drm/mgag200: fix G200ER pll picking algorithm

The original code was misported from the X driver,

a) an int went to unsigned int, breaking the downward counting testm code
b) the port did the vco/computed clock bits completely wrong.

This fixes an infinite loop on modprobe on some Dell servers with the G200ER
chipset variant.

Found in internal testing.

Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c
index a4d7c500c97b..b69642d5d850 100644
--- a/drivers/gpu/drm/mgag200/mgag200_mode.c
+++ b/drivers/gpu/drm/mgag200/mgag200_mode.c
@@ -468,10 +468,11 @@ static int mga_g200er_set_plls(struct mga_device *mdev, long clock)
 {
         unsigned int vcomax, vcomin, pllreffreq;
         unsigned int delta, tmpdelta;
-        unsigned int testr, testn, testm, testo;
+        int testr, testn, testm, testo;
         unsigned int p, m, n;
-        unsigned int computed;
+        unsigned int computed, vco;
         int tmp;
+        const unsigned int m_div_val[] = { 1, 2, 4, 8 };
 
         m = n = p = 0;
         vcomax = 1488000;
@@ -490,12 +491,13 @@ static int mga_g200er_set_plls(struct mga_device *mdev, long clock)
                                 if (delta == 0)
                                         break;
                                 for (testo = 5; testo < 33; testo++) {
-                                        computed = pllreffreq * (testn + 1) /
+                                        vco = pllreffreq * (testn + 1) /
                                                 (testr + 1);
-                                        if (computed < vcomin)
+                                        if (vco < vcomin)
                                                 continue;
-                                        if (computed > vcomax)
+                                        if (vco > vcomax)
                                                 continue;
+                                        computed = vco / (m_div_val[testm] * (testo + 1));
                                         if (computed > clock)
                                                 tmpdelta = computed - clock;
                                         else