ARM: 6975/1: pmu: reject duplicate PMU registrations

Currently, the PMU reservation framework allows for multiple PMUs of
the same type to register themselves. This can lead to a bug with the
sequence:

register_pmu(pmu1);
reserve_pmu(pmu_type);
register_pmu(pmu2);
release_pmu(pmu1);

Here, pmu1 cannot be released, and pmu2 cannot be reserved.

This patch modifies register_pmu to reject registrations where a PMU is
already present, preventing this problem. PMUs which can have multiple
instances should not use the PMU reservation framework.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/pmu.c

@@ -34,13 +34,13 @@ static int __devinit pmu_register(struct platform_device *pdev,
 		return -EINVAL;
 	}
 
-	if (pmu_devices[type])
-		pr_warning("registering new PMU device type %d overwrites "
-				"previous registration!\n", type);
-	else
-		pr_info("registered new PMU device of type %d\n",
-				type);
+	if (pmu_devices[type]) {
+		pr_warning("rejecting duplicate registration of PMU device "
+			"type %d.", type);
+		return -ENOSPC;
+	}
 
+	pr_info("registered new PMU device of type %d\n", type);
 	pmu_devices[type] = pdev;
 	return 0;
 }