====== Transmitting frames with iwn(4) ======

NOTE: this is just a series of notes as iwn(4) hacking is done. It doesn't at all qualify as complete documentation.

===== Overview =====

The iwn(4) hardware has a series of transmit rings which map to access categories. (TODO: there are other rings too - beacons? management? Do some digging!)

Each transmit ring entry contains a transmit descriptor and then multiple segments containing the data payload.

In the driver, iwn_tx_data() is the main transmit path and iwn_tx_data_raw() is for the raw data (management, etc) transmit path.

===== Rate selection =====

(TODO: talk about how this is done with PLCP and how antenna selection also goes into this.)

(.. and CCK/HT flags.)

===== Link Quality / Multi-rate retry =====

The firmware supports trying multiple transmit attempts. iwn_set_link_quality() configures the rates in the transmit table. There are 16 slots to fill. There's one table per associated station. Upon association and rate control negotiation, iwn_set_link_quality() populate the transmit table from highest available transmit rate to lowest available rate - if there's less than 16 rates available, the final slots just contain the lowest transmit rate.

Then during iwn_tx_data() and iwn_tx_data_raw(), the TX descriptor can have a linkq value that points to a particular slot in the table to start transmitting from. Each attempt will occur from the linkq value all the way to 0 (ie, counting backwards.) That way it tries successive values until it hits the lowest rate. It tells the firmware to do this by setting the IWN_TX_LINKQ flag in the TX descriptor flag value.

Now, this means that the TX descriptor rate field needs to exist in the link quality rate table **and** it likely should match the linkq value given.

So for now this is disabled in iwn(4), at least until the link quality table use can be refactored out and better planned.