[PATCH 26/59] drm/i915/cnl: Simplify dco_fraction calculation.

[Timo Aaltonen]

From: Rodrigo Vivi <rodrigo.vivi at intel.com>

BugLink: http://bugs.launchpad.net/bugs/1757573

I confess I never fully understood that previous calculation,
so this is not a "fix". But let's simplify this math
so poor brains like mine can read and make some sense of
it in the future.

v2: Don't follow the spec since that gives invalid
    values and it is also confusing. This Ville's
    version is much simpler.
v3: Use u64 cast instead of declaring a u64 dco. (Ville).

Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Mika Kahola <mika.kahola at intel.com>
Cc: Manasi Navare <manasi.d.navare at intel.com>
Cc: James Ausmus <james.ausmus at intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115184257.8633-1-rodrigo.vivi@intel.com
(cherry picked from commit 8a00678a096445716861d2c0bd2a7007ad373f4d)
Signed-off-by: Timo Aaltonen <timo.aaltonen at canonical.com>
---
 drivers/gpu/drm/i915/intel_dpll_mgr.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dpll_mgr.c b/drivers/gpu/drm/i915/intel_dpll_mgr.c
index 579e4814773b..3507e27af9ab 100644
--- a/drivers/gpu/drm/i915/intel_dpll_mgr.c
+++ b/drivers/gpu/drm/i915/intel_dpll_mgr.c
@@ -2169,6 +2169,8 @@ static void cnl_wrpll_params_populate(struct skl_wrpll_params *params,
 				      u32 dco_freq, u32 ref_freq,
 				      int pdiv, int qdiv, int kdiv)
 {
+	u32 dco;
+
 	switch (kdiv) {
 	case 1:
 		params->kdiv = 1;
@@ -2205,9 +2207,10 @@ static void cnl_wrpll_params_populate(struct skl_wrpll_params *params,
 	params->qdiv_ratio = qdiv;
 	params->qdiv_mode = (qdiv == 1) ? 0 : 1;
 
-	params->dco_integer = div_u64(dco_freq, ref_freq);
-	params->dco_fraction = div_u64((div_u64((uint64_t)dco_freq<<15, (uint64_t)ref_freq) -
-					((uint64_t)params->dco_integer<<15)) * 0x8000, 0x8000);
+	dco = div_u64((u64)dco_freq << 15, ref_freq);
+
+	params->dco_integer = dco >> 15;
+	params->dco_fraction = dco & 0x7fff;
 }
 
 static bool
-- 
2.15.1