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1	PHY SUBSYSTEM
2	Kishon Vijay Abraham I <kishon@ti.com>
3
4	This document explains the Generic PHY Framework along with the APIs provided,
5	and how-to-use.
6
7	1. Introduction
8
9	*PHY* is the abbreviation for physical layer. It is used to connect a device
10	to the physical medium e.g., the USB controller has a PHY to provide functions
11	such as serialization, de-serialization, encoding, decoding and is responsible
12	for obtaining the required data transmission rate. Note that some USB
13	controllers have PHY functionality embedded into it and others use an external
14	PHY. Other peripherals that use PHY include Wireless LAN, Ethernet,
15	SATA etc.
16
17	The intention of creating this framework is to bring the PHY drivers spread
18	all over the Linux kernel to drivers/phy to increase code re-use and for
19	better code maintainability.
20
21	This framework will be of use only to devices that use external PHY (PHY
22	functionality is not embedded within the controller).
23
24	2. Registering/Unregistering the PHY provider
25
26	PHY provider refers to an entity that implements one or more PHY instances.
27	For the simple case where the PHY provider implements only a single instance of
28	the PHY, the framework provides its own implementation of of_xlate in
29	of_phy_simple_xlate. If the PHY provider implements multiple instances, it
30	should provide its own implementation of of_xlate. of_xlate is used only for
31	dt boot case.
32
33	#define of_phy_provider_register(dev, xlate) \
34	__of_phy_provider_register((dev), NULL, THIS_MODULE, (xlate))
35
36	#define devm_of_phy_provider_register(dev, xlate) \
37	__devm_of_phy_provider_register((dev), NULL, THIS_MODULE, (xlate))
38
39	of_phy_provider_register and devm_of_phy_provider_register macros can be used to
40	register the phy_provider and it takes device and of_xlate as
41	arguments. For the dt boot case, all PHY providers should use one of the above
42	2 macros to register the PHY provider.
43
44	Often the device tree nodes associated with a PHY provider will contain a set
45	of children that each represent a single PHY. Some bindings may nest the child
46	nodes within extra levels for context and extensibility, in which case the low
47	level of_phy_provider_register_full() and devm_of_phy_provider_register_full()
48	macros can be used to override the node containing the children.
49
50	#define of_phy_provider_register_full(dev, children, xlate) \
51	__of_phy_provider_register(dev, children, THIS_MODULE, xlate)
52
53	#define devm_of_phy_provider_register_full(dev, children, xlate) \
54	__devm_of_phy_provider_register_full(dev, children, THIS_MODULE, xlate)
55
56	void devm_of_phy_provider_unregister(struct device *dev,
57	struct phy_provider *phy_provider);
58	void of_phy_provider_unregister(struct phy_provider *phy_provider);
59
60	devm_of_phy_provider_unregister and of_phy_provider_unregister can be used to
61	unregister the PHY.
62
63	3. Creating the PHY
64
65	The PHY driver should create the PHY in order for other peripheral controllers
66	to make use of it. The PHY framework provides 2 APIs to create the PHY.
67
68	struct phy *phy_create(struct device *dev, struct device_node *node,
69	const struct phy_ops *ops);
70	struct phy *devm_phy_create(struct device *dev, struct device_node *node,
71	const struct phy_ops *ops);
72
73	The PHY drivers can use one of the above 2 APIs to create the PHY by passing
74	the device pointer and phy ops.
75	phy_ops is a set of function pointers for performing PHY operations such as
76	init, exit, power_on and power_off.
77
78	Inorder to dereference the private data (in phy_ops), the phy provider driver
79	can use phy_set_drvdata() after creating the PHY and use phy_get_drvdata() in
80	phy_ops to get back the private data.
81
82	4. Getting a reference to the PHY
83
84	Before the controller can make use of the PHY, it has to get a reference to
85	it. This framework provides the following APIs to get a reference to the PHY.
