[PATCH 13/20] drm/amd/display: Correct rounding calcs in mod_freesync_is_valid_range

[Harry Wentland]

From: Anthony Koo <Anthony.Koo at amd.com>

Signed-off-by: Anthony Koo <Anthony.Koo at amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr at amd.com>
Acked-by: Harry Wentland <harry.wentland at amd.com>
---
 .../amd/display/modules/freesync/freesync.c   | 39 ++++++++++++++++---
 1 file changed, 34 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/amd/display/modules/freesync/freesync.c b/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
index 349387eb9fe6..769f46777a1d 100644
--- a/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
+++ b/drivers/gpu/drm/amd/display/modules/freesync/freesync.c
@@ -887,12 +887,41 @@ bool mod_freesync_is_valid_range(struct mod_freesync *mod_freesync,
 	unsigned long long nominal_field_rate_in_uhz =
 			mod_freesync_calc_nominal_field_rate(stream);
 
-	/* Allow for some rounding error of actual video timing by taking ceil.
-	 * For example, 144 Hz mode timing may actually be 143.xxx Hz when
-	 * calculated from pixel rate and vertical/horizontal totals, but
-	 * this should be allowed instead of blocking FreeSync.
+	/* Typically nominal refresh calculated can have some fractional part.
+	 * Allow for some rounding error of actual video timing by taking floor
+	 * of caps and request. Round the nominal refresh rate.
+	 *
+	 * Dividing will convert everything to units in Hz although input
+	 * variable name is in uHz!
+	 *
+	 * Also note, this takes care of rounding error on the nominal refresh
+	 * so by rounding error we only expect it to be off by a small amount,
+	 * such as < 0.1 Hz. i.e. 143.9xxx or 144.1xxx.
+	 *
+	 * Example 1. Caps    Min = 40 Hz, Max = 144 Hz
+	 *            Request Min = 40 Hz, Max = 144 Hz
+	 *                    Nominal = 143.5x Hz rounded to 144 Hz
+	 *            This function should allow this as valid request
+	 *
+	 * Example 2. Caps    Min = 40 Hz, Max = 144 Hz
+	 *            Request Min = 40 Hz, Max = 144 Hz
+	 *                    Nominal = 144.4x Hz rounded to 144 Hz
+	 *            This function should allow this as valid request
+	 *
+	 * Example 3. Caps    Min = 40 Hz, Max = 144 Hz
+	 *            Request Min = 40 Hz, Max = 144 Hz
+	 *                    Nominal = 120.xx Hz rounded to 120 Hz
+	 *            This function should return NOT valid since the requested
+	 *            max is greater than current timing's nominal
+	 *
+	 * Example 4. Caps    Min = 40 Hz, Max = 120 Hz
+	 *            Request Min = 40 Hz, Max = 120 Hz
+	 *                    Nominal = 144.xx Hz rounded to 144 Hz
+	 *            This function should return NOT valid since the nominal
+	 *            is greater than the capability's max refresh
 	 */
-	nominal_field_rate_in_uhz = div_u64(nominal_field_rate_in_uhz, 1000000);
+	nominal_field_rate_in_uhz =
+			div_u64(nominal_field_rate_in_uhz + 500000, 1000000);
 	min_refresh_cap_in_uhz /= 1000000;
 	max_refresh_cap_in_uhz /= 1000000;
 	min_refresh_request_in_uhz /= 1000000;
-- 
2.17.0