coresight: always use stashed trace id value in etm4_trace_id

etm4_trace_id is not guaranteed to be executed on the CPU whose ETM is
being accessed. This leads to exception similar to below one if the
CPU whose ETM is being accessed is in deeper idle states. So it must
be executed on the CPU whose ETM is being accessed.

Unhandled fault: synchronous external abort (0x96000210) at 0xffff000008db4040
Internal error: : 96000210 [#1] PREEMPT SMP
Modules linked in:
CPU: 5 PID: 5979 Comm: etm.sh Not tainted 4.7.0-rc3 torvalds#159
Hardware name: ARM Juno development board (r2) (DT)
task: ffff80096dd34b00 ti: ffff80096dfe4000 task.ti: ffff80096dfe4000
PC is at etm4_trace_id+0x5c/0x90
LR is at etm4_trace_id+0x3c/0x90
Call trace:
 etm4_trace_id+0x5c/0x90
 coresight_id_match+0x78/0xa8
 bus_for_each_dev+0x60/0xa0
 coresight_enable+0xc0/0x1b8
 enable_source_store+0x3c/0x70
 dev_attr_store+0x18/0x28
 sysfs_kf_write+0x48/0x58
 kernfs_fop_write+0x14c/0x1e0
 __vfs_write+0x1c/0x100
 vfs_write+0xa0/0x1b8
 SyS_write+0x44/0xa0
 el0_svc_naked+0x24/0x28

However, TRCTRACEIDR is not guaranteed to hold the previous programmed
trace id if it enters deeper idle states. Further, the trace id that is
computed in etm4_init_trace_id is programmed into TRCTRACEIDR only in
etm4_enable_hw which happens much later in the sequence after
coresight_id_match is executed from enable_source_store.

This patch simplifies etm4_trace_id by returning the stashed trace id
value similar to etm4_cpu_id.

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>

drivers/hwtracing/coresight/coresight-etm4x.c

@@ -79,22 +79,8 @@ static int etm4_cpu_id(struct coresight_device *csdev)
 static int etm4_trace_id(struct coresight_device *csdev)
 {
 	struct etmv4_drvdata *drvdata = dev_get_drvdata(csdev->dev.parent);
-	unsigned long flags;
-	int trace_id = -1;
 
-	if (!local_read(&drvdata->mode))
-		return drvdata->trcid;
-
-	spin_lock_irqsave(&drvdata->spinlock, flags);
-
-	CS_UNLOCK(drvdata->base);
-	trace_id = readl_relaxed(drvdata->base + TRCTRACEIDR);
-	trace_id &= ETM_TRACEID_MASK;
-	CS_LOCK(drvdata->base);
-
-	spin_unlock_irqrestore(&drvdata->spinlock, flags);
-
-	return trace_id;
+	return drvdata->trcid;
 }
 
 static void etm4_enable_hw(void *info)