drm/i915/dsb: Improve the indexed reg write checks

Currently intel_dsb_indexed_reg_write() just assumes the previous
instructions is also an indexed register write, and thus only
checks the register offset. Make the check more robust by
actually checking the instruction opcode as well.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216003810.13338-8-ville.syrjala@linux.intel.comReviewed-by: Animesh Manna <animesh.manna@intel.com>

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

@@ -102,6 +102,23 @@ static void intel_dsb_emit(struct intel_dsb *dsb, u32 ldw, u32 udw)
 	buf[dsb->free_pos++] = udw;
 }
 
+static bool intel_dsb_prev_ins_is_write(struct intel_dsb *dsb,
+					u32 opcode, i915_reg_t reg)
+{
+	const u32 *buf = dsb->cmd_buf;
+	u32 prev_opcode, prev_reg;
+
+	prev_opcode = buf[dsb->ins_start_offset + 1] >> DSB_OPCODE_SHIFT;
+	prev_reg = buf[dsb->ins_start_offset + 1] & DSB_REG_VALUE_MASK;
+
+	return prev_opcode == opcode && prev_reg == i915_mmio_reg_offset(reg);
+}
+
+static bool intel_dsb_prev_ins_is_indexed_write(struct intel_dsb *dsb, i915_reg_t reg)
+{
+	return intel_dsb_prev_ins_is_write(dsb, DSB_OPCODE_INDEXED_WRITE, reg);
+}
+
 /**
  * intel_dsb_indexed_reg_write() -Write to the DSB context for auto
  * increment register.
@@ -119,7 +136,6 @@ void intel_dsb_indexed_reg_write(struct intel_dsb *dsb,
 				 i915_reg_t reg, u32 val)
 {
 	u32 *buf = dsb->cmd_buf;
-	u32 reg_val;
 
 	if (!assert_dsb_has_room(dsb))
 		return;
@@ -140,8 +156,7 @@ void intel_dsb_indexed_reg_write(struct intel_dsb *dsb,
 	 * we are writing odd no of dwords, Zeros will be added in the end for
 	 * padding.
 	 */
-	reg_val = buf[dsb->ins_start_offset + 1] & DSB_REG_VALUE_MASK;
-	if (reg_val != i915_mmio_reg_offset(reg)) {
+	if (!intel_dsb_prev_ins_is_indexed_write(dsb, reg)) {
 		/* Every instruction should be 8 byte aligned. */
 		dsb->free_pos = ALIGN(dsb->free_pos, 2);