86
87	struct phy *phy_get(struct device *dev, const char *string);
88	struct phy *phy_optional_get(struct device *dev, const char *string);
89	struct phy *devm_phy_get(struct device *dev, const char *string);
90	struct phy *devm_phy_optional_get(struct device *dev, const char *string);
91	struct phy *devm_of_phy_get_by_index(struct device *dev, struct device_node *np,
92	int index);
93
94	phy_get, phy_optional_get, devm_phy_get and devm_phy_optional_get can
95	be used to get the PHY. In the case of dt boot, the string arguments
96	should contain the phy name as given in the dt data and in the case of
97	non-dt boot, it should contain the label of the PHY. The two
98	devm_phy_get associates the device with the PHY using devres on
99	successful PHY get. On driver detach, release function is invoked on
100	the the devres data and devres data is freed. phy_optional_get and
101	devm_phy_optional_get should be used when the phy is optional. These
102	two functions will never return -ENODEV, but instead returns NULL when
103	the phy cannot be found.Some generic drivers, such as ehci, may use multiple
104	phys and for such drivers referencing phy(s) by name(s) does not make sense. In
105	this case, devm_of_phy_get_by_index can be used to get a phy reference based on
106	the index.
107
108	It should be noted that NULL is a valid phy reference. All phy
109	consumer calls on the NULL phy become NOPs. That is the release calls,
110	the phy_init() and phy_exit() calls, and phy_power_on() and
111	phy_power_off() calls are all NOP when applied to a NULL phy. The NULL
112	phy is useful in devices for handling optional phy devices.
113
114	5. Releasing a reference to the PHY
115
116	When the controller no longer needs the PHY, it has to release the reference
117	to the PHY it has obtained using the APIs mentioned in the above section. The
118	PHY framework provides 2 APIs to release a reference to the PHY.
119
120	void phy_put(struct phy *phy);
121	void devm_phy_put(struct device *dev, struct phy *phy);
122
123	Both these APIs are used to release a reference to the PHY and devm_phy_put
124	destroys the devres associated with this PHY.
125
126	6. Destroying the PHY
127
128	When the driver that created the PHY is unloaded, it should destroy the PHY it
129	created using one of the following 2 APIs.
130
131	void phy_destroy(struct phy *phy);
132	void devm_phy_destroy(struct device *dev, struct phy *phy);
133
134	Both these APIs destroy the PHY and devm_phy_destroy destroys the devres
135	associated with this PHY.
136
137	7. PM Runtime
138
139	This subsystem is pm runtime enabled. So while creating the PHY,
140	pm_runtime_enable of the phy device created by this subsystem is called and
141	while destroying the PHY, pm_runtime_disable is called. Note that the phy
142	device created by this subsystem will be a child of the device that calls
143	phy_create (PHY provider device).
144
145	So pm_runtime_get_sync of the phy_device created by this subsystem will invoke
146	pm_runtime_get_sync of PHY provider device because of parent-child relationship.
147	It should also be noted that phy_power_on and phy_power_off performs
148	phy_pm_runtime_get_sync and phy_pm_runtime_put respectively.
149	There are exported APIs like phy_pm_runtime_get, phy_pm_runtime_get_sync,
150	phy_pm_runtime_put, phy_pm_runtime_put_sync, phy_pm_runtime_allow and
151	phy_pm_runtime_forbid for performing PM operations.
152
153	8. PHY Mappings
154
155	In order to get reference to a PHY without help from DeviceTree, the framework
156	offers lookups which can be compared to clkdev that allow clk structures to be
157	bound to devices. A lookup can be made be made during runtime when a handle to
158	the struct phy already exists.
159
160	The framework offers the following API for registering and unregistering the
161	lookups.
162
163	int phy_create_lookup(struct phy *phy, const char *con_id, const char *dev_id);
164	void phy_remove_lookup(struct phy *phy, const char *con_id, const char *dev_id);
165
166	9. DeviceTree Binding
167
168	The documentation for PHY dt binding can be found @
169	Documentation/devicetree/bindings/phy/phy-bindings.txt
170