1System Trace Module 2=================== 3 4System Trace Module (STM) is a device described in MIPI STP specs as 5STP trace stream generator. STP (System Trace Protocol) is a trace 6protocol multiplexing data from multiple trace sources, each one of 7which is assigned a unique pair of master and channel. While some of 8these masters and channels are statically allocated to certain 9hardware trace sources, others are available to software. Software 10trace sources are usually free to pick for themselves any 11master/channel combination from this pool. 12 13On the receiving end of this STP stream (the decoder side), trace 14sources can only be identified by master/channel combination, so in 15order for the decoder to be able to make sense of the trace that 16involves multiple trace sources, it needs to be able to map those 17master/channel pairs to the trace sources that it understands. 18 19For instance, it is helpful to know that syslog messages come on 20master 7 channel 15, while arbitrary user applications can use masters 2148 to 63 and channels 0 to 127. 22 23To solve this mapping problem, stm class provides a policy management 24mechanism via configfs, that allows defining rules that map string 25identifiers to ranges of masters and channels. If these rules (policy) 26are consistent with what decoder expects, it will be able to properly 27process the trace data. 28 29This policy is a tree structure containing rules (policy_node) that 30have a name (string identifier) and a range of masters and channels 31associated with it, located in "stp-policy" subsystem directory in 32configfs. The topmost directory's name (the policy) is formatted as 33the STM device name to which this policy applies and and arbitrary 34string identifier separated by a stop. From the examle above, a rule 35may look like this: 36 37$ ls /config/stp-policy/dummy_stm.my-policy/user 38channels masters 39$ cat /config/stp-policy/dummy_stm.my-policy/user/masters 4048 63 41$ cat /config/stp-policy/dummy_stm.my-policy/user/channels 420 127 43 44which means that the master allocation pool for this rule consists of 45masters 48 through 63 and channel allocation pool has channels 0 46through 127 in it. Now, any producer (trace source) identifying itself 47with "user" identification string will be allocated a master and 48channel from within these ranges. 49 50These rules can be nested, for example, one can define a rule "dummy" 51under "user" directory from the example above and this new rule will 52be used for trace sources with the id string of "user/dummy". 53 54Trace sources have to open the stm class device's node and write their 55trace data into its file descriptor. In order to identify themselves 56to the policy, they need to do a STP_POLICY_ID_SET ioctl on this file 57descriptor providing their id string. Otherwise, they will be 58automatically allocated a master/channel pair upon first write to this 59file descriptor according to the "default" rule of the policy, if such 60exists. 61 62Some STM devices may allow direct mapping of the channel mmio regions 63to userspace for zero-copy writing. One mappable page (in terms of 64mmu) will usually contain multiple channels' mmios, so the user will 65need to allocate that many channels to themselves (via the 66aforementioned ioctl() call) to be able to do this. That is, if your 67stm device's channel mmio region is 64 bytes and hardware page size is 684096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with 69width==64, you should be able to mmap() one page on this file 70descriptor and obtain direct access to an mmio region for 64 channels. 71 72For kernel-based trace sources, there is "stm_source" device 73class. Devices of this class can be connected and disconnected to/from 74stm devices at runtime via a sysfs attribute. 75 76Examples of STM devices are Intel(R) Trace Hub [1] and Coresight STM 77[2]. 78 79[1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf 80[2] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html 81