MIPS: pm-cps: Block system suspend when a JTAG probe is present

If a JTAG probe is connected to a MIPS cluster, then the CPC detects it
and latches the CPC.STAT_CONF.EJTAG_PROBE bit to 1. While set,
attempting to send a power-down command to a core will be blocked, and
the CPC will instead send the core to clock-off state. This can
interfere with systems fully entering a low power state where all cores,
CM, GIC, etc are powered down.

Detect that a JTAG probe is / has been connected to the cluster and
block the suspend attempt.

Attempting to suspend the system while a JTAG probe is connected now
yields:
 # echo mem > /sys/power/state
 [   11.654000] PM: Syncing filesystems ... done.
 [   11.658000] JTAG probe is connected - abort suspend
 -sh: echo: write error: Operation not permitted
 #

To restore suspend, the JTAG probe should be disconnected or put into
quiescent state. Platform code can then clear the
CPC.STAT_CONF.EJTAG_PROBE bit.
Reported-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18641/Signed-off-by: James Hogan <jhogan@kernel.org>

arch/mips/kernel/pm-cps.c

@@ -12,6 +12,7 @@
 #include <linux/init.h>
 #include <linux/percpu.h>
 #include <linux/slab.h>
+#include <linux/suspend.h>
 
 #include <asm/asm-offsets.h>
 #include <asm/cacheflush.h>
@@ -670,6 +671,34 @@ static int cps_pm_online_cpu(unsigned int cpu)
 	return 0;
 }
 
+static int cps_pm_power_notifier(struct notifier_block *this,
+				 unsigned long event, void *ptr)
+{
+	unsigned int stat;
+
+	switch (event) {
+	case PM_SUSPEND_PREPARE:
+		stat = read_cpc_cl_stat_conf();
+		/*
+		 * If we're attempting to suspend the system and power down all
+		 * of the cores, the JTAG detect bit indicates that the CPC will
+		 * instead put the cores into clock-off state. In this state
+		 * a connected debugger can cause the CPU to attempt
+		 * interactions with the powered down system. At best this will
+		 * fail. At worst, it can hang the NoC, requiring a hard reset.
+		 * To avoid this, just block system suspend if a JTAG probe
+		 * is detected.
+		 */
+		if (stat & CPC_Cx_STAT_CONF_EJTAG_PROBE) {
+			pr_warn("JTAG probe is connected - abort suspend\n");
+			return NOTIFY_BAD;
+		}
+		return NOTIFY_DONE;
+	default:
+		return NOTIFY_DONE;
+	}
+}
+
 static int __init cps_pm_init(void)
 {
 	/* A CM is required for all non-coherent states */
@@ -705,6 +734,8 @@ static int __init cps_pm_init(void)
 		pr_warn("pm-cps: no CPC, clock & power gating unavailable\n");
 	}
 
+	pm_notifier(cps_pm_power_notifier, 0);
+
 	return cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "mips/cps_pm:online",
 				 cps_pm_online_cpu, NULL);
 }