Introduction
============
This driver implements Session Request Protocol (SRP) and Host negotiation
Protocol (HNP) described in On-The-Go (OTG) Supplement to the USB 2.0
Specification. It also provides support for Accessory Charger Adapter (ACA)
defined in the Battery Charging Specification 1.1.

These protocols provide a means for USB host devices to intelligently manage
power on VBUS and USB peripheral devices to become the host when paired with
another OTG device.

Hardware description
====================
USB hardware found in Qualcomm chipsets like MSM7x27, MSM7x30, QSD8x50 and
MSM8660 is compliant to USB 2.0 high speed On-The-Go protocol.
The transceiver, aka PHY is integrated on the chip and ULPI interface is used for
communication.

USB hardware interfaces to the system memory via AHB BUS. DMA engine is included
to move all of the data to be transferred over the USB between USB core and
system memory. Device controller can support 16 endpoints of all types
(control/bulk/interrupt /isochronous) defined in USB 2.0 specification. The
host controller is compliant to EHCI specification. Directly connected USB 1.1
Full/Low speed devices are supported without a companion controller by having
inbuilt Transaction Translator (TT).

USB_HS_CLK, USB_HS_PCLK and USB_HS_CCLK are required for USB operation.
Phy feeds 60MHZ HS_CLK to link when ULPI interface is used. This clock needs to
be turned on only while resetting the link. HS_PCLK (Pbus clock) is required to
move data to/from hardware FIFO. This clock may not be required on targets like
MSM8660 where USB is part of smart peripheral subsystem. AXI bus frequency needs
to be kept at maximum value while USB data transfers are happening. HS_CCLK
(core clock) is introduced in MSM7x30 to get rid of dependency on AXI bus
frequency.

The same irq line is  shared across OTG, Device controller and Host controller
drivers. Phy is integrated on the chip and no gpios are required to connect link
and PHY.

Phy can monitor VBUS and ID lines while operating in low power mode (LPM). But
leaving comparators ON in LPM increases power consumption. Hence VBUS line is
routed to PMIC hardware which can generate interrupt (when accessed by Apps
processor) or send RPC callback. This is also useful when an External LDO to
power up 3.3V of PHY is not installed. An internal LDO is turned upon
receiving notification from PMIC. Id line is not routed to PMIC. Hence OTG mode
can not be supported with this configuration and External LDO must be present.

Hardware can generate interrupt when voltage on VBUS line is reached
above/below A-VBUS Valid and B-Session Valid threshold values defined in OTG
specification. Interrupt is generated when Id line is grounded i.e Micro-A
cable is connected.

The following hardware features help in meeting the SRP and HNP protocol
timings.

Hardware Assist Data-pulse (HADP):
---------------------------------
When software programs HADP, Hardware start a data pulse of approximately 7ms
in duration and then automatically ceases the data pulsing. This automation
relieves software from controlling the data-pulse duration. This assist will
ensure data pulsing meets the OTG requirement of > 5ms and < 10ms.

Hardware Assist Auto-Reset (HAAR):
---------------------------------
When software programs HAAR, Hardware will automatically start a reset after
a connect event. This shortcuts the normal process where software is notified
of the connect event and starts the reset. Software will still receive
notification of the connect event but should not write the reset bit when the
HAAR is set. Software will be notified again after the reset is complete via
the enable change bit in the PORTSC register which cause a port change
interrupt.

Hardware Assist B-Disconnect to A-Connect (HABA):
------------------------------------------------
During Host negotiation Protocol(HNP), A-Device must enable pull-up on D+ as
soon as possible after detecting disconnect from B-device.

When Software programs HABA, the Host Controller port is in suspend mode, and
the B-device disconnects, then this hardware assist begins.
1. Reset the OTG core
2. Write the OTG core into device mode.
3. Write the device run bit to a '1' and enable necessary interrupts including:
	* USB Reset Enable (URE) : enables interrupt on usb bus reset to device
        * Sleep Enable (SLE) : enables interrupt on device suspend
	* Port Change Detect Enable (PCE) : enables interrupt on device connect

When software has enabled this hardware assist, it must not interfere during the
transition and should not write any register in the core until it gets an
interrupt from the device controller signifying that a reset interrupt has
occurred or at least first verify that the core has entered device mode.

