[PATCH v4 2/2] tty/sysrq: Dump printk ring buffer messages via sysrq

[Sreenath Vijayan <sreenath.vijayan@xxxxxxxx>]

When terminal is unresponsive, one cannot use dmesg to view printk
ring buffer messages. Also, syslog services may be disabled,
to check the messages after a reboot, especially on embedded systems.
In this scenario, dump the printk ring buffer messages via sysrq
by pressing sysrq+D.

Signed-off-by: Sreenath Vijayan <sreenath.vijayan@xxxxxxxx>
Signed-off-by: Shimoyashiki Taichi <taichi.shimoyashiki@xxxxxxxx>
---
 Documentation/admin-guide/sysrq.rst |  2 ++
 drivers/tty/sysrq.c                 | 20 +++++++++++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/sysrq.rst b/Documentation/admin-guide/sysrq.rst
index 2f2e5bd440f9..c634e8b4cea2 100644
--- a/Documentation/admin-guide/sysrq.rst
+++ b/Documentation/admin-guide/sysrq.rst
@@ -161,6 +161,8 @@ Command	    Function
             will be printed to your console. (``0``, for example would make
             it so that only emergency messages like PANICs or OOPSes would
             make it to your console.)
+
+``D``	    Dump the printk ring buffer
 =========== ===================================================================
 
 Okay, so what can I use them for?
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 02217e3c916b..365f7fa145f0 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -450,6 +450,24 @@ static const struct sysrq_key_op sysrq_unrt_op = {
 	.enable_mask	= SYSRQ_ENABLE_RTNICE,
 };
 
+static void dmesg_dump_callback(struct work_struct *work)
+{
+	dump_printk_buffer();
+}
+
+static DECLARE_WORK(sysrq_dmesg_work, dmesg_dump_callback);
+
+static void sysrq_handle_dmesg_dump(u8 key)
+{
+	queue_work(system_unbound_wq, &sysrq_dmesg_work);
+}
+static struct sysrq_key_op sysrq_dmesg_dump_op = {
+	.handler        = sysrq_handle_dmesg_dump,
+	.help_msg       = "dump-dmesg(D)",
+	.action_msg     = "Dump dmesg",
+	.enable_mask    = SYSRQ_ENABLE_DUMP,
+};
+
 /* Key Operations table and lock */
 static DEFINE_SPINLOCK(sysrq_key_table_lock);
 
@@ -505,7 +523,7 @@ static const struct sysrq_key_op *sysrq_key_table[62] = {
 	NULL,				/* A */
 	NULL,				/* B */
 	NULL,				/* C */
-	NULL,				/* D */
+	&sysrq_dmesg_dump_op,		/* D */
 	NULL,				/* E */
 	NULL,				/* F */
 	NULL,				/* G */
-- 
2.43.0