drm/amd/display: Correct rounding calcs in mod_freesync_is_valid_range

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

1 files 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;