Fixed MTP to work with TWRP

Documentation/networking/caif/README

@@ -0,0 +1,109 @@
+Copyright (C) ST-Ericsson AB 2010
+Author: Sjur Brendeland/ sjur.brandeland@stericsson.com
+License terms: GNU General Public License (GPL) version 2
+---------------------------------------------------------
+
+=== Start ===
+If you have compiled CAIF for modules do:
+
+$modprobe crc_ccitt
+$modprobe caif
+$modprobe caif_socket
+$modprobe chnl_net
+
+
+=== Preparing the setup with a STE modem ===
+
+If you are working on integration of CAIF you should make sure
+that the kernel is built with module support.
+
+There are some things that need to be tweaked to get the host TTY correctly
+set up to talk to the modem.
+Since the CAIF stack is running in the kernel and we want to use the existing
+TTY, we are installing our physical serial driver as a line discipline above
+the TTY device.
+
+To achieve this we need to install the N_CAIF ldisc from user space.
+The benefit is that we can hook up to any TTY.
+
+The use of Start-of-frame-extension (STX) must also be set as
+module parameter "ser_use_stx".
+
+Normally Frame Checksum is always used on UART, but this is also provided as a
+module parameter "ser_use_fcs".
+
+$ modprobe caif_serial ser_ttyname=/dev/ttyS0 ser_use_stx=yes
+$ ifconfig caif_ttyS0 up
+
+PLEASE NOTE: 	There is a limitation in Android shell.
+		It only accepts one argument to insmod/modprobe!
+
+=== Trouble shooting ===
+
+There are debugfs parameters provided for serial communication.
+/sys/kernel/debug/caif_serial/<tty-name>/
+
+* ser_state:   Prints the bit-mask status where
+  - 0x02 means SENDING, this is a transient state.
+  - 0x10 means FLOW_OFF_SENT, i.e. the previous frame has not been sent
+	and is blocking further send operation. Flow OFF has been propagated
+	to all CAIF Channels using this TTY.
+
+* tty_status: Prints the bit-mask tty status information
+  - 0x01 - tty->warned is on.
+  - 0x02 - tty->low_latency is on.
+  - 0x04 - tty->packed is on.
+  - 0x08 - tty->flow_stopped is on.
+  - 0x10 - tty->hw_stopped is on.
+  - 0x20 - tty->stopped is on.
+
+* last_tx_msg: Binary blob Prints the last transmitted frame.
+	This can be printed with
+	$od --format=x1 /sys/kernel/debug/caif_serial/<tty>/last_rx_msg.
+	The first two tx messages sent look like this. Note: The initial
+	byte 02 is start of frame extension (STX) used for re-syncing
+	upon errors.
+
+  - Enumeration:
+        0000000  02 05 00 00 03 01 d2 02
+                 |  |     |  |  |  |
+                 STX(1)   |  |  |  |
+                    Length(2)|  |  |
+                          Control Channel(1)
+                             Command:Enumeration(1)
+                                Link-ID(1)
+                                    Checksum(2)
+  - Channel Setup:
+        0000000  02 07 00 00 00 21 a1 00 48 df
+                 |  |     |  |  |  |  |  |
+                 STX(1)   |  |  |  |  |  |
+                    Length(2)|  |  |  |  |
+                          Control Channel(1)
+                             Command:Channel Setup(1)
+                                Channel Type(1)
+                                    Priority and Link-ID(1)
+				      Endpoint(1)
+					  Checksum(2)
+
+* last_rx_msg: Prints the last transmitted frame.
+	The RX messages for LinkSetup look almost identical but they have the
+	bit 0x20 set in the command bit, and Channel Setup has added one byte
+	before Checksum containing Channel ID.
+	NOTE: Several CAIF Messages might be concatenated. The maximum debug
+	buffer size is 128 bytes.
+
+== Error Scenarios:
+- last_tx_msg contains channel setup message and last_rx_msg is empty ->
+  The host seems to be able to send over the UART, at least the CAIF ldisc get
+  notified that sending is completed.
+
+- last_tx_msg contains enumeration message and last_rx_msg is empty ->
+  The host is not able to send the message from UART, the tty has not been
+  able to complete the transmit operation.
+
+- if /sys/kernel/debug/caif_serial/<tty>/tty_status is non-zero there
+  might be problems transmitting over UART.
+  E.g. host and modem wiring is not correct you will typically see
+  tty_status = 0x10 (hw_stopped) and ser_state = 0x10 (FLOW_OFF_SENT).
+  You will probably see the enumeration message in last_tx_message
+  and empty last_rx_message.