drm/i915/dsb: Introduce intel_dsb_wait_usec()

Add a function to emit the DSB "wait usecs" instruction.
This is just a usleep() for the DSB.

As a lower bound it seems pretty accurate, but the upper bound
seemed oddly relaxed (ie. sometimes I've seen waits that are
quite a bit longer than specified, not sure why).

Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240930170415.23841-7-ville.syrjala@linux.intel.com

Author: vsyrjala

drivers/gpu/drm/i915/display/intel_dsb.c

@@ -393,6 +393,12 @@ void intel_dsb_interrupt(struct intel_dsb *dsb)
 		       DSB_OPCODE_INTERRUPT << DSB_OPCODE_SHIFT);
 }
 
+void intel_dsb_wait_usec(struct intel_dsb *dsb, int count)
+{
+	intel_dsb_emit(dsb, count,
+		       DSB_OPCODE_WAIT_USEC << DSB_OPCODE_SHIFT);
+}
+
 static void intel_dsb_emit_wait_dsl(struct intel_dsb *dsb,
 				    u32 opcode, int lower, int upper)
 {

drivers/gpu/drm/i915/display/intel_dsb.h

@@ -40,6 +40,7 @@ void intel_dsb_noop(struct intel_dsb *dsb, int count);
 void intel_dsb_nonpost_start(struct intel_dsb *dsb);
 void intel_dsb_nonpost_end(struct intel_dsb *dsb);
 void intel_dsb_interrupt(struct intel_dsb *dsb);
+void intel_dsb_wait_usec(struct intel_dsb *dsb, int count);
 void intel_dsb_wait_scanline_in(struct intel_atomic_state *state,
 				struct intel_dsb *dsb,
 				int lower, int upper);