docs: Add comments on mpm.conf

This adds some more details on mpm.conf, what it does, how it works, and
which keys it accepts.

host/docs/configfiles.dox

@@ -44,7 +44,50 @@ ext_adc_self_test=1
 ~~~~
 
 This will act as if this key/value pair had been passed to the device args
-directly.
+directly. This means the valid key/value pairs for the config files are the
+same as those for the device args (see \ref config_devaddr).
+
+\section configfiles_usrps_dpdk Configuring DPDK
+
+DPDK is a special case. Because there are so many things to configure for
+DPDK, it is highly recommended to use config files. The section headers for
+DPDK configurations depend on on the network device used, not the USRP device.
+For more details, see \ref dpdk_nic_config.
+
+\section configfiles_usrps_mpm Configuring MPM
+
+MPM devices (N3xx, E3xx) have their own configuration file (`mpm.conf`)
+which is read once during initialization of MPM. It controls different things
+than the `uhd.conf` file (both can be used for a session, but the UHD
+configuration will be used at every UHD session, whereas the MPM configuration
+file is only read once during MPM initialization). To reload the MPM
+configuration file, a restart of MPM is required.
+
+The `mpm.conf` files are structured a bit differently from `uhd.conf`, so here's
+an example:
+
+~~~~{.ini}
+; This section is for any MPM behaviour
+[mpm]
+; Default log level for journald, can be overwritten by -v when starting manually
+log_level=info
+; Number of log records to buffer for the next get_log_buf() API call
+log_buf_size=100
+
+; Device-specific behaviour is set here. This allows having the same file for
+; different device types, e.g., when a fleet of different devices are
+; managed via Salt or similar tools. The key here is the same as what you would
+; use in device args as a type value.
+[n3xx]
+KEY=VALUE
+
+; Alternatively, specify a product value.
+[n310]
+KEY=VALUE
+~~~~
+
+The key/value pairs for the `type` or `product` sections are documented
+in \ref config_mpm.
 
 \section configfiles_location Location of configuration files
 

host/docs/configuration.dox

@@ -37,7 +37,6 @@ and possible more options.
  recover_mb_eeprom   | Disable version checks. Can damage hardware. Only recommended for recovering devices with corrupted EEPROMs. | X3x0 | recover_mb_eeprom=1
  skip_dram           | Ignore DRAM FIFO block. Connect TX streamers straight into DUC or radio.     | X3x0, N3xx         | skip_dram=1
 
-
 In addition, many of the streaming-related options can be set per-device at configuration time.
 See \ref config_stream_args and \ref page_transport for more details.
 
@@ -261,5 +260,26 @@ simple examples of channel mappings with subdev specs:
   USRP, this is a more versatile solution than setting the subdev globally to
   `B:0`.
 
+\section config_mpm Device Configuration through MPM configuration files
+
+For more detail on how MPM is configured, see also \ref configfiles_usrps_mpm.
+
+The following key/value pairs are understood:
+
+ Key                 | Description                                                                  | Supported Devices | Example Value
+---------------------|------------------------------------------------------------------------------|-------------------|---------------------
+ enable_gps          | Enable/disable power to the GPSDO.                                           | N3xx, E320        | enable_gps=0
+ enable_fp_gpio      | Enable/disable power to the front-panel GPIOs.                               | N3xx, E320        | enable_fp_gpio=0
+ skip_boot_init      | Don't init the device during MPM boot, but on the first UHD run.             | N3xx              | skip_boot_init=1
+ clock_source        | Default clock source for this device (can be overridden by UHD).             | N3xx, E320, E31x  | clock_source=external
+ time_source         | Default time source for this device (can be overridden by UHD).              | N3xx, E320, E31x  | time_source=external
+ rx_lo_source        | Default source for the RX LO (can be overridden by UHD).                     | N310              | rx_lo_source=external
+ tx_lo_source        | Default source for the TX LO (can be overridden by UHD).                     | N310              | tx_lo_source=external
+ pps_export          | Default PPS export value (can be overridden by UHD).                         | N3xx              | pps_export=0
+ forward_eth         | Set forwarding policy for Ethernet packets                                   | N3xx              | forward_eth=1
+ forward_bcast       | Set forwarding policy for broadcast packets                                  | N3xx              | forward_bcast=1
+ no_reload_fpga      | Don't load an idle image after session terminates                            | E31x              | no_reload_fpga=1
+ master_clock_rate   | Default master clock rate (can be overridden by UHD)                         | N3xx, E320, E31x  | master_clock_rate=1e6
+
 */
 // vim:ft=doxygen:

mpm/python/usrp_mpm/prefs.py

@@ -34,6 +34,7 @@ class _MPMPrefs(configparser.ConfigParser):
     Said dictionary needs to be compatible with the ConfigParser.read_dict()
     API call (refer to the Python documentation).
     """
+    # Note: When changing this, you might also want to update host/docs/configfiles.dox
     default_prefs = {
         'mpm': {
             'log_level': MPM_DEFAULT_LOG_LEVEL,