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+.. SPDX-License-Identifier: GPL-2.0
+
+======================================
+HiSilicon PCIe Tune and Trace device
+======================================
+
+Introduction
+============
+
+HiSilicon PCIe tune and trace device (PTT) is a PCIe Root Complex
+integrated Endpoint (RCiEP) device, providing the capability
+to dynamically monitor and tune the PCIe link's events (tune),
+and trace the TLP headers (trace). The two functions are independent,
+but is recommended to use them together to analyze and enhance the
+PCIe link's performance.
+
+On Kunpeng 930 SoC, the PCIe Root Complex is composed of several
+PCIe cores. Each PCIe core includes several Root Ports and a PTT
+RCiEP, like below. The PTT device is capable of tuning and
+tracing the links of the PCIe core.
+::
+ +--------------Core 0-------+
+ | | [ PTT ] |
+ | | [Root Port]---[Endpoint]
+ | | [Root Port]---[Endpoint]
+ | | [Root Port]---[Endpoint]
+ Root Complex |------Core 1-------+
+ | | [ PTT ] |
+ | | [Root Port]---[ Switch ]---[Endpoint]
+ | | [Root Port]---[Endpoint] `-[Endpoint]
+ | | [Root Port]---[Endpoint]
+ +---------------------------+
+
+The PTT device driver registers one PMU device for each PTT device.
+The name of each PTT device is composed of 'hisi_ptt' prefix with
+the id of the SICL and the Core where it locates. The Kunpeng 930
+SoC encapsulates multiple CPU dies (SCCL, Super CPU Cluster) and
+IO dies (SICL, Super I/O Cluster), where there's one PCIe Root
+Complex for each SICL.
+::
+ /sys/devices/hisi_ptt<sicl_id>_<core_id>
+
+Tune
+====
+
+PTT tune is designed for monitoring and adjusting PCIe link parameters (events).
+Currently we support events in 4 classes. The scope of the events
+covers the PCIe core to which the PTT device belongs.
+
+Each event is presented as a file under $(PTT PMU dir)/tune, and
+a simple open/read/write/close cycle will be used to tune the event.
+::
+ $ cd /sys/devices/hisi_ptt<sicl_id>_<core_id>/tune
+ $ ls
+ qos_tx_cpl qos_tx_np qos_tx_p
+ tx_path_rx_req_alloc_buf_level
+ tx_path_tx_req_alloc_buf_level
+ $ cat qos_tx_dp
+ 1
+ $ echo 2 > qos_tx_dp
+ $ cat qos_tx_dp
+ 2
+
+Current value (numerical value) of the event can be simply read
+from the file, and the desired value written to the file to tune.
+
+1. Tx path QoS control
+------------------------
+
+The following files are provided to tune the QoS of the tx path of
+the PCIe core.
+
+- qos_tx_cpl: weight of Tx completion TLPs
+- qos_tx_np: weight of Tx non-posted TLPs
+- qos_tx_p: weight of Tx posted TLPs
+
+The weight influences the proportion of certain packets on the PCIe link.
+For example, for the storage scenario, increase the proportion
+of the completion packets on the link to enhance the performance as
+more completions are consumed.
+
+The available tune data of these events is [0, 1, 2].
+Writing a negative value will return an error, and out of range
+values will be converted to 2. Note that the event value just
+indicates a probable level, but is not precise.
+
+2. Tx path buffer control
+-------------------------
+
+Following files are provided to tune the buffer of tx path of the PCIe core.
+
+- tx_path_rx_req_alloc_buf_level: watermark of Rx requested
+- tx_path_tx_req_alloc_buf_level: watermark of Tx requested
+
+These events influence the watermark of the buffer allocated for each
+type. Rx means the inbound while Tx means outbound. The packets will
+be stored in the buffer first and then transmitted either when the
+watermark reached or when timed out. For a busy direction, you should
+increase the related buffer watermark to avoid frequently posting and
+thus enhance the performance. In most cases just keep the default value.
+
+The available tune data of above events is [0, 1, 2].
+Writing a negative value will return an error, and out of range
+values will be converted to 2. Note that the event value just
+indicates a probable level, but is not precise.
