drm/nva3/clk: minor improvements to fractional N calculation

Helps us to get identical numbers to the binary driver for (at least)
Kepler memory PLLs, and fixes a rounding error.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

1 files changed, 11 insertions, 3 deletions

diff --git a/drivers/gpu/drm/nouveau/core/subdev/clock/pllnva3.c b/drivers/gpu/drm/nouveau/core/subdev/clock/pllnva3.c
index 4497378ba1e8..2fe1f712eefa 100644
--- a/drivers/gpu/drm/nouveau/core/subdev/clock/pllnva3.c
+++ b/drivers/gpu/drm/nouveau/core/subdev/clock/pllnva3.c
@@ -50,8 +50,15 @@ nva3_pll_calc(struct nouveau_subdev *subdev, struct nvbios_pll *info,
                 u32 tmp = freq * *P * M;
                 N  = tmp / info->refclk;
                 fN = tmp % info->refclk;
-                if (!pfN && fN >= info->refclk / 2)
-                        N++;
+
+                if (!pfN) {
+                        if (fN >= info->refclk / 2)
+                                N++;
+                } else {
+                        if (fN <  info->refclk / 2)
+                                N--;
+                        fN = tmp - (N * info->refclk);
+                }
 
                 if (N < info->vco1.min_n)
                         continue;
@@ -66,7 +73,8 @@ nva3_pll_calc(struct nouveau_subdev *subdev, struct nvbios_pll *info,
                 }
 
                 if (pfN) {
-                        *pfN = (((fN << 13) / info->refclk) - 4096) & 0xffff;
+                        *pfN = ((fN << 13) + info->refclk / 2) / info->refclk;
+                        *pfN = (*pfN - 4096) & 0xffff;
                         return freq;
                 }
         }