remoteproc: qcom: q6v5-mss: Improve readability of reset_assert

Define AXI_GATING_VALID_OVERRIDE and fixup comments to improve readability
of Q6 modem reset sequence on SC7180 SoCs.

Reviewed-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Link: https://lore.kernel.org/r/20200123131236.1078-3-sibis@codeaurora.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

drivers/remoteproc/qcom_q6v5_mss.c

@@ -71,6 +71,7 @@
 #define NAV_AXI_HALTREQ_BIT		BIT(0)
 #define NAV_AXI_HALTACK_BIT		BIT(1)
 #define NAV_AXI_IDLE_BIT		BIT(2)
+#define AXI_GATING_VALID_OVERRIDE	BIT(0)
 
 #define HALT_ACK_TIMEOUT_US		100000
 #define NAV_HALT_ACK_TIMEOUT_US		200
@@ -415,16 +416,24 @@ static int q6v5_reset_assert(struct q6v5 *qproc)
 		ret = reset_control_reset(qproc->mss_restart);
 		reset_control_deassert(qproc->pdc_reset);
 	} else if (qproc->has_halt_nav) {
-		/* SWAR using CONN_BOX_SPARE_0 for pipeline glitch issue */
+		/*
+		 * When the AXI pipeline is being reset with the Q6 modem partly
+		 * operational there is possibility of AXI valid signal to
+		 * glitch, leading to spurious transactions and Q6 hangs. A work
+		 * around is employed by asserting the AXI_GATING_VALID_OVERRIDE
+		 * BIT before triggering Q6 MSS reset. Both the HALTREQ and
+		 * AXI_GATING_VALID_OVERRIDE are withdrawn post MSS assert
+		 * followed by a MSS deassert, while holding the PDC reset.
+		 */
 		reset_control_assert(qproc->pdc_reset);
 		regmap_update_bits(qproc->conn_map, qproc->conn_box,
-				   BIT(0), BIT(0));
+				   AXI_GATING_VALID_OVERRIDE, 1);
 		regmap_update_bits(qproc->halt_nav_map, qproc->halt_nav,
 				   NAV_AXI_HALTREQ_BIT, 0);
 		reset_control_assert(qproc->mss_restart);
 		reset_control_deassert(qproc->pdc_reset);
 		regmap_update_bits(qproc->conn_map, qproc->conn_box,
-				   BIT(0), 0);
+				   AXI_GATING_VALID_OVERRIDE, 0);
 		ret = reset_control_deassert(qproc->mss_restart);
 	} else {
 		ret = reset_control_assert(qproc->mss_restart);