1=================== 2System Trace Module 3=================== 4 5System Trace Module (STM) is a device described in MIPI STP specs as 6STP trace stream generator. STP (System Trace Protocol) is a trace 7protocol multiplexing data from multiple trace sources, each one of 8which is assigned a unique pair of master and channel. While some of 9these masters and channels are statically allocated to certain 10hardware trace sources, others are available to software. Software 11trace sources are usually free to pick for themselves any 12master/channel combination from this pool. 13 14On the receiving end of this STP stream (the decoder side), trace 15sources can only be identified by master/channel combination, so in 16order for the decoder to be able to make sense of the trace that 17involves multiple trace sources, it needs to be able to map those 18master/channel pairs to the trace sources that it understands. 19 20For instance, it is helpful to know that syslog messages come on 21master 7 channel 15, while arbitrary user applications can use masters 2248 to 63 and channels 0 to 127. 23 24To solve this mapping problem, stm class provides a policy management 25mechanism via configfs, that allows defining rules that map string 26identifiers to ranges of masters and channels. If these rules (policy) 27are consistent with what decoder expects, it will be able to properly 28process the trace data. 29 30This policy is a tree structure containing rules (policy_node) that 31have a name (string identifier) and a range of masters and channels 32associated with it, located in "stp-policy" subsystem directory in 33configfs. The topmost directory's name (the policy) is formatted as 34the STM device name to which this policy applies and and arbitrary 35string identifier separated by a stop. From the examle above, a rule 36may look like this:: 37 38 $ ls /config/stp-policy/dummy_stm.my-policy/user 39 channels masters 40 $ cat /config/stp-policy/dummy_stm.my-policy/user/masters 41 48 63 42 $ cat /config/stp-policy/dummy_stm.my-policy/user/channels 43 0 127 44 45which means that the master allocation pool for this rule consists of 46masters 48 through 63 and channel allocation pool has channels 0 47through 127 in it. Now, any producer (trace source) identifying itself 48with "user" identification string will be allocated a master and 49channel from within these ranges. 50 51These rules can be nested, for example, one can define a rule "dummy" 52under "user" directory from the example above and this new rule will 53be used for trace sources with the id string of "user/dummy". 54 55Trace sources have to open the stm class device's node and write their 56trace data into its file descriptor. In order to identify themselves 57to the policy, they need to do a STP_POLICY_ID_SET ioctl on this file 58descriptor providing their id string. Otherwise, they will be 59automatically allocated a master/channel pair upon first write to this 60file descriptor according to the "default" rule of the policy, if such 61exists. 62 63Some STM devices may allow direct mapping of the channel mmio regions 64to userspace for zero-copy writing. One mappable page (in terms of 65mmu) will usually contain multiple channels' mmios, so the user will 66need to allocate that many channels to themselves (via the 67aforementioned ioctl() call) to be able to do this. That is, if your 68stm device's channel mmio region is 64 bytes and hardware page size is 694096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with 70width==64, you should be able to mmap() one page on this file 71descriptor and obtain direct access to an mmio region for 64 channels. 72 73Examples of STM devices are Intel(R) Trace Hub [1] and Coresight STM 74[2]. 75 76stm_source 77========== 78 79For kernel-based trace sources, there is "stm_source" device 80class. Devices of this class can be connected and disconnected to/from 81stm devices at runtime via a sysfs attribute called "stm_source_link" 82by writing the name of the desired stm device there, for example:: 83 84 $ echo dummy_stm.0 > /sys/class/stm_source/console/stm_source_link 85 86For examples on how to use stm_source interface in the kernel, refer 87to stm_console, stm_heartbeat or stm_ftrace drivers. 88 89Each stm_source device will need to assume a master and a range of 90channels, depending on how many channels it requires. These are 91allocated for the device according to the policy configuration. If 92there's a node in the root of the policy directory that matches the 93stm_source device's name (for example, "console"), this node will be 94used to allocate master and channel numbers. If there's no such policy 95node, the stm core will pick the first contiguous chunk of channels 96within the first available master. Note that the node must exist 97before the stm_source device is connected to its stm device. 98 99stm_console 100=========== 101 102One implementation of this interface also used in the example above is 103the "stm_console" driver, which basically provides a one-way console 104for kernel messages over an stm device. 105 106To configure the master/channel pair that will be assigned to this 107console in the STP stream, create a "console" policy entry (see the 108beginning of this text on how to do that). When initialized, it will 109consume one channel. 110 111stm_ftrace 112========== 113 114This is another "stm_source" device, once the stm_ftrace has been 115linked with an stm device, and if "function" tracer is enabled, 116function address and parent function address which Ftrace subsystem 117would store into ring buffer will be exported via the stm device at 118the same time. 119 120Currently only Ftrace "function" tracer is supported. 121 122* [1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf 123* [2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html 124