drm/radeon/kms: add PLL flag to prefer frequencies <= the target freq

This is needed when using fractional feedback dividers on some IGP
chips.
Signed-off-by: NAlex Deucher <alexdeucher@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_display.c

@@ -491,7 +491,11 @@ void radeon_compute_pll(struct radeon_pll *pll,
 					tmp += (uint64_t)pll->reference_freq * 1000 * frac_feedback_div;
 					current_freq = radeon_div(tmp, ref_div * post_div);
 
-					error = abs(current_freq - freq);
+					if (flags & RADEON_PLL_PREFER_CLOSEST_LOWER) {
+						error = freq - current_freq;
+						error = error < 0 ? 0xffffffff : error;
+					} else
+						error = abs(current_freq - freq);
 					vco_diff = abs(vco - best_vco);
 
 					if ((best_vco == 0 && error < best_error) ||

drivers/gpu/drm/radeon/radeon_mode.h

@@ -124,6 +124,7 @@ struct radeon_tmds_pll {
 #define RADEON_PLL_PREFER_LOW_POST_DIV  (1 << 8)
 #define RADEON_PLL_PREFER_HIGH_POST_DIV (1 << 9)
 #define RADEON_PLL_USE_FRAC_FB_DIV      (1 << 10)
+#define RADEON_PLL_PREFER_CLOSEST_LOWER (1 << 11)
 
 struct radeon_pll {
 	uint16_t reference_freq;