+
+Trace
+=====
+
+PTT trace is designed for dumping the TLP headers to the memory, which
+can be used to analyze the transactions and usage condition of the PCIe
+Link. You can choose to filter the traced headers by either requester ID,
+or those downstream of a set of Root Ports on the same core of the PTT
+device. It's also supported to trace the headers of certain type and of
+certain direction.
+
+You can use the perf command `perf record` to set the parameters, start
+trace and get the data. It's also supported to decode the trace
+data with `perf report`. The control parameters for trace is inputted
+as event code for each events, which will be further illustrated later.
+An example usage is like
+::
+ $ perf record -e hisi_ptt0_2/filter=0x80001,type=1,direction=1,
+ format=1/ -- sleep 5
+
+This will trace the TLP headers downstream root port 0000:00:10.1 (event
+code for event 'filter' is 0x80001) with type of posted TLP requests,
+direction of inbound and traced data format of 8DW.
+
+1. filter
+---------
+
+The TLP headers to trace can be filtered by the Root Ports or the requester
+ID of the endpoints, which are located on the same core of the PTT device.
+You can set the filter by specifying the `filter` parameter which is required
+to start the trace. The parameter value is 20 bit. The supported filters and
+related values are outputted through `available_root_port_filters` and
+`available_requester_filters` sysfs attributes for Root Ports and Requesters
+respectively.
+::
+ $ cat available_root_port_filters
+ 0000:00:10.0 0x80001
+ 0000:00:11.0 0x80004
+ $ cat available_requester_filters
+ 0000:01:00.0 0x00100
+ 0000:01:00.1 0x00101
+
+Note that multiple Root Ports can be specified at one time, but only
+one Endpoint function can be specified in one trace. Specifying both
+Root Port and function at the same time is not supported.
+
+If no filter is available, reading the related filter sysfs attribute
+will get an empty string.
+::
+ $ cat available_root_port_filters
+
+ $ cat available_requester_filters
+
+The available filters can be dynamically updated, which means you can always
+get correct filter information when hotplug events happen, or when you manually
+remove/rescan the devices.
+
+2. type
+-------
+
+You can trace the TLP headers of certain types by specifying the `type`
+parameter, which is required to start the trace. The parameter value is
+8 bit. Current supported types and related values are shown below:
+
+8'b00000001: posted requests (P)
+8'b00000010: non-posted requests (NP)
+8'b00000100: completions (CPL)
+
+You can specify multiple types when tracing inbound TLP headers, but can only
+specify one when tracing outbound TLP headers.
+
+3. direction
+------------
+
+You can trace the TLP headers from certain direction, which is relative
+to the Root Port or the PCIe core, by specifying the `direction` parameter.
+This is optional and the default parameter is inbound. The parameter value
+is 4 bit. When the desired format is 4DW, directions and related values
+supported are shown below:
+
+4'b0000: inbound TLPs (P, NP, CPL)
+4'b0001: outbound TLPs (P, NP, CPL)
+4'b0010: outbound TLPs (P, NP, CPL) and inbound TLPs (P, NP, CPL B)
+4'b0011: outbound TLPs (P, NP, CPL) and inbound TLPs (CPL A)
+
+When the desired format is 8DW, directions and related values supported are
+shown below:
+
+4'b0000: reserved
+4'b0001: outbound TLPs (P, NP, CPL)
+4'b0010: inbound TLPs (P, NP, CPL B)
+4'b0011: inbound TLPs (CPL A)
+
+Inbound completions are classified into two types:
+
+completion A (CPL A): completion of CHI/DMA/Native non-posted requests, except for CPL B
+completion B (CPL B): completion of DMA remote2local and P2P non-posted requests
+
+4. format
+--------------
+
+You can change the format of the traced TLP headers by specifying the
+`format` parameter. The default format is 4DW. The parameter value is 4 bit.
+Current supported formats and related values are shown below:
+
+4'b0000: 4DW length per TLP header
+4'b0001: 8DW length per TLP header
+
+The traced TLP header format is different from the PCIe standard.
