+static u32 check_clock(u32 rate, u32 a, u32 b, u32 c,
+ u32 *best, u32 *best_diff, u32 *reg)
+{
+ /* a is 10-bit, b is 10-bit, c is 12-bit */
+ u64 new_rate;
+ u32 new_diff;
+
+ new_rate = ixp4xx_timer_freq * (u64)(c + 1);
+ do_div(new_rate, a * (c + 1) + b + 1);
+ new_diff = abs((u32)new_rate - rate);
+
+ if (new_diff < *best_diff) {
+ *best = new_rate;
+ *best_diff = new_diff;
+ *reg = (a << 22) | (b << 12) | c;
+ }
+ return new_diff;
+}
+
+static void find_best_clock(u32 rate, u32 *best, u32 *reg)
+{
+ u32 a, b, diff = 0xFFFFFFFF;
+
+ a = ixp4xx_timer_freq / rate;
+
+ if (a > 0x3FF) { /* 10-bit value - we can go as slow as ca. 65 kb/s */
+ check_clock(rate, 0x3FF, 1, 1, best, &diff, reg);
+ return;
+ }
+ if (a == 0) { /* > 66.666 MHz */
+ a = 1; /* minimum divider is 1 (a = 0, b = 1, c = 1) */
+ rate = ixp4xx_timer_freq;
+ }
+
+ if (rate * a == ixp4xx_timer_freq) { /* don't divide by 0 later */
+ check_clock(rate, a - 1, 1, 1, best, &diff, reg);
+ return;
+ }
+
+ for (b = 0; b < 0x400; b++) {
+ u64 c = (b + 1) * (u64)rate;
+ do_div(c, ixp4xx_timer_freq - rate * a);
+ c--;
+ if (c >= 0xFFF) { /* 12-bit - no need to check more 'b's */
+ if (b == 0 && /* also try a bit higher rate */
+ !check_clock(rate, a - 1, 1, 1, best, &diff, reg))
+ return;
+ check_clock(rate, a, b, 0xFFF, best, &diff, reg);
+ return;
+ }
+ if (!check_clock(rate, a, b, c, best, &diff, reg))
+ return;
+ if (!check_clock(rate, a, b, c + 1, best, &diff, reg))
+ return;
+ }
+}