powerpc/fadump: sysfs for fadump memory reservation

Add a sys interface to allow querying the memory reserved by FADump for
saving the crash dump.

Also added Documentation/ABI for the new sysfs file.
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191211160910.21656-7-sourabhjain@linux.ibm.com

Documentation/ABI/testing/sysfs-kernel-fadump

@@ -31,3 +31,10 @@ Description:	write only
 		the system is booted to capture the vmcore using FADump.
 		It is used to release the memory reserved by FADump to
 		save the crash dump.
+
+What:		/sys/kernel/fadump/mem_reserved
+Date:		Dec 2019
+Contact:	linuxppc-dev@lists.ozlabs.org
+Description:	read only
+		Provide information about the amount of memory reserved by
+		FADump to save the crash dump in bytes.

Documentation/powerpc/firmware-assisted-dump.rst

@@ -268,6 +268,11 @@ Here is the list of files under kernel sysfs:
     be handled and vmcore will not be captured. This interface can be
     easily integrated with kdump service start/stop.
 
+ /sys/kernel/fadump/mem_reserved
+
+   This is used to display the memory reserved by FADump for saving the
+   crash dump.
+
  /sys/kernel/fadump_release_mem
     This file is available only when FADump is active during
     second kernel. This is used to release the reserved memory

arch/powerpc/kernel/fadump.c

@@ -1369,6 +1369,13 @@ static ssize_t enabled_show(struct kobject *kobj,
 	return sprintf(buf, "%d\n", fw_dump.fadump_enabled);
 }
 
+static ssize_t mem_reserved_show(struct kobject *kobj,
+				 struct kobj_attribute *attr,
+				 char *buf)
+{
+	return sprintf(buf, "%ld\n", fw_dump.reserve_dump_area_size);
+}
+
 static ssize_t registered_show(struct kobject *kobj,
 			       struct kobj_attribute *attr,
 			       char *buf)
@@ -1433,10 +1440,12 @@ static int fadump_region_show(struct seq_file *m, void *private)
 static struct kobj_attribute release_attr = __ATTR_WO(release_mem);
 static struct kobj_attribute enable_attr = __ATTR_RO(enabled);
 static struct kobj_attribute register_attr = __ATTR_RW(registered);
+static struct kobj_attribute mem_reserved_attr = __ATTR_RO(mem_reserved);
 
 static struct attribute *fadump_attrs[] = {
 	&enable_attr.attr,
 	&register_attr.attr,
+	&mem_reserved_attr.attr,
 	NULL,
 };