+
+When using the 8DW data format, the entire TLP header is logged
+(Header DW0-3 shown below). For example, the TLP header for Memory
+Reads with 64-bit addresses is shown in PCIe r5.0, Figure 2-17;
+the header for Configuration Requests is shown in Figure 2.20, etc.
+
+In addition, 8DW trace buffer entries contain a timestamp and
+possibly a prefix for a PASID TLP prefix (see Figure 6-20, PCIe r5.0).
+Otherwise this field will be all 0.
+
+The bit[31:11] of DW0 is always 0x1fffff, which can be
+used to distinguish the data format. 8DW format is like
+::
+ bits [ 31:11 ][ 10:0 ]
+ |---------------------------------------|-------------------|
+ DW0 [ 0x1fffff ][ Reserved (0x7ff) ]
+ DW1 [ Prefix ]
+ DW2 [ Header DW0 ]
+ DW3 [ Header DW1 ]
+ DW4 [ Header DW2 ]
+ DW5 [ Header DW3 ]
+ DW6 [ Reserved (0x0) ]
+ DW7 [ Time ]
+
+When using the 4DW data format, DW0 of the trace buffer entry
+contains selected fields of DW0 of the TLP, together with a
+timestamp. DW1-DW3 of the trace buffer entry contain DW1-DW3
+directly from the TLP header.
+
+4DW format is like
+::
+ bits [31:30] [ 29:25 ][24][23][22][21][ 20:11 ][ 10:0 ]
+ |-----|---------|---|---|---|---|-------------|-------------|
+ DW0 [ Fmt ][ Type ][T9][T8][TH][SO][ Length ][ Time ]
+ DW1 [ Header DW1 ]
+ DW2 [ Header DW2 ]
+ DW3 [ Header DW3 ]
+
+5. memory management
+--------------------
+
+The traced TLP headers will be written to the memory allocated
+by the driver. The hardware accepts 4 DMA address with same size,
+and writes the buffer sequentially like below. If DMA addr 3 is
+finished and the trace is still on, it will return to addr 0.
+::
+ +->[DMA addr 0]->[DMA addr 1]->[DMA addr 2]->[DMA addr 3]-+
+ +---------------------------------------------------------+
+
+Driver will allocate each DMA buffer of 4MiB. The finished buffer
+will be copied to the perf AUX buffer allocated by the perf core.
+Once the AUX buffer is full while the trace is still on, driver
+will commit the AUX buffer first and then apply for a new one with
+the same size. The size of AUX buffer is default to 16MiB. User can
+adjust the size by specifying the `-m` parameter of the perf command.
+
+Note that there is a gap between committing the old AUX buffer and
+applying a new one, which means the trace is stopped during the
+moment and TLPs transferred in the moment cannot be traced. To avoid
+this situation, you should begin the trace with large AUX buffer
+enough to avoid this gap.
+
+6. decoding
+-----------
+
+You can decode the traced data with `perf report -D` command (currently
+only support to dump the raw trace data). The traced data will be decoded
+according to the format described previously (take 8DW as an example):
+::
+ [...perf headers and other information]
+ . ... HISI PTT data: size 4194304 bytes
+ . 00000000: 00 00 00 00 Prefix
+ . 00000004: 01 00 00 60 Header DW0
+ . 00000008: 0f 1e 00 01 Header DW1
+ . 0000000c: 04 00 00 00 Header DW2
+ . 00000010: 40 00 81 02 Header DW3
+ . 00000014: 33 c0 04 00 Time
+ . 00000020: 00 00 00 00 Prefix
+ . 00000024: 01 00 00 60 Header DW0
+ . 00000028: 0f 1e 00 01 Header DW1
+ . 0000002c: 04 00 00 00 Header DW2
+ . 00000030: 40 00 81 02 Header DW3
+ . 00000034: 02 00 00 00 Time
+ . 00000040: 00 00 00 00 Prefix
+ . 00000044: 01 00 00 60 Header DW0
+ . 00000048: 0f 1e 00 01 Header DW1
+ . 0000004c: 04 00 00 00 Header DW2
+ . 00000050: 40 00 81 02 Header DW3
+ [...]