The following hardware feature helps in supporting Accessory Charger Adapter:

PHY Support for ID_A/B/C:
------------------------
Accessory Charger Adapter has three ports to attach an OTG, charger and A or
B-device. So, based on what all device are attached to the ACA, it outputs a
state on the ID pin (i.e GROUND, ID_A, ID_B, ID_C, FLOAT).
USB PHY has support for these ID states. Once software enables this support,
PHY sets corresponding bit in its INTS register based on any changes in the
ID state.

Software description
====================

This driver provides OTG functionality when Device controller driver (DCD) and
Host controller driver (HCD) are enabled. It is enabled even when one of the DCD
or HCD is enabled to use PHY initialization, clock management, register memory
mapping, low power mode (LPM) functionalities.

Session Request Protocol (SRP): A-device may turn off power on VBUS upon user
request or A_WAIT_BCON timeout. SRP detection interrupt is enabled and
hardware is put into LPM. If Data pulse is detected, A-device starts a new
session by applying power on VBUS. Hardware Auto Assist Data pulse feature is
used to program Data pulse
When acting as a B-device, if SRP initial conditions (SE0 condition for
TB_SE0_SRP min and previous session was ended before TB_SSEND_SRP) are met, SRP
is initiated upon user request. Hardware Auto Assist Data pulse feature is
used to program Data pulse. If remote device does not turn on VBUS before
TB_SRP_FAIL, an error is reported to user space.

Host Negotiation Protocol (HNP): A-device periodically polls B-device to check
host request status. When B-device returns true, A-device shall enable HNP and
suspend the bus with in THOST_REQ_SUSP. HNP polling is implemented in USB core
software. HCD registers a start_hnp callback method with EHCI framework. This
method is called after suspending the port if HNP is enabled. HCD notifies OTG
that B-device is suspended. A_AIDL_BDIS timer is kicked and waits for B-device
disconnection. If B-device does not signal disconnect within TA_AIDL_BDIS
timeout, session is closed by powering down VBUS. Otherwise A-device stops HCD
and starts DCD to enable pull-up. A-device again resumes host role if it had
observed bus idle for TA_BIDL_ADIS time.
B-device signals host_request true upon user request. DCD notifies OTG that
HNP is enabled and bus is idle. OTG driver disable pull-up by stopping DCD and
kick B_ASE0_BRST timer. If A-device does not signal connect with in
TB_ASE0_BRST, B-device resumes in peripheral role. Otherwise B-device assert
the bus reset and enumerate the A-device.

MSM chipsets which have 45nm integrated PHY supports Attach Detection Protocol.
(A protocol which enables an OTG device to detect when a remote device has been
attached or detached without supplying VBUS). ADP support needs to be
implemented to efficiently supply/request power on VBUS. Leakage currents (i.e
VBUS applied but no peripheral is connected) are very less on MSM hardware. So
VBUS can be applied when Id becomes false. ADP may be never implemented in
this driver due to this reason.

The state machine is implemented as described in the OTG specification.
A few exceptions are mentioned below:

1. Host session request i.e a_bus_request input variable is automatically
asserted when Id becomes false and SRP is detected.
It is de-asserted When HCD suspends the bus and asserted again in case of
remote device remote wakeup.
2. Output variables like drv_vbus, loc_conn, loc_sof, adp_prb are not
maintained in the state machine as they serve no purpose.
3. Bus power down request i.e a_bus_drop is cleared when Micro-A cable is
connected so that non OTG device can be detected when Micro-A cable is
connected next time.
4. Input variables that determine SRP initial condition status b_se0_srp and
b_ssend_srp are not maintained in state machine processing. When a session is
ended i.e VBUS falls below B-Session Valid threshold, time stamp is taken and
is compared against the current time at the time of SRP initiation.


Controller gives interrupt for every 1 msec if 1MSIE (1 Msec interrupt enable)
is set. Timers used in OTG state machine can be implementing using 1MSEC
timer as a source.  But hrtimer on MSM hardware can give at least 1/32KHZ
precision. So hrtimer framework is used to implement OTG timers.  No two OTG
timers run in parallel. Hence one hrtimer is used for all OTG timers.

OTG state machine processing can not be done in atomic context. Hence a worker
thread is created for processing the state machine events. A separate worker
thread is created instead of using default worker thread to meet OTG
specification timings.

