drm/i915/dsb: Move the +1 usec adjustment into dsb_wait_usec()

The "wait usec" DSB command doesn't quite seem to able to
guarantee that it always waits at least the specified
amount of usecs. Some of that could be just because it
supposedly just does some kind of dumb timestamp comparison
internally. But I also see cases where two hardware timestamps
sampled on each side of the "wait usec" command come out one
less than expected. So it looks like we always need at least a
+1 to guarantee that we never wait less than specified. Always
apply that adjustment in dsb_wait_usec().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250207223159.14132-2-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c b/drivers/gpu/drm/i915/display/intel_dsb.c
index 2f2812c23972550c81cd5adebc77ac0a57e8ef75..f8bd6fad0c871f528e0aee87a93e6b2098851f9e 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_dsb.c
+++ b/drivers/gpu/drm/i915/display/intel_dsb.c
@@ -369,7 +369,8 @@ void intel_dsb_interrupt(struct intel_dsb *dsb)
 
 void intel_dsb_wait_usec(struct intel_dsb *dsb, int count)
 {
-       intel_dsb_emit(dsb, count,
+       /* +1 to make sure we never wait less time than asked for */
+       intel_dsb_emit(dsb, count + 1,
                       DSB_OPCODE_WAIT_USEC << DSB_OPCODE_SHIFT);
 }
 
@@ -622,7 +623,7 @@ void intel_dsb_wait_vblank_delay(struct intel_atomic_state *state,
        const struct intel_crtc_state *crtc_state =
                intel_pre_commit_crtc_state(state, crtc);
        int usecs = intel_scanlines_to_usecs(&crtc_state->hw.adjusted_mode,
-                                            dsb_vblank_delay(state, crtc)) + 1;
+                                            dsb_vblank_delay(state, crtc));
 
        intel_dsb_wait_usec(dsb, usecs);
 }