Commit 7bd1d409 authored by Alexander Shishkin's avatar Alexander Shishkin Committed by Greg Kroah-Hartman
Browse files

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: default avatarMathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: default avatarAlexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 9b76968d

Documentation/ABI/testing/configfs-stp-policy

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+48 −0
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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.

Documentation/ABI/testing/sysfs-class-stm

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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.

Documentation/ABI/testing/sysfs-class-stm_source

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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.

Documentation/ioctl/ioctl-number.txt

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@@ -81,6 +81,9 @@ Code Seq#(hex) Include File Comments 
0x22	all	scsi/sg.h

0x22	all	scsi/sg.h

'#'	00-3F	IEEE 1394 Subsystem	Block for the entire subsystem

'#'	00-3F	IEEE 1394 Subsystem	Block for the entire subsystem

'$'	00-0F	linux/perf_counter.h, linux/perf_event.h

'$'	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

'&'	00-07	drivers/firewire/nosy-user.h

'1'	00-1F	<linux/timepps.h>	PPS kit from Ulrich Windl

'1'	00-1F	<linux/timepps.h>	PPS kit from Ulrich Windl

					<ftp://ftp.de.kernel.org/pub/linux/daemons/ntp/PPS/>

					<ftp://ftp.de.kernel.org/pub/linux/daemons/ntp/PPS/>

Documentation/trace/stm.txt

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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

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