coresight: etm4x: request to retain power to the trace unit when active

The Coresight ETMv4 architecture provides a way to request to keep the
power to the trace unit. This might help to collect the traces without
the need to disable the CPU power management(entering/exiting deeper
idle states).

Trace PowerDown Control Register provides powerup request bit which when
set requests the system to retain power to the trace unit and emulate
the powerdown request.

Typically, a trace unit drives a signal to the power controller to
request that the trace unit core power domain is powered up. However,
if the trace unit and the CPU are in the same power domain then the
implementation might combine the trace unit power up status with a
signal from the CPU.

This patch requests to retain power to the trace unit when active and
to remove when inactive. Note this change will only request but the
behaviour depends on the implementation. However, it matches the
exact behaviour expected when the external debugger is connected with
respect to CPU power states.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-etm4x.c

@@ -164,6 +164,13 @@ static void etm4_enable_hw(void *info)
 	writel_relaxed(config->vmid_mask0, drvdata->base + TRCVMIDCCTLR0);
 	writel_relaxed(config->vmid_mask1, drvdata->base + TRCVMIDCCTLR1);
 
+	/*
+	 * Request to keep the trace unit powered and also
+	 * emulation of powerdown
+	 */
+	writel_relaxed(readl_relaxed(drvdata->base + TRCPDCR) | TRCPDCR_PU,
+		       drvdata->base + TRCPDCR);
+
 	/* Enable the trace unit */
 	writel_relaxed(1, drvdata->base + TRCPRGCTLR);
 
@@ -294,6 +301,11 @@ static void etm4_disable_hw(void *info)
 
 	CS_UNLOCK(drvdata->base);
 
+	/* power can be removed from the trace unit now */
+	control = readl_relaxed(drvdata->base + TRCPDCR);
+	control &= ~TRCPDCR_PU;
+	writel_relaxed(control, drvdata->base + TRCPDCR);
+
 	control = readl_relaxed(drvdata->base + TRCPRGCTLR);
 
 	/* EN, bit[0] Trace unit enable bit */

drivers/hwtracing/coresight/coresight-etm4x.h

@@ -183,6 +183,9 @@
 #define TRCSTATR_IDLE_BIT		0
 #define ETM_DEFAULT_ADDR_COMP		0
 
+/* PowerDown Control Register bits */
+#define TRCPDCR_PU			BIT(3)
+
 /* secure state access levels */
 #define ETM_EXLEVEL_S_APP		BIT(8)
 #define ETM_EXLEVEL_S_OS		BIT(9)