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: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

2 files changed, 6 insertions, 1 deletions

diff --git a/drivers/gpu/drm/radeon/radeon_display.c b/drivers/gpu/drm/radeon/radeon_display.c
index 3efcf1a526be..bc312f3d9a0a 100644
--- a/drivers/gpu/drm/radeon/radeon_display.c
+++ b/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) ||
diff --git a/drivers/gpu/drm/radeon/radeon_mode.h b/drivers/gpu/drm/radeon/radeon_mode.h
index 86f766e868e7..38c1dd082441 100644
--- a/drivers/gpu/drm/radeon/radeon_mode.h
+++ b/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;