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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 link 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 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
+mostly 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 posted 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 illustracted 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 locates on the same core of the PTT device.
+You can set the filter by spedifying the `filter` parameter which is required
+to start the trace. The parameter value is 20 bit. The supported filters and
+related values is outputted through `available_filters` sysfs attribute
+under related PTT PMU directory, classified as Root Ports and Requesters
+respectively.
+::
+    $ cat available_filters
+    #### Root Ports ####
+    0000:00:10.0	0x80001
+    0000:00:11.0	0x80004
+    #### Requesters ####
+    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 available_filters will get the hint.
+::
+    $ cat available_filters
+    #### No available filter ####
+
+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 classifed 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. This is optional and 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 buflet 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 (we call it buflet) of 4MiB.
+The finished buflet 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
+    [...]
