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+This page describes how to make calls to the firmware api.
+
+How to call
+===========
+
+The preferred calling convention is known as the firmware mailbox. The
+mailboxes are basically a fixed length array that serves as the call-stack.
+
+Firmware mailboxes can be located by searching the encoder and decoder memory
+for a 16 byte signature. That signature will be located on a 256-byte boundary.
+
+Signature:
+0x78, 0x56, 0x34, 0x12, 0x12, 0x78, 0x56, 0x34,
+0x34, 0x12, 0x78, 0x56, 0x56, 0x34, 0x12, 0x78
+
+The firmware implements 20 mailboxes of 20 32-bit words. The first 10 are
+reserved for API calls. The second 10 are used by the firmware for event
+notification.
+
+  Index  Name
+  -----  ----
+  0      Flags
+  1      Command
+  2      Return value
+  3      Timeout
+  4-19   Parameter/Result
+
+
+The flags are defined in the following table. The direction is from the
+perspective of the firmware.
+
+  Bit  Direction  Purpose
+  ---  ---------  -------
+  2    O          Firmware has processed the command.
+  1    I          Driver has finished setting the parameters.
+  0    I          Driver is using this mailbox.
+
+
+The command is a 32-bit enumerator. The API specifics may be found in the
+fw-*-api.txt documents.
+
+The return value is a 32-bit enumerator. Only two values are currently defined:
+0=success and -1=command undefined.
+
+There are 16 parameters/results 32-bit fields. The driver populates these fields
+with values for all the parameters required by the call. The driver overwrites
+these fields with result values returned by the call. The API specifics may be
+found in the fw-*-api.txt documents.
+
+The timeout value protects the card from a hung driver thread. If the driver
+doesn't handle the completed call within the timeout specified, the firmware
+will reset that mailbox.
+
+To make an API call, the driver iterates over each mailbox looking for the
+first one available (bit 0 has been cleared). The driver sets that bit, fills
+in the command enumerator, the timeout value and any required parameters. The
+driver then sets the parameter ready bit (bit 1). The firmware scans the
+mailboxes for pending commands, processes them, sets the result code, populates
+the result value array with that call's return values and sets the call
+complete bit (bit 2). Once bit 2 is set, the driver should retrieve the results
+and clear all the flags. If the driver does not perform this task within the
+time set in the timeout register, the firmware will reset that mailbox.
+
+Event notifications are sent from the firmware to the host. The host tells the
+firmware which events it is interested in via an API call. That call tells the
+firmware which notification mailbox to use. The firmware signals the host via
+an interrupt. Only the 16 Results fields are used, the Flags, Command, Return
+value and Timeout words are not used.
+