Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 System Trace Module 2 =================== 3 4 System Trace Module (STM) is a device described in MIPI STP specs as 5 STP trace stream generator. STP (System Trace Protocol) is a trace 6 protocol multiplexing data from multiple trace sources, each one of 7 which is assigned a unique pair of master and channel. While some of 8 these masters and channels are statically allocated to certain 9 hardware trace sources, others are available to software. Software 10 trace sources are usually free to pick for themselves any 11 master/channel combination from this pool. 12 13 On the receiving end of this STP stream (the decoder side), trace 14 sources can only be identified by master/channel combination, so in 15 order for the decoder to be able to make sense of the trace that 16 involves multiple trace sources, it needs to be able to map those 17 master/channel pairs to the trace sources that it understands. 18 19 For instance, it is helpful to know that syslog messages come on 20 master 7 channel 15, while arbitrary user applications can use masters 21 48 to 63 and channels 0 to 127. 22 23 To solve this mapping problem, stm class provides a policy management 24 mechanism via configfs, that allows defining rules that map string 25 identifiers to ranges of masters and channels. If these rules (policy) 26 are consistent with what decoder expects, it will be able to properly 27 process the trace data. 28 29 This policy is a tree structure containing rules (policy_node) that 30 have a name (string identifier) and a range of masters and channels 31 associated with it, located in "stp-policy" subsystem directory in 32 configfs. The topmost directory's name (the policy) is formatted as 33 the STM device name to which this policy applies and and arbitrary 34 string identifier separated by a stop. From the examle above, a rule 35 may look like this: 36 37 $ ls /config/stp-policy/dummy_stm.my-policy/user 38 channels masters 39 $ cat /config/stp-policy/dummy_stm.my-policy/user/masters 40 48 63 41 $ cat /config/stp-policy/dummy_stm.my-policy/user/channels 42 0 127 43 44 which means that the master allocation pool for this rule consists of 45 masters 48 through 63 and channel allocation pool has channels 0 46 through 127 in it. Now, any producer (trace source) identifying itself 47 with "user" identification string will be allocated a master and 48 channel from within these ranges. 49 50 These rules can be nested, for example, one can define a rule "dummy" 51 under "user" directory from the example above and this new rule will 52 be used for trace sources with the id string of "user/dummy". 53 54 Trace sources have to open the stm class device's node and write their 55 trace data into its file descriptor. In order to identify themselves 56 to the policy, they need to do a STP_POLICY_ID_SET ioctl on this file 57 descriptor providing their id string. Otherwise, they will be 58 automatically allocated a master/channel pair upon first write to this 59 file descriptor according to the "default" rule of the policy, if such 60 exists. 61 62 Some STM devices may allow direct mapping of the channel mmio regions 63 to userspace for zero-copy writing. One mappable page (in terms of 64 mmu) will usually contain multiple channels' mmios, so the user will 65 need to allocate that many channels to themselves (via the 66 aforementioned ioctl() call) to be able to do this. That is, if your 67 stm device's channel mmio region is 64 bytes and hardware page size is 68 4096 bytes, after a successful STP_POLICY_ID_SET ioctl() call with 69 width==64, you should be able to mmap() one page on this file 70 descriptor and obtain direct access to an mmio region for 64 channels. 71 72 Examples of STM devices are Intel(R) Trace Hub  and Coresight STM 73 . 74 75 stm_source 76 ========== 77 78 For kernel-based trace sources, there is "stm_source" device 79 class. Devices of this class can be connected and disconnected to/from 80 stm devices at runtime via a sysfs attribute called "stm_source_link" 81 by writing the name of the desired stm device there, for example: 82 83 $ echo dummy_stm.0 > /sys/class/stm_source/console/stm_source_link 84 85 For examples on how to use stm_source interface in the kernel, refer 86 to stm_console, stm_heartbeat or stm_ftrace drivers. 87 88 Each stm_source device will need to assume a master and a range of 89 channels, depending on how many channels it requires. These are 90 allocated for the device according to the policy configuration. If 91 there's a node in the root of the policy directory that matches the 92 stm_source device's name (for example, "console"), this node will be 93 used to allocate master and channel numbers. If there's no such policy 94 node, the stm core will pick the first contiguous chunk of channels 95 within the first available master. Note that the node must exist 96 before the stm_source device is connected to its stm device. 97 98 stm_console 99 =========== 100 101 One implementation of this interface also used in the example above is 102 the "stm_console" driver, which basically provides a one-way console 103 for kernel messages over an stm device. 104 105 To configure the master/channel pair that will be assigned to this 106 console in the STP stream, create a "console" policy entry (see the 107 beginning of this text on how to do that). When initialized, it will 108 consume one channel. 109 110 stm_ftrace 111 ========== 112 113 This is another "stm_source" device, once the stm_ftrace has been 114 linked with an stm device, and if "function" tracer is enabled, 115 function address and parent function address which Ftrace subsystem 116 would store into ring buffer will be exported via the stm device at 117 the same time. 118 119 Currently only Ftrace "function" tracer is supported. 120 121  https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf 122  http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0444b/index.html