stm class: Introduce an abstraction for System Trace Module devices

A System Trace Module (STM) is a device exporting data in System Trace
Protocol (STP) format as defined by MIPI STP standards. Examples of such
devices are Intel(R) Trace Hub and Coresight STM.

This abstraction provides a unified interface for software trace sources
to send their data over an STM device to a debug host. In order to do
that, such a trace source needs to be assigned a pair of master/channel
identifiers that all the data from this source will be tagged with. The
STP decoder on the debug host side will use these master/channel tags to
distinguish different trace streams from one another inside one STP
stream.

This abstraction provides a configfs-based policy management mechanism
for dynamic allocation of these master/channel pairs based on trace
source-supplied string identifier. It has the flexibility of being
defined at runtime and at the same time (provided that the policy
definition is aligned with the decoding end) consistency.

For userspace trace sources, this abstraction provides write()-based and
mmap()-based (if the underlying stm device allows this) output mechanism.

For kernel-side trace sources, we provide "stm_source" device class that
can be connected to an stm device at run time.

Cc: linux-api@vger.kernel.org
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

5 files changed, 156 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/configfs-stp-policy b/Documentation/ABI/testing/configfs-stp-policy
new file mode 100644
index 000000000000..421ce6825c66
--- /dev/null
+++ b/Documentation/ABI/testing/configfs-stp-policy
@@ -0,0 +1,48 @@
+What:		/config/stp-policy
+Date:		June 2015
+KernelVersion:	4.3
+Description:
+		This group contains policies mandating Master/Channel allocation
+		for software sources wishing to send trace data over an STM
+		device.
+
+What:		/config/stp-policy/<device>.<policy>
+Date:		June 2015
+KernelVersion:	4.3
+Description:
+		This group is the root of a policy; its name is a concatenation
+		of an stm device name to which this policy applies and an
+		arbitrary string. If <device> part doesn't match an existing
+		stm device, mkdir will fail with ENODEV; if that device already
+		has a policy assigned to it, mkdir will fail with EBUSY.
+
+What:		/config/stp-policy/<device>.<policy>/device
+Date:		June 2015
+KernelVersion:	4.3
+Description:
+		STM device to which this policy applies, read only. Same as the
+		<device> component of its parent directory.
+
+What:		/config/stp-policy/<device>.<policy>/<node>
+Date:		June 2015
+KernelVersion:	4.3
+Description:
+		Policy node is a string identifier that software clients will
+		use to request a master/channel to be allocated and assigned to
+		them.
+
+What:		/config/stp-policy/<device>.<policy>/<node>/masters
+Date:		June 2015
+KernelVersion:	4.3
+Description:
+		Range of masters from which to allocate for users of this node.
+		Write two numbers: the first master and the last master number.
+
+What:		/config/stp-policy/<device>.<policy>/<node>/channels
+Date:		June 2015
+KernelVersion:	4.3
+Description:
+		Range of channels from which to allocate for users of this node.
+		Write two numbers: the first channel and the last channel
+		number.
+
diff --git a/Documentation/ABI/testing/sysfs-class-stm b/Documentation/ABI/testing/sysfs-class-stm
new file mode 100644
index 000000000000..c9aa4f3fc9a7
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-stm
@@ -0,0 +1,14 @@
+What:		/sys/class/stm/<stm>/masters
+Date:		June 2015
+KernelVersion:	4.3
+Contact:	Alexander Shishkin <alexander.shishkin@linux.intel.com>
+Description:
+		Shows first and last available to software master numbers on
+		this STM device.
+
+What:		/sys/class/stm/<stm>/channels
+Date:		June 2015
+KernelVersion:	4.3
+Contact:	Alexander Shishkin <alexander.shishkin@linux.intel.com>
+Description:
+		Shows the number of channels per master on this STM device.
diff --git a/Documentation/ABI/testing/sysfs-class-stm_source b/Documentation/ABI/testing/sysfs-class-stm_source
new file mode 100644
index 000000000000..57b8dd39bbf7
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-stm_source
@@ -0,0 +1,11 @@
+What:		/sys/class/stm_source/<stm_source>/stm_source_link
+Date:		June 2015
+KernelVersion:	4.3
+Contact:	Alexander Shishkin <alexander.shishkin@linux.intel.com>
+Description:
+		stm_source device linkage to stm device, where its tracing data
+		is directed. Reads return an existing connection or "<none>" if
+		this stm_source is not connected to any stm device yet.
+		Write an existing (registered) stm device's name here to
+		connect that device. If a device is already connected to this
+		stm_source, it will first be disconnected.
