. It could
be done to avoid enabling the second DPLL at all in certain cases, but
I'm not sure that's such a huge win. So let's not do it for SNB, at
least for now. On ILK it never makes sense as the DPLLs can't be shared.

v2: Just always enable the high speed clock to keep things simple (Daniel)
    Beef up the commit message a bit (Daniel)

Cc: Nick Yamane <nick.diego at gmail.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: stable at vger.kernel.org
Tested-by: Nick Yamane <nick.diego at gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97204
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/intel_display.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index e5ad1010c8b1..45ff5007544c 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -9512,6 +9512,24 @@ static void ironlake_compute_dpll(struct intel_crtc *intel_crtc,
 	if (intel_crtc_has_dp_encoder(crtc_state))
 		dpll |= DPLL_SDVO_HIGH_SPEED;
 
+	/*
+	 * The high speed IO clock is only really required for
+	 * SDVO/HDMI/DP, but we also enable it for CRT to make it
+	 * possible to share the DPLL between CRT and HDMI. Enabling
+	 * the clock needlessly does no real harm, except use up a
+	 * bit of power potentially.
+	 *
+	 * We'll limit this to IVB with 3 pipes, since it has only two
+	 * DPLLs and so DPLL sharing is the only way to get three pipes
+	 * driving PCH ports at the same time. On SNB we could do this,
+	 * and potentially avoid enabling the second DPLL, but it's not
+	 * clear if it''s a win or loss power wise. No point in doing
+	 * this on ILK at all since it has a fixed DPLL<->pipe mapping.
+	 */
+	if (INTEL_INFO(dev_priv)->num_pipes == 3 &&
+	    intel_crtc_has_type(crtc_state, INTEL_OUTPUT_ANALOG))
+		dpll |= DPLL_SDVO_HIGH_SPEED;
+
 	/* compute bitmask from p1 value */
 	dpll |= (1 << (crtc_state->dpll.p1 - 1)) << DPLL_FPA01_P1_POST_DIV_SHIFT;
 	/* also FPA1 */
-- 
2.7.4