From c528dd16da88ea9e12b04b594995b911e39b0b32 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@linux.intel.com>
Date: Thu, 14 May 2020 15:38:37 +0300
Subject: drm/i915: Document our lackluster FSB frequency readout
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Git-commit: 42ab330530b47b44070af592e2f84a2a0b61532e
Patch-mainline: v5.9-rc1
References: jsc#SLE-12680, jsc#SLE-12880, jsc#SLE-12882, jsc#SLE-12883, jsc#SLE-13496, jsc#SLE-15322
Document the fact that we aren't reading out the actual FSB
frequency but rather just the state of the FSB straps.
Some BIOSen allow you to configure the two independently.
So if someone sets the two up in an inconsistent manner
we'll get the wrong answer here and thus will end up with
incorrect aux/pps clock dividers. Alas, proper docs are no
longer around so we can't do any better.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200514123838.3017-2-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Patrik Jakobsson <pjakobsson@suse.de>
---
drivers/gpu/drm/i915/display/intel_cdclk.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
index c17cf611625c..d57dfec7e9a5 100644
--- a/drivers/gpu/drm/i915/display/intel_cdclk.c
+++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
@@ -2704,7 +2704,16 @@ static int g4x_hrawclk(struct drm_i915_private *dev_priv)
{
u32 clkcfg;
- /* hrawclock is 1/4 the FSB frequency */
+ /*
+ * hrawclock is 1/4 the FSB frequency
+ *
+ * Note that this only reads the state of the FSB
+ * straps, not the actual FSB frequency. Some BIOSen
+ * let you configure each independently. Ideally we'd
+ * read out the actual FSB frequency but sadly we
+ * don't know which registers have that information,
+ * and all the relevant docs have gone to bit heaven :(
+ */
clkcfg = intel_de_read(dev_priv, CLKCFG) & CLKCFG_FSB_MASK;
/* ELK seems to redefine some of the values */
--
2.29.2