drm/i915: prevent FIFO calculation overflows on 32 bits with high dotclocks

A very high dotclock (e.g. 229500kHz as reported by Anton) can cause
the entries_required variable to overflow, potentially leading to a
FIFO watermark value that's too low to support the given mode.  Split
the division across the calculation to avoid this.

Cc: stable@kernel.org
Reported-by: Anton Khirnov <wyskas@gmail.com>
Tested-by: Anton Khirnov <wyskas@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

drivers/gpu/drm/i915/intel_display.c

@@ -2119,7 +2119,14 @@ static unsigned long intel_calculate_wm(unsigned long clock_in_khz,
 {
 	long entries_required, wm_size;
 
-	entries_required = (clock_in_khz * pixel_size * latency_ns) / 1000000;
+	/*
+	 * Note: we need to make sure we don't overflow for various clock &
+	 * latency values.
+	 * clocks go from a few thousand to several hundred thousand.
+	 * latency is usually a few thousand
+	 */
+	entries_required = ((clock_in_khz / 1000) * pixel_size * latency_ns) /
+		1000;
 	entries_required /= wm->cacheline_size;
 
 	DRM_DEBUG("FIFO entries required for mode: %d\n", entries_required);