kmsg - add Documentation/ABI/testing/dev-kmsg

Signed-off-by: Kay Sievers <kay@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 92 insertions, 1 deletions

diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg
new file mode 100644
index 000000000000..281ecc5f9709
--- /dev/null
+++ b/Documentation/ABI/testing/dev-kmsg
@@ -0,0 +1,90 @@
+What:           /dev/kmsg
+Date:           Mai 2012
+KernelVersion:  3.5
+Contact:        Kay Sievers <kay@vrfy.org>
+Description:    The /dev/kmsg character device node provides userspace access
+                to the kernel's printk buffer.
+
+                Injecting messages:
+                Every write() to the opened device node places a log entry in
+                the kernel's printk buffer.
+
+                The logged line can be prefixed with a <N> syslog prefix, which
+                carries the syslog priority and facility. The single decimal
+                prefix number is composed of the 3 lowest bits being the syslog
+                priority and the higher bits the syslog facility number.
+
+                If no prefix is given, the priority number is the default kernel
+                log priority and the facility number is set to LOG_USER (1). It
+                is not possible to inject messages from userspace with the
+                facility number LOG_KERN (0), to make sure that the origin of
+                the messages can always be reliably determined.
+
+                Accessing the buffer:
+                Every read() from the opened device node receives one record
+                of the kernel's printk buffer.
+
+                The first read() directly following an open() always returns
+                first message in the buffer; there is no kernel-internal
+                persistent state; many readers can concurrently open the device
+                and read from it, without affecting other readers.
+
+                Every read() will receive the next available record. If no more
+                records are available read() will block, or if O_NONBLOCK is
+                used -EAGAIN returned.
+
+                Messages in the record ring buffer get overwritten as whole,
+                there are never partial messages received by read().
+
+                In case messages get overwritten in the circular buffer while
+                the device is kept open, the next read() will return -EPIPE,
+                and the seek position be updated to the next available record.
+                Subsequent reads() will return available records again.
+
+                Unlike the classic syslog() interface, the 64 bit record
+                sequence numbers allow to calculate the amount of lost
+                messages, in case the buffer gets overwritten. And they allow
+                to reconnect to the buffer and reconstruct the read position
+                if needed, without limiting the interface to a single reader.
+
+                The device supports seek with the following parameters:
+                SEEK_SET, 0
+                  seek to the first entry in the buffer
+                SEEK_END, 0
+                  seek after the last entry in the buffer
+                SEEK_DATA, 0
+                  seek after the last record available at the time
+                  the last SYSLOG_ACTION_CLEAR was issued.
+
+                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.
+                The values are separated by a ','. Future extensions might
+                add more comma separated values before the terminating ';'.
+                Unknown values should be gracefully ignored.
+
+                The human readable text string starts directly after the ';'
+                and is terminated by a '\n'. Untrusted values derived from
+                hardware or other facilities are printed, therefore
+                all non-printable characters in the log message are escaped
+                by "\x00" C-style hex encoding.
+
+                A line starting with ' ', is a continuation line, adding
+                key/value pairs to the log message, which provide the machine
+                readable context of the message, for reliable processing in
+                userspace.
+
+                Example:
+                7,160,424069;pci_root PNP0A03:00: host bridge window [io  0x0000-0x0cf7] (ignored)
+                 SUBSYSTEM=acpi
+                 DEVICE=+acpi:PNP0A03:00
+                6,339,5140900;NET: Registered protocol family 10
+                30,340,5690716;udevd[80]: starting version 181
+
+                The DEVICE= key uniquely identifies devices the following way:
+                  b12:8        - block dev_t
+                  c127:3       - char dev_t
+                  n8           - netdev ifindex
+                  +sound:card0 - subsystem:devname
+
+Users:          dmesg(1), userspace kernel log consumers
diff --git a/Documentation/devices.txt b/Documentation/devices.txt
index 00383186d8fb..5941f5136c6b 100644
--- a/Documentation/devices.txt
+++ b/Documentation/devices.txt
@@ -98,7 +98,8 @@ Your cooperation is appreciated.
                   8 = /dev/random       Nondeterministic random number gen.
                   9 = /dev/urandom      Faster, less secure random number gen.
                  10 = /dev/aio          Asynchronous I/O notification interface
-                 11 = /dev/kmsg         Writes to this come out as printk's
+                 11 = /dev/kmsg         Writes to this come out as printk's, reads
+                                        export the buffered printk records.
                  12 = /dev/oldmem       Used by crashdump kernels to access
                                         the memory of the kernel that crashed.