diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg
index 1e6c28b1942b..a917efc289a2 100644
--- a/Documentation/ABI/testing/dev-kmsg
+++ b/Documentation/ABI/testing/dev-kmsg
@@ -61,6 +61,17 @@ Description:	The /dev/kmsg character device node provides userspace access
 		SEEK_CUR is not supported, returning -ESPIPE (invalid seek) to
 		errno whenever requested.
 
+		Other seek operations or offsets are not supported because of
+		the special behavior this device has. The device allows to read
+		or write only whole variable length messages (records) that are
+		stored in a ring buffer.
+
+		Because of the non-standard behavior also the error values are
+		non-standard. -ESPIPE is returned for non-zero offset. -EINVAL
+		is returned for other operations, e.g. SEEK_CUR. This behavior
+		and values are historical and could not be modified without the
+		risk of breaking userspace.
+
 		The output format consists of a prefix carrying the syslog
 		prefix including priority and facility, the 64 bit message
 		sequence number and the monotonic timestamp in microseconds,
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8c14835be46c..5f6eca65dd9a 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -943,6 +943,14 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
 	return ret;
 }
 
+/*
+ * Be careful when modifying this function!!!
+ *
+ * Only few operations are supported because the device works only with the
+ * entire variable length messages (records). Non-standard values are
+ * returned in the other cases and has been this way for quite some time.
+ * User space applications might depend on this behavior.
+ */
 static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
 {
 	struct devkmsg_user *user = file->private_data;