[PATCH v7 12/12] Documentation: power: Add document on debugging shutdown hangs

["Mario Limonciello (AMD)" <superm1@kernel.org>]

The kernel will attempt hibernation callbacks before shutdown callbacks.
If there is any problem with this, ideally a UART log should be captured
to debug the problem.  However if one isn't available users can use the
pstore functionality to retrieve logs.  Add a document explaining how
this works.

Signed-off-by: Mario Limonciello (AMD) <superm1@kernel.org>
---
v7:
 * New patch
---
 Documentation/power/index.rst              |  1 +
 Documentation/power/shutdown-debugging.rst | 55 ++++++++++++++++++++++
 2 files changed, 56 insertions(+)
 create mode 100644 Documentation/power/shutdown-debugging.rst

diff --git a/Documentation/power/index.rst b/Documentation/power/index.rst
index a0f5244fb4279..ea70633d9ce6c 100644
--- a/Documentation/power/index.rst
+++ b/Documentation/power/index.rst
@@ -19,6 +19,7 @@ Power Management
     power_supply_class
     runtime_pm
     s2ram
+    shutdown-debugging
     suspend-and-cpuhotplug
     suspend-and-interrupts
     swsusp-and-swap-files
diff --git a/Documentation/power/shutdown-debugging.rst b/Documentation/power/shutdown-debugging.rst
new file mode 100644
index 0000000000000..d4bf12000c1cd
--- /dev/null
+++ b/Documentation/power/shutdown-debugging.rst
@@ -0,0 +1,55 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Debugging Kernel Shutdown Hangs with pstore
++++++++++++++++++++++++++++++++++++++++++++
+
+Overview
+========
+When the system is shut down to either a halt or power off, the kernel will
+attempt to run hibernation calls for all devices. If this fails, the kernel will
+fall back to shutdown callbacks. If this process fails and the system hangs
+the kernel logs will need to be retrieved to debug the issue.
+
+On systems that have a UART available, it is best to configure the kernel to use
+this UART for kernel console output.
+
+If a UART isn't available, the ``pstore`` subsystem provides a mechanism to
+persist this data across a system reset, allowing it to be retrieved on the next
+boot.
+
+Kernel Configuration
+====================
+To enable ``pstore`` and enable saving kernel ring buffer logs, set the
+following kernel configuration options:
+
+* ``CONFIG_PSTORE=y``
+* ``CONFIG_PSTORE_CONSOLE=y``
+
+Additionally, enable a backend to store the data. Depending upon your platform
+some options include:
+
+* ``CONFIG_EFI_VARS_PSTORE=y``
+* ``CONFIG_PSTORE_RAM=y``
+* ``CONFIG_PSTORE_FIRMWARE=y``
+* ``CONFIG_PSTORE_BLK=y``
+
+Kernel Command-line Parameters
+==============================
+Add these parameters to your kernel command line:
+
+* ``printk.always_kmsg_dump=Y``
+	* Forces the kernel to dump the entire message buffer to pstore during
+		shutdown
+* ``efi_pstore.pstore_disable=N``
+	* For EFI-based systems, ensures the EFI backend is active
+
+Userspace Interaction and Log Retrieval
+=======================================
+On the next boot after a hang, pstore logs will be available in the pstore
+filesystem (``/sys/fs/pstore``) and can be retrieved by userspace.
+
+On systemd systems, the ``systemd-pstore`` service will help do the following:
+
+#. Locate pstore data in ``/sys/fs/pstore``
+#. Read and save it to ``/var/lib/systemd/pstore``
+#. Clear pstore data for the next event
-- 
2.43.0