coresight: etm4x: Add necessary synchronization for sysreg access

As per the specification any update to the TRCPRGCTLR must be synchronized
by a context synchronization event (in our case an explicist ISB) before
the TRCSTATR is checked.

Link: https://lore.kernel.org/r/20210110224850.1880240-22-suzuki.poulose@arm.com
Cc: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210201181351.1475223-24-mathieu.poirier@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-etm4x-core.c

@@ -284,6 +284,15 @@ static int etm4_enable_hw(struct etmv4_drvdata *drvdata)
 	/* Disable the trace unit before programming trace registers */
 	etm4x_relaxed_write32(csa, 0, TRCPRGCTLR);
 
+	/*
+	 * If we use system instructions, we need to synchronize the
+	 * write to the TRCPRGCTLR, before accessing the TRCSTATR.
+	 * See ARM IHI0064F, section
+	 * "4.3.7 Synchronization of register updates"
+	 */
+	if (!csa->io_mem)
+		isb();
+
 	/* wait for TRCSTATR.IDLE to go up */
 	if (coresight_timeout(csa, TRCSTATR, TRCSTATR_IDLE_BIT, 1))
 		dev_err(etm_dev,
@@ -362,6 +371,10 @@ static int etm4_enable_hw(struct etmv4_drvdata *drvdata)
 	/* Enable the trace unit */
 	etm4x_relaxed_write32(csa, 1, TRCPRGCTLR);
 
+	/* Synchronize the register updates for sysreg access */
+	if (!csa->io_mem)
+		isb();
+
 	/* wait for TRCSTATR.IDLE to go back down to '0' */
 	if (coresight_timeout(csa, TRCSTATR, TRCSTATR_IDLE_BIT, 0))
 		dev_err(etm_dev,