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>

diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c b/drivers/gpu/drm/mgag200/mgag200_mode.c
index a4d7c500c97b7f43d049a8ed8cc6bca59b20c100..b69642d5d850581694fe8741056a5d0529f4eeed 100644 (file)
--- 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