OTG supplement Revision 2.0 has made HNP timings less stringent compared to
Revision 1.3. TA_BDIS_ACON (The time with in A-Device should enable pull-up
upon B-device signals disconnect) has been changed to 150 msec from 3 msec.
DCD can be easily activated within 150 msec. Hence HABA is not used.
TB_ACON_BSE0 (The time with in B-device should reset the A-device) has been
changed to 150 msec from 1 sec. Host software would easily meet this timing
given that de-bounce delays and root hub port power stabilization delays are
not required during HNP.

Accessory Charger Adapter (ACA): To support ACA there must be support in the
USB hardware (Controller and PHY) for the ID_A/B/C states. It should be able
to interrupt software for any ID state changes. On receiving this interrupt,
interrupt handler checks the current ID state and invokes OTG state machine
for further handling. Even if the USB Controller doesn't support these ID_A/B/C
states, driver can still detect the ID state transitions by depending on USB
PHY if the PHY supports these ID states. For this scenario, driver relies
on polling of PHY register to determine the ID state changes as long as an
ACA is attached to the system. This polling is implemented by using a timer
running at a frequency of 1 sec. This timer checks for the current ID state
and on detecting any change it invokes OTG state machine for further handling.

Following are the actions performed by the driver as per the ID state:
* ID_FLOAT: Configure device to act as peripheral and allow charging if VBUS
  is present, else move it to LPM (low power mode).
* ID_GROUND: Configure device to act as host and supply VBUS.
* ID_A: Configure device to act as host and don't supply VBUS. In this state
  the device can charge as well.
* ID_B: Keep the device in IDLE state and allow charging.
* ID_C: Configure device to act as peripheral and allow charging.

Design
======

The following goals are kept in mind while designing OTG state machine.

1. Avoid User intervention when operating as a standard Host or standard
peripheral
2. Support host only and peripheral only modes
3. Put Hardware in LPM when ever possible
4. Pass OTG compliance tests
5. Report notification/error messages to user space
6. With ACA, allow charging in Host mode as well
7. Disable LPM as long as ID state polling is happening

Power Management
================

System suspend is negated by acquiring wakelock while processing OTG state
machine, or while polling for the PHY ID state in case of ACA.
Wakelock is released:
1. After activating the DCD/HCD. It is the responsibility of DCD/HCD to
acquire wakelock if required.
2. After putting hardware in LPM.
3. No state machine events and timers are pending. This would cover scenarios
mentioned in (1) and (2).
4. After driver stops polling for ID state in case of ACA.

Wake lock is re-acquired when state machine work is scheduled, which can
happen from interrupt (exiting LPM), sysfs entries (initiate SRP, clear
error, bus drop, etc), or from ID state polling routine.

OTG driver provides set_suspend method for DCD/HCD to put hardware in LPM. DCD
can use this method upon bus suspend. HCD can use this method upon suspending
the root hub.

LPM entering procedure:
1. Clear PHY interrupt latch registers.
2. Enable PHY comparators to detect Id, B-Session Valid interrupts while hardware
is in LPM.
3. Turn off PLL block on the PHY to achieve maximum power savings.
4. Put PHY in suspend mode by setting PHCD bit in PORTSC register.
5. Enable asynchronous interrupt so that PHY can generate interrupt when
clocks are disabled.
6. Disable all USB clocks.

LPM exit procedure:
1. Enable USB clocks.
2. Disable asynchronous interrupt.
3. Put PHY out of suspend mode. This is not required when LPM is exited due to
hardware activity i.e asynchronous interrupt.

SMP/multi-core
==============

OTG state machine inputs like bus request, bus drop, srp_detect etc accessed
from interrupt context, and multiple process contexts. Hence atomic bit ops are
used. ulpi_read and ulpi_write functions can now be accessed from multiple
context, hence, these are protected using a spin_lock.

Interface
=========
This driver provides the following methods to DCD and HCD.

set_peripheral: DCD use this methods to register/unregister USB gadget.

set_host: HCD use this method to register/unregister USB bus. Unlike gadget
framework, there are no standard methods to start/stop HCD. Hence start_host
method is introduced and must be initialized by HCD prior to registration.

set_clk: HCD and DCD use this method to turn on/off USB_HS_CLK clk which is
required only while resetting the controller.

start_srp: DCD use this method to initiate Session Request Protocol (SRP).
SRP may be initiated when function drivers use remote wakeup facility, when
B-Device wishes to become host. OTG driver programs Data-Pulsing if initial
condition of SRP are met. Otherwise proper error code is returned.

