diff --git a/drivers/gpu/drm/i915/intel_guc.c b/drivers/gpu/drm/i915/intel_guc.c
index 4c61eb94527a..8660af3fd755 100644
--- a/drivers/gpu/drm/i915/intel_guc.c
+++ b/drivers/gpu/drm/i915/intel_guc.c
@@ -50,7 +50,8 @@ void intel_guc_init_send_regs(struct intel_guc *guc)
 	unsigned int i;
 
 	guc->send_regs.base = i915_mmio_reg_offset(SOFT_SCRATCH(0));
-	guc->send_regs.count = SOFT_SCRATCH_COUNT - 1;
+	guc->send_regs.count = GUC_MAX_MMIO_MSG_LEN;
+	BUILD_BUG_ON(GUC_MAX_MMIO_MSG_LEN > SOFT_SCRATCH_COUNT);
 
 	for (i = 0; i < guc->send_regs.count; i++) {
 		fw_domains |= intel_uncore_forcewake_for_reg(dev_priv,
diff --git a/drivers/gpu/drm/i915/intel_guc_fwif.h b/drivers/gpu/drm/i915/intel_guc_fwif.h
index d1bbaba6e012..ef79e0f144d7 100644
--- a/drivers/gpu/drm/i915/intel_guc_fwif.h
+++ b/drivers/gpu/drm/i915/intel_guc_fwif.h
@@ -601,7 +601,9 @@ struct guc_shared_ctx_data {
  * registers, where first register holds data treated as message header,
  * and other registers are used to hold message payload.
  *
- * For Gen9+, GuC uses software scratch registers 0xC180-0xC1B8
+ * For Gen9+, GuC uses software scratch registers 0xC180-0xC1B8,
+ * but no H2G command takes more than 8 parameters and the GuC FW
+ * itself uses an 8-element array to store the H2G message.
  *
  *      +-----------+---------+---------+---------+
  *      |  MMIO[0]  | MMIO[1] |   ...   | MMIO[n] |
@@ -633,6 +635,8 @@ struct guc_shared_ctx_data {
  *   field.
  */
 
+#define GUC_MAX_MMIO_MSG_LEN		8
+
 #define INTEL_GUC_MSG_TYPE_SHIFT	28
 #define INTEL_GUC_MSG_TYPE_MASK		(0xF << INTEL_GUC_MSG_TYPE_SHIFT)
 #define INTEL_GUC_MSG_DATA_SHIFT	16