misc: pvpanic: introduce device capability

According to pvpanic spec:
https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/specs/pvpanic.txt

The guest should determine pvpanic capability by RDPT, so initialize
capability during device probing. There is no need to register panic
notifier callback function if no events supported.

Before sending event to host side, check capability firstly.

Suggested by Greg KH, use sysfs to expose capability to user space,
also add new sysfs attribute in document.
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20210110115358.79100-2-pizhenwei@bytedance.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-pci-devices-pvpanic

+What:		/sys/devices/pci0000:00/*/QEMU0001:00/capability
+Date:		Jan 2021
+Contact:	zhenwei pi <pizhenwei@bytedance.com>
+Description:
+		Read-only attribute. Capabilities of pvpanic device which
+		are supported by QEMU.
+
+		Format: %x.
+
+		Detailed bit definition refers to section <Bit Definition>
+		from pvpanic device specification:
+		https://git.qemu.org/?p=qemu.git;a=blob_plain;f=docs/specs/pvpanic.txt

drivers/misc/pvpanic.c

@@ -19,6 +19,20 @@
 #include <uapi/misc/pvpanic.h>
 
 static void __iomem *base;
+static unsigned int capability = PVPANIC_PANICKED | PVPANIC_CRASH_LOADED;
+
+static ssize_t capability_show(struct device *dev,
+			       struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%x", capability);
+}
+static DEVICE_ATTR_RO(capability);
+
+static struct attribute *pvpanic_dev_attrs[] = {
+	&dev_attr_capability.attr,
+	NULL
+};
+ATTRIBUTE_GROUPS(pvpanic_dev);
 
 MODULE_AUTHOR("Hu Tao <hutao@cn.fujitsu.com>");
 MODULE_DESCRIPTION("pvpanic device driver");
@@ -27,6 +41,7 @@ MODULE_LICENSE("GPL");
 static void
 pvpanic_send_event(unsigned int event)
 {
+	if (event & capability)
 		iowrite8(event, base);
 }
 
@@ -73,6 +88,10 @@ static int pvpanic_mmio_probe(struct platform_device *pdev)
 		return -EINVAL;
 	}
 
+	/* initlize capability by RDPT */
+	capability &= ioread8(base);
+
+	if (capability)
 		atomic_notifier_chain_register(&panic_notifier_list,
 					       &pvpanic_panic_nb);
 
@@ -82,6 +101,7 @@ static int pvpanic_mmio_probe(struct platform_device *pdev)
 static int pvpanic_mmio_remove(struct platform_device *pdev)
 {
 
+	if (capability)
 		atomic_notifier_chain_unregister(&panic_notifier_list,
 						 &pvpanic_panic_nb);
 
@@ -104,6 +124,7 @@ static struct platform_driver pvpanic_mmio_driver = {
 		.name = "pvpanic-mmio",
 		.of_match_table = pvpanic_mmio_match,
 		.acpi_match_table = pvpanic_device_ids,
+		.dev_groups = pvpanic_dev_groups,
 	},
 	.probe = pvpanic_mmio_probe,
 	.remove = pvpanic_mmio_remove,