set_suspend: DCD call this method when controller generates suspend
interrupt or generates reset/port change interrupt before HNP and during HNP.
If device is in B_PERIPHERAL state, HNP is initiated if host had enabled it.
If device is in A_PERIPHERAL state, A_BIDL_ADIS timer is kicked in case of
suspend interrupt. If this timer expires, A-device take back it's host role
and continue previous session. This timer is deleted in case of
reset/port change interrupts.
HCD call this method after suspending the root hub. Hardware is put into LPM.

start_hnp: A-device needs to enable pull-up on D+ within TA_BIDL_ADIS after
suspending the bus i.e putting port in suspend state. EHCI stack can use this
method to notify OTG right after suspending the OTG port. OTG driver schedule
a work to stop host and enable pull-up on D+.

send_event: USB core, DCD and HCD can use otg_send_event() API to send OTG
notification/error messages to user space. send_event method defined in
otg_transceiver is invoked by otg_send_event() API. An uevent is sent
with SUBSYSTEM=platform, MODULE=msm_otg and EVENT=<event>, where event could
be one of the following events.

OTG_EVENT_DEV_CONN_TMOUT: Device connection timeout or device not responding.
OTG_EVENT_NO_RESP_FOR_HNP_ENABLE: Device is not responding to B_HNP_ENABLE
	feature request.
OTG_EVENT_HUB_NOT_SUPPORTED: Host does not support HUB class peripherals.
OTG_EVENT_DEV_NOT_SUPPORTED: Host does not support the attached peripheral.
OTG_EVENT_HNP_FAILED: HNP failed due to not meeting protocol timings.
OTG_EVENT_NO_RESP_FOR_SRP: No Response for B-device SRP request.

set_power: DCD can use otg_set_power() API to specify about the current that
can be withdrawn from the VBUS for charging. Based on the current OTG state
and whether ACA is attached or not, OTG driver makes a decision about the
charging current and calls the charging APIs.

The following sysfs nodes are provided at /sys/devices/platform/msm_otg

pwr_down: This node can be used to control A-device session. a_bus_drop and
a_bus_req state machine input variables are altered to start/stop session.
Write to this node is invalid when device operating as a B-device.

start_srp: This node can be used for requesting session. If all initial
conditions of SRP are met, SRP is initiated. Write to this node is invalid
when device operating as an A-device.

clr_err: This node can be used to clear over-current condition. Write to this
node is invalid when operating as an B-device. Error condition is
automatically cleared when Id becomes false.

The following sysfs nodes are provided at /sys/devices/platform/msm_hsusb/otg

host_request: This node can be used for requesting host role. A-device shall
select a_hnp_support feature prior to configuration and poll B-device for host
request. When '1' is written to this node, host request is asserted. This node
can also be used for taking host role when A-device operating as a peripheral.

hnp_avail: User space can check this node before requesting the host role.
Gadget controller driver asserts its internal variable hnp_avail when HNP
polling request is send by the Host.

Dependencies
============

If USB clocks are controlled by modem processor, proc_comm interface is used
to turn on/off clocks.

If VBUS power is controlled by modem processor, RPC interface is used to turn
on/off VBUS power.

Config options
==============

CONFIG_USB_MSM_ACA: Enable support for Accessory Charger Adapter (ACA)
CONFIG_ENABLE_MSM_OTG_A_WAIT_BCON_TIMEOUT: Enable A_WAIT_BCON timeout. VBUS
will be turned off and SRP detection is enabled upon this timeout. If this
config is not selected, VBUS will not be turned off when Mini/Micro-A cable
is connected. But hardware is put into LPM.

Other
=====
On-The-Go and Embedded Host Supplement to the USB Revision 2.0 Specification
(Revision 2.0) found at http://www.usb.org/developers/onthego

Known issues
============
Phy instability issues are observed when vbus_valid interrupt is enabled.
Hence a_vbus_vld state machine variable is explicitly asserted after
a_wait_vrise timer expiration.

Spurious interrupt is seen when trying to put PHY in Low Power Mode with
ID_A/B/C interrupts enabled in the PHY. As a result of which PHY doesn't stay
in LPM. Hence, ID_A/B/C interrupts are disabled before entering LPM, and
enabled while returning.

To do
=====

Verify SRP detection on all targets.

Phy instability issues are observed when A-Vbus Valid interrupt is enabled.
But without this interrupt over current condition can not be determined. Root
cause analysis for PHY instability issue and alternative methods like PMIC
interrupt are being pursued.