mlxsw: Add support of latency TLV

Amit Cohen writes:

Ethernet Management Datagrams (EMADs) are Ethernet packets sent between the driver and device's firmware. They are used to pass various configurations to the device, but also to get events (e.g., port up) from it. After the Ethernet header, these packets are built in a TLV format.

This is the structure of EMADs:
* Ethernet header
* Operation TLV
* String TLV (optional)
* Latency TLV (optional)
* Reg TLV
* End TLV

The latency of each EMAD is measured by firmware. The driver can get the measurement via latency TLV which can be added to each EMAD. This TLV is optional, when EMAD is sent with this TLV, the EMAD's response will include the TLV and will contain the firmware measurement.

Add support for Latency TLV and use it by default for all EMADs (see more information in commit messages). The latency measurements can be processed using BPF program for example, to create a histogram and average of the latency per register. In addition, it is possible to measure the end-to-end latency, so then the latency of the software overhead can be calculated. This information can be useful to improve the driver performance.

See an example of output of BPF tool which presents these measurements:

$ ./emadlatency -f -a
    Tracing EMADs... Hit Ctrl-C to end.
    Register write = RALUE (0x8013)
    E2E Measurements:
    average = 23 usecs, total = 32052693 usecs, count = 1337061
         usecs               : count    distribution
             0 -> 1          : 0        |                                 |
             2 -> 3          : 0        |                                 |
             4 -> 7          : 0        |                                 |
             8 -> 15         : 0        |                                 |
            16 -> 31         : 1290814  |*********************************|
            32 -> 63         : 45339    |*                                |
            64 -> 127        : 532      |                                 |
           128 -> 255        : 247      |                                 |
           256 -> 511        : 57       |                                 |
           512 -> 1023       : 26       |                                 |
          1024 -> 2047       : 33       |                                 |
          2048 -> 4095       : 0        |                                 |
          4096 -> 8191       : 10       |                                 |
          8192 -> 16383      : 1        |                                 |
         16384 -> 32767      : 1        |                                 |
         32768 -> 65535      : 1        |                                 |

    Firmware Measurements:
    average = 10 usecs, total = 13884128 usecs, count = 1337061
         usecs               : count    distribution
             0 -> 1          : 0        |                                 |
             2 -> 3          : 0        |                                 |
             4 -> 7          : 0        |                                 |
             8 -> 15         : 1337035  |*********************************|
            16 -> 31         : 17       |                                 |
            32 -> 63         : 7        |                                 |
            64 -> 127        : 0        |                                 |
           128 -> 255        : 2        |                                 |

    Diff between measurements: 13 usecs

Patch set overview:
Patches #1-#3 add support for querying MGIR, to know if string TLV and latency TLV are supported
Patches #4-#5 add some relevant fields to support latency TLV Patch #6 adds support of latency TLV

Amit Cohen (6):
mlxsw: reg: Add TLV related fields to MGIR register
mlxsw: Enable string TLV usage according to MGIR output
mlxsw: core: Do not worry about changing 'enable_string_tlv' while
sending EMADs
mlxsw: emad: Add support for latency TLV
mlxsw: core: Define latency TLV fields
mlxsw: Add support of latency TLV

 drivers/net/ethernet/mellanox/mlxsw/core.c    | 108 ++++++++++++++----
 drivers/net/ethernet/mellanox/mlxsw/core.h    |   2 -
 drivers/net/ethernet/mellanox/mlxsw/emad.h    |   4 +
 drivers/net/ethernet/mellanox/mlxsw/reg.h     |  12 ++
 .../net/ethernet/mellanox/mlxsw/spectrum.c    |   1 -
 5 files changed, 103 insertions(+), 24 deletions(-)

2.39.0