drm/i915/guc: doorbell reset should avoid used doorbells

guc_init_doorbell_hw() borrows the (currently single) GuC client to use
in reinitialising ALL the doorbell registers (as the hardware doesn't
reset them when the GuC is reset). As a prerequisite for accommodating
multiple clients, it should only reset doorbells that are supposed to be
disabled, avoiding those that are marked as in use by any client.
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_guc_submission.c

@@ -697,7 +697,7 @@ guc_client_free(struct drm_i915_private *dev_priv,
 }
 
 /*
- * Borrow the first client to set up & tear down every doorbell
+ * Borrow the first client to set up & tear down each unused doorbell
  * in turn, to ensure that all doorbell h/w is (re)initialised.
  */
 static void guc_init_doorbell_hw(struct intel_guc *guc)
@@ -713,6 +713,9 @@ static void guc_init_doorbell_hw(struct intel_guc *guc)
 		i915_reg_t drbreg = GEN8_DRBREGL(i);
 		u32 value = I915_READ(drbreg);
 
+		if (test_bit(i, guc->doorbell_bitmap))
+			continue;
+
 		err = guc_update_doorbell_id(guc, client, i);
 
 		/* Report update failure or unexpectedly active doorbell */
@@ -731,6 +734,9 @@ static void guc_init_doorbell_hw(struct intel_guc *guc)
 		i915_reg_t drbreg = GEN8_DRBREGL(i);
 		u32 value = I915_READ(drbreg);
 
+		if (test_bit(i, guc->doorbell_bitmap))
+			continue;
+
 		if (i != db_id && (value & GUC_DOORBELL_ENABLED))
 			DRM_DEBUG_DRIVER("Doorbell %d (reg 0x%x) finally 0x%x\n",
 					  i, drbreg.reg, value);