drm/i915: Make vlv_find_best_dpll() ppm calculation safe

Use div_u64() to make the ppm calculation in vlv_find_best_dpll() safe
against interger overflows.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 617b963..dc50775 100644 (file)
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -672,13 +672,11 @@ vlv_find_best_dpll(const intel_limit_t *limit, struct drm_crtc *crtc,
        u32 p1, p2, m1, m2, vco, bestn, bestm1, bestm2, bestp1, bestp2;
        u32 m, n, fastclk;
        u32 updrate, minupdate, p;
-       unsigned long bestppm, ppm, absppm;
+       unsigned int bestppm = 1000000;
        int dotclk, flag;
 
        flag = 0;
        dotclk = target * 1000;
-       bestppm = 1000000;
-       ppm = absppm = 0;
        fastclk = dotclk / (2*100);
        updrate = 0;
        minupdate = 19200;
@@ -695,6 +693,8 @@ vlv_find_best_dpll(const intel_limit_t *limit, struct drm_crtc *crtc,
                                p = p1 * p2;
                                /* based on hardware requirement, prefer bigger m1,m2 values */
                                for (m1 = limit->m1.min; m1 <= limit->m1.max; m1++) {
+                                       unsigned int ppm, diff;
+
                                        m2 = DIV_ROUND_CLOSEST(fastclk * p * n, refclk * m1);
                                        m = m1 * m2;
                                        vco = updrate * m;
@@ -702,14 +702,14 @@ vlv_find_best_dpll(const intel_limit_t *limit, struct drm_crtc *crtc,
                                        if (vco < limit->vco.min || vco >= limit->vco.max)
                                                continue;
 
-                                       ppm = 1000000 * ((vco / p) - fastclk) / fastclk;
-                                       absppm = (ppm > 0) ? ppm : (-ppm);
-                                       if (absppm < 100 && ((p1 * p2) > (bestp1 * bestp2))) {
+                                       diff = abs(vco / p - fastclk);
+                                       ppm = div_u64(1000000ULL * diff, fastclk);
+                                       if (ppm < 100 && ((p1 * p2) > (bestp1 * bestp2))) {
                                                bestppm = 0;
                                                flag = 1;
                                        }
-                                       if (absppm < bestppm - 10) {
-                                               bestppm = absppm;
+                                       if (ppm < bestppm - 10) {
+                                               bestppm = ppm;
                                                flag = 1;
                                        }
                                        if (flag) {