OEM IPMI EthStats

[Patrick Venture]

Out of a desire to export link health information, we implemented an
OEM IPMI command that lets one grab ethernet statistics from a named
ethernet device on the BMC.  (The NCSI statistics IPMI commands were a
bit clunky and didn't do what we needed).

The OEM IPMI commands spec is pretty short on this one:

### Ethernet Statistics Command (0x30) {#ethernet-statistics-command-0x30}

There is the need to read a specific ethernet-level statistic value
from the BMC.  This is driven primarily to detect link errors that
require hardware swaps during manufacturing.

This command will be well structured such that there is a request and
response which mirror to some extent.

The request will specify the ethernet interface by name, as a
length-prepended string, and the field they're requesting by
identifier (an unsigned byte).

If the command is not supported, the IPMI OEM handler will respond
accordingly, however, if the field is not supported or not recognized,
the command will return 0xcc (invalid field).

The current ethernet statistics available (all future additions must append):

|Identifier |Human Readable Name
|-----------|--------------------
|0          |rx_bytes
|1          |rx_compressed
|2          |rx_crc_errors
|3          |rx_dropped
|4          |rx_errors
|5          |rx_fifo_errors
|6          |rx_frame_errors
|7          |rx_length_errors
|8          |rx_missed_errors
|9          |rx_nohandler
|10         |rx_over_errors
|11         |rx_packets
|12         |tx_aborted_errors
|13         |tx_bytes
|14         |tx_carrier_errors
|15         |tx_compressed
|16         |tx_dropped
|17         |tx_errors
|18         |tx_fifo_errors
|19         |tx_heartbeat_errors
|20         |tx_packets
|21         |tx_window_errors

Request

|Byte(s) |Value |Data
|--------|------|----
|0x00    |Statistic ID |The identifier of the desired statistic
|0x01    |Length       |Where you expect the cable, eth0 or eth1, etc
|0x02..  |The name     |The string, not null-terminated

Response

|Byte(s) |Value  |Data
|--------|-------|----
|0x00    |Stat ID|The identifier of the desired statistic
|0x01....|Uint64 |The value.  Because these are counters we don't
anticipate negative values, and we don't expect overflow.