[PATCH v6 2/3] powerpc/fadump: add hotplug_ready sysfs interface

[Sourabh Jain]

The elfcorehdr describes the CPUs and memory of the crashed kernel to
the kernel that captures the dump, known as the second or fadump kernel.
The elfcorehdr needs to be updated if the system's memory changes due to
memory hotplug or online/offline events.

Currently, memory hotplug events are monitored in userspace by udev
rules, and fadump is re-registered, which recreates the elfcorehdr with
the latest available memory in the system.

However, the previous patch ("powerpc: make fadump resilient with memory
add/remove events") moved the creation of elfcorehdr to the second or
fadump kernel. This eliminates the need to regenerate the elfcorehdr
during memory hotplug or online/offline events.

Create a sysfs entry at /sys/kernel/fadump/hotplug_ready to let
userspace know that fadump re-registration is not required for memory
add/remove events.

Signed-off-by: Sourabh Jain <sourabhjain at linux.ibm.com>
Cc: Aditya Gupta <adityag at linux.ibm.com>
Cc: Aneesh Kumar K.V <aneesh.kumar at kernel.org>
Cc: Hari Bathini <hbathini at linux.ibm.com>
Cc: Mahesh Salgaonkar <mahesh at linux.ibm.com>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Cc: Naveen N Rao <naveen at kernel.org>
---
 Documentation/ABI/testing/sysfs-kernel-fadump | 11 +++++++++++
 arch/powerpc/kernel/fadump.c                  | 14 ++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-kernel-fadump b/Documentation/ABI/testing/sysfs-kernel-fadump
index 8f7a64a81783..971934b2891e 100644
--- a/Documentation/ABI/testing/sysfs-kernel-fadump
+++ b/Documentation/ABI/testing/sysfs-kernel-fadump
@@ -38,3 +38,14 @@ Contact:	linuxppc-dev at lists.ozlabs.org
 Description:	read only
 		Provide information about the amount of memory reserved by
 		FADump to save the crash dump in bytes.
+
+What:		/sys/kernel/fadump/hotplug_ready
+Date:		Sep 2023
+Contact:	linuxppc-dev at lists.ozlabs.org
+Description:	read only
+		Kdump udev rule re-registers fadump on memory add/remove events,
+		primarily to update the elfcorehdr. This sysfs indicates the
+		kdump udev rule that fadump re-registration is not required on
+		memory add/remove events because elfcorehdr is now prepared in
+		the second/fadump kernel.
+User:		kexec-tools
diff --git a/arch/powerpc/kernel/fadump.c b/arch/powerpc/kernel/fadump.c
index eb9132538268..a55dd9bf754c 100644
--- a/arch/powerpc/kernel/fadump.c
+++ b/arch/powerpc/kernel/fadump.c
@@ -1455,6 +1455,18 @@ static ssize_t enabled_show(struct kobject *kobj,
 	return sprintf(buf, "%d\n", fw_dump.fadump_enabled);
 }
 
+/*
+ * /sys/kernel/fadump/hotplug_ready sysfs node returns 1, which inidcates
+ * to usersapce that fadump re-registration is not required on memory
+ * hotplug events.
+ */
+static ssize_t hotplug_ready_show(struct kobject *kobj,
+				      struct kobj_attribute *attr,
+				      char *buf)
+{
+	return sprintf(buf, "%d\n", 1);
+}
+
 static ssize_t mem_reserved_show(struct kobject *kobj,
 				 struct kobj_attribute *attr,
 				 char *buf)
@@ -1527,11 +1539,13 @@ 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 kobj_attribute hotplug_ready_attr = __ATTR_RO(hotplug_ready);
 
 static struct attribute *fadump_attrs[] = {
 	&enable_attr.attr,
 	&register_attr.attr,
 	&mem_reserved_attr.attr,
+	&hotplug_ready_attr.attr,
 	NULL,
 };
 
-- 
2.41.0