3.2. mango_wlan
Kernel Module¶
The mango_wlan
kernel module is a Linux mac80211
driver for the Mango 802.11 MAC/PHY IP. The driver implements all required ops
for the mac80211
API.
Supports:
- Monitor mode (including control and management Rx) with Tx injection
- STA client mode
- AP mode
- RTS/CTS (Request To Send / Clear To Send)
- CTS-to-self
- DSSS, NONHT, HTMF advertised rates in management frames
- Virtual interfaces (up to 4)
- Firmware-supported beaconing for tight TBTT timing
- Custom statistics reporting through
debugfs
MAC Interface¶
The mango_wlan
module communicates with the MAC via a pair of message FIFOs with addresses and interrupt IDs defined in the device tree. These message FIFOs are used primarily for MAC/PHY configuration. Tx/Rx packets are handled by dedicated DMAs and do not transit the message FIFOs.
The mango_wlan
exchanges atomic messages with the MAC for all interface configuration commands. The list of message IDs and formats varies between releases. It is essential the mango_wlan
driver and MAC software versions match.
debugfs
Statistics¶
Once the module is loaded, /sys/kernel/debug/ieee80211/mango-wlan-phy
will be populated with a number of debugfs
files. Most are automatically populated with data from the mac80211
framework. mango_wlan
provides the following additional files:
netdev:<virtual interface>/stations/<station MAC address>/mango_wlan
¶
This file provides Tx and Rx statistics about the link to a particular station. These statistics are organized according to PHY rate and MCS. It also provides statistics on the number of retries needed in each rate stage of the Minstel rate control algorithm.
netdev:<virtual interface>/mango_latency_debugfs
¶
This file provides information about important system latencies measured by the driver and MAC at runtime.
Beacon Latency¶
When operating in AP mode, the MAC firmware handles periodic beaconing in order to abide by the strict Target Beacon Transmission Time (TBTT) requirements from the 802.11 specification, but mac80211
provides the payload of each beacon. To do this, the MAC firmware requests a new beacon payload from the driver. There is a tradeoff in the timing of this request. Too early and the beacon contents may represent stale state by the time it is actually transmitted. Too late and a beacon payload update may not occur in time for the TBTT. The default heuristic used by the mango_wlan
driver is that beacon update triggers are sent \(2/3\) of the way through a beacon interval. A typical beacon interval is 100 TU, or 102,400 microseconds. In this case, we anticipate that a new beacon payload will be requested approximately 34,133 microseconds before the beacon must be sent.
An example output from mango_latency_debugfs
confirms that beacon payload updates are occuring in a timely manner:
mango_wlan Latency Statistics
----------------------------------------------------
Beacon Update Slack:
Min: 23907 usec
Avg: 34733 usec
Max: 34754 usec
----------------------------------------------------