diff --git a/Documentation/ioctl/ioctl-number.txt b/Documentation/ioctl/ioctl-number.txt
index df1b25eb8382..3785b7e131f8 100644
--- a/Documentation/ioctl/ioctl-number.txt
+++ b/Documentation/ioctl/ioctl-number.txt
@@ -81,6 +81,9 @@ Code  Seq#(hex)	Include File		Comments
 0x22	all	scsi/sg.h
 '#'	00-3F	IEEE 1394 Subsystem	Block for the entire subsystem
 '$'	00-0F	linux/perf_counter.h, linux/perf_event.h
+'%'	00-0F	include/uapi/linux/stm.h
+					System Trace Module subsystem
+					<mailto:alexander.shishkin@linux.intel.com>
 '&'	00-07	drivers/firewire/nosy-user.h
 '1'	00-1F	<linux/timepps.h>	PPS kit from Ulrich Windl
 					<ftp://ftp.de.kernel.org/pub/linux/daemons/ntp/PPS/>
diff --git a/Documentation/trace/stm.txt b/Documentation/trace/stm.txt
new file mode 100644
index 000000000000..ea035f9dbfd7
--- /dev/null
+++ b/Documentation/trace/stm.txt
@@ -0,0 +1,80 @@
+System Trace Module
+===================
+
+System Trace Module (STM) is a device described in MIPI STP specs as
+STP trace stream generator. STP (System Trace Protocol) is a trace
+protocol multiplexing data from multiple trace sources, each one of
+which is assigned a unique pair of master and channel. While some of
+these masters and channels are statically allocated to certain
+hardware trace sources, others are available to software. Software
+trace sources are usually free to pick for themselves any
+master/channel combination from this pool.
+
+On the receiving end of this STP stream (the decoder side), trace
+sources can only be identified by master/channel combination, so in
+order for the decoder to be able to make sense of the trace that
+involves multiple trace sources, it needs to be able to map those
+master/channel pairs to the trace sources that it understands.
+
+For instance, it is helpful to know that syslog messages come on
+master 7 channel 15, while arbitrary user applications can use masters
+48 to 63 and channels 0 to 127.
+
+To solve this mapping problem, stm class provides a policy management
+mechanism via configfs, that allows defining rules that map string
+identifiers to ranges of masters and channels. If these rules (policy)
+are consistent with what decoder expects, it will be able to properly
+process the trace data.
+
+This policy is a tree structure containing rules (policy_node) that
+have a name (string identifier) and a range of masters and channels
+associated with it, located in "stp-policy" subsystem directory in
+configfs. The topmost directory's name (the policy) is formatted as
+the STM device name to which this policy applies and and arbitrary
+string identifier separated by a stop. From the examle above, a rule
+may look like this:
+
+$ ls /config/stp-policy/dummy_stm.my-policy/user
+channels masters
+$ cat /config/stp-policy/dummy_stm.my-policy/user/masters
+48 63
+$ cat /config/stp-policy/dummy_stm.my-policy/user/channels
+0 127
+
+which means that the master allocation pool for this rule consists of
+masters 48 through 63 and channel allocation pool has channels 0
+through 127 in it. Now, any producer (trace source) identifying itself
+with "user" identification string will be allocated a master and
+channel from within these ranges.
+
+These rules can be nested, for example, one can define a rule "dummy"
+under "user" directory from the example above and this new rule will
+be used for trace sources with the id string of "user/dummy".
+
+Trace sources have to open the stm class device's node and write their
+trace data into its file descriptor. In order to identify themselves
+to the policy, they need to do a STP_POLICY_ID_SET ioctl on this file
+descriptor providing their id string. Otherwise, they will be
+automatically allocated a master/channel pair upon first write to this
+file descriptor according to the "default" rule of the policy, if such
+exists.
+
+Some STM devices may allow direct mapping of the channel mmio regions
+to userspace for zero-copy writing. One mappable page (in terms of
+mmu) will usually contain multiple channels' mmios, so the user will
+need to allocate that many channels to themselves (via the
+aforementioned ioctl() call) to be able to do this. That is, if your
+stm device's channel mmio region is 64 bytes and hardware page size is
+4096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with
+width==64, you should be able to mmap() one page on this file
+descriptor and obtain direct access to an mmio region for 64 channels.
+
+For kernel-based trace sources, there is "stm_source" device
+class. Devices of this class can be connected and disconnected to/from
+stm devices at runtime via a sysfs attribute.
+
+Examples of STM devices are Intel(R) Trace Hub [1] and Coresight STM
+[2].
+
+[1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf
+[2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html