From c2b63374461c0986147902f719c26412d1f26fbc Mon Sep 17 00:00:00 2001 From: =?utf8?q?Ville=20Syrj=C3=A4l=C3=A4?= Date: Wed, 7 Oct 2015 22:08:25 +0300 Subject: [PATCH] drm/i915: Enable DPLL VGA mode before P1/P2 divider write MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Apparently writing the DPLL register P1/P2 divider fields won't trigger an actual change in the DPLL output unless VGA mode is enabled for prior to the register write that changes the P1/P2 dividers. The write with the new P1/P2 divider can itself disable VGA mode again without problems. I tested the behaviour on my 946GZ, and when manually frobbing the register with the display on, the behaviour is very clear. However I can't explain why this machine actually works. The P1/P2 divider changes caused by normal modesets do seem to make it through to the hardware somehow since I get a stable picture on the monitor with any resolution. Maybe it's the "three times for luck" stuff that somehow masks the problem, or something. But apparently there are machines (eg. Nick Bowler's G45) where that isn't the case and we fail to get the correct clock from the DPLL. Things used to work because we enabled VGA mode for disabled DPLLs, so when re-enabling the DPLL VGA mode was enabled just prior to the first register write, and hence the P1/P2 change went through without a hitch. That got changed in b8afb9113c51 drm/i915: Keep GMCH DPLL VGA mode always disabled in the name of consistency. In order to keep the consistency part, leave VGA mode disabled for disabled DPLLs, but turn it on just prior to updating the P1/P2 dividers to make sure the hardware picks up on the new values. Cc: Nick Bowler Reported-by: Nick Bowler Tested-by: Nick Bowler Signed-off-by: Ville Syrjälä Reviewed-by: Daniel Vetter Signed-off-by: Jani Nikula --- drivers/gpu/drm/i915/intel_display.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c index bdfac53dd945..96e6c41783df 100644 --- a/drivers/gpu/drm/i915/intel_display.c +++ b/drivers/gpu/drm/i915/intel_display.c @@ -1724,6 +1724,13 @@ static void i9xx_enable_pll(struct intel_crtc *crtc) I915_READ(DPLL(!crtc->pipe)) | DPLL_DVO_2X_MODE); } + /* + * Apparently we need to have VGA mode enabled prior to changing + * the P1/P2 dividers. Otherwise the DPLL will keep using the old + * dividers, even though the register value does change. + */ + I915_WRITE(reg, 0); + I915_WRITE(reg, dpll); /* Wait for the clocks to stabilize. */ -- 2.34.1