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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 core is composed of several root ports, RCiEPs, and one
+PTT device, 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 cannot be loaded if debugfs is not mounted.
+Each PTT device will be presented under /sys/kernel/debugfs/hisi_ptt
+as its root directory, with name of its BDF number.
+::
+
+    /sys/kernel/debug/hisi_ptt/<domain>:<bus>:<device>.<function>
+
+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 with which the PTT device belongs to.
+
+Each event is presented as a file under $(PTT root dir)/$(BDF)/tune, and
+mostly a simple open/read/write/close cycle will be used to tune
+the event.
+::
+    $ cd /sys/kernel/debug/hisi_ptt/$(BDF)/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.
+Tuning multiple events at the same time is not permitted, which means
+you cannot read or write more than one tune file at one time.
+
+1. Tx path QoS control
+------------------------
+
+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. For a busy
+direction, you should increase the related buffer watermark to enhance
+the performance.
+
+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 chose 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 support to trace the headers of certain type and of
+certain direction.
+
+In order to start trace, you need to configure the parameters first.
+The parameters files is provided under $(PTT root dir)/$(BDF)/trace.
+::
+    $ cd /sys/kernel/debug/hisi_ptt/$(BDF)/trace
+    $ ls
+    free_buffer     filter      buflet_nums     buflet_size
+    direction       type        data            trace_on
+    data_format
+
+1. filter
+---------
+
+You can configure the filter of TLP headers through the file. The filter
+is provided as BDF numbers of either root port or subordinates, which
+belong to the same PCIe core. You can get the filters available and
+currently configured by read the file, and write the desired BDF to the
+file to set the filters. There is no default filter, which means you
+must specifiy at least one filter before start tracing.
+Write invalid BDF(not in the available list) will return
+a failure.
+::
+    $ echo 0000:80:04.0 > filter
+    $ cat filter
+    #### Root Ports ####
+    0000:80:00.0
+    [0000:80:04.0]
+    #### Functions ####
+    0000:81:00.0
+    0000:81:00.1
+    0000:82:00.0
+
+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, read the filter will get the hint.
+::
+    $ cat filter
+    #### No available filter ####
+
+The filter can be dynamically updated, which means you can always
+get correct filter information when hotplug events happens, or
+manually remove/rescan the devices.
+
+2. type
+-------
+
+You can trace the TLP headers of certain types by configure the file.
+Read the file will get available types and current setting, and write
+the desired type to the file to configure. The default type is
+`posted_request` and write types not in the available list will return
+a failure.
+::
+    $ echo completion > type
+    $ cat type
+    all  posted_request  non-posted_request  [completion]
+
+3. direction
+------------
+
+You can trace the TLP headers from certain direction, which is relative
+to the root port or the PCIe core. Read the file to get available
+directions and current configurition, and write the desired direction
+to configure. The default value is `rx` and any invalid direction will
+return a failure. Note `rxtx_no_dma_p2p` means the headers of both
+directions, but not include P2P DMA access.
+::
+    $ echo rxtx > direction
+    $ cat direction
+    rx  tx  [rxtx]  rxtx_no_dma_p2p
+
+4. buflet_size
+--------------
+
+The traced TLP headers will be written to the memory allocated
+by the driver. The hardware accept 4 DMA address with same size,
+and write the buflet sequentially like below. If DMA addr 3 is
+finished and the trace is still on, it will return to addr 0.
+Driver will allocated each DMA buffer (we call it buflet).
+The finished buflet will be replaced with a new one, so
+a long time trace can be achieved.
+::
+    +->[DMA addr 0]->[DMA addr 1]->[DMA addr 2]->[DMA addr 3]-+
+    +---------------------------------------------------------+
+
+You should both configure the buflet_size and buflet_nums to
+configure the `trace buffer` to receive the TLP headers. The
+total trace buffer size is buflet_size * buflet_nums. Note
+that the trace buffer will not be allocated immediately after you
+configure the parameters, but will be allocated right before
+the trace starts.
+
+This file configures the buflet size. Read the file will get
+available buflet size and size set currently, write the desired
+size to the file to configure. The default size is 2 MiB and any
+invalid size written will return a failure.
+::
+    $ cat buflet_size
+    [2 MiB]     4 MiB
+    $ echo 4 > buflet_size
+    $ cat buflet_size
+    2 MiB     [4 MiB]
+
+5. buflet_nums
+--------------
+
+You can write the desired buflet count to the file to configure,
+and read the file to get current buflet count. The default
+value is 64. And any positive value is valid. Note that big value
+may lead to DMA memory allocation failure, and you will not be
+able to start tracing. If it happens, you should consider adjusting
+buflet_nums or buflet_size.
+::
+    $ cat buflet_nums
+    64
+    $ echo 128 > buflet_nums
+    $ cat buflet_nums
+    128
+
+6. data
+-------
+
+The file to access the traced data. You can read the file to get the
+binary blob of traced TLP headers. The format of the headers is
+4 Dword length and is just as defined by the PCIe Spec r4.0,
+Sec 2.2.4.1, or 8 Dword length with additional 4 Dword extra
+information.
+
+echo "" > data will free all the trace buffers allocated as well as
+the traced datas.
+
+7. trace_on
+-----------
+
+Start or end the trace by simple writing to the file, and monitor the
+trace status by reading the file.
+::
+    $ echo 1 > trace_on     # start trace
+    $ cat trace_on          # get the trace status
+    1
+    $ echo 0 > trace_on     # manually end trace
+
+The read value of the trace_on will be auto cleared if the buffer
+allocated is full. 1 indicates the trace is running and 0 for
+stopped. Write any non-zero value to the file can start trace.
+
+8. free_buffer
+--------------
+
+File to indicate the trace buffer status and to manually free the
+trace buffer. The read value of 1 indicates the trace buffer has
+been allocated and exists in the memory, while 0 indicates there
+is no buffer allocated. Write 1 to the file to free the trace
+buffer as well as the traced datas.
+::
+    $ cat free_buffer
+    1                       # indicate the buffer exists
+    $ echo 1 > free_buffer  # free the trace buffer
+    $ cat free_buffer
+    0
+
+9. data_format
+--------------
+
+File to indicate the format of the traced TLP headers. User can also
+specify the desired format of traced TLP headers. Available formats
+are 4DW, 8DW which indicates the length of each TLP headers traced.
+::
+    $ cat data_format
+    [4DW]    8DW
+    $ echo 8 > data_format
+    $ cat data_format
+    4DW     [8DW]
+
+The traced TLP header format is different from the PCIe standard.
+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                            ]
+
+For 8DW format, 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                               ]
+
+All the fields of the traced TLP header is defined by the PCIe Specification.
+While 'Header DWx' means standard TLP header DWord x, and 'Time' is the
+timestamp of the traced header.
