198 lines
7.8 KiB
Plaintext
198 lines
7.8 KiB
Plaintext
=============
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PHY subsystem
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=============
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:Author: Kishon Vijay Abraham I <kishon@ti.com>
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This document explains the Generic PHY Framework along with the APIs provided,
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and how-to-use.
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Introduction
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============
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*PHY* is the abbreviation for physical layer. It is used to connect a device
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to the physical medium e.g., the USB controller has a PHY to provide functions
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such as serialization, de-serialization, encoding, decoding and is responsible
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for obtaining the required data transmission rate. Note that some USB
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controllers have PHY functionality embedded into it and others use an external
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PHY. Other peripherals that use PHY include Wireless LAN, Ethernet,
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SATA etc.
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The intention of creating this framework is to bring the PHY drivers spread
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all over the Linux kernel to drivers/phy to increase code re-use and for
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better code maintainability.
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This framework will be of use only to devices that use external PHY (PHY
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functionality is not embedded within the controller).
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Registering/Unregistering the PHY provider
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==========================================
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PHY provider refers to an entity that implements one or more PHY instances.
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For the simple case where the PHY provider implements only a single instance of
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the PHY, the framework provides its own implementation of of_xlate in
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of_phy_simple_xlate. If the PHY provider implements multiple instances, it
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should provide its own implementation of of_xlate. of_xlate is used only for
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dt boot case.
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::
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#define of_phy_provider_register(dev, xlate) \
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__of_phy_provider_register((dev), NULL, THIS_MODULE, (xlate))
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#define devm_of_phy_provider_register(dev, xlate) \
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__devm_of_phy_provider_register((dev), NULL, THIS_MODULE,
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(xlate))
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of_phy_provider_register and devm_of_phy_provider_register macros can be used to
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register the phy_provider and it takes device and of_xlate as
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arguments. For the dt boot case, all PHY providers should use one of the above
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2 macros to register the PHY provider.
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Often the device tree nodes associated with a PHY provider will contain a set
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of children that each represent a single PHY. Some bindings may nest the child
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nodes within extra levels for context and extensibility, in which case the low
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level of_phy_provider_register_full() and devm_of_phy_provider_register_full()
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macros can be used to override the node containing the children.
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::
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#define of_phy_provider_register_full(dev, children, xlate) \
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__of_phy_provider_register(dev, children, THIS_MODULE, xlate)
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#define devm_of_phy_provider_register_full(dev, children, xlate) \
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__devm_of_phy_provider_register_full(dev, children,
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THIS_MODULE, xlate)
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void devm_of_phy_provider_unregister(struct device *dev,
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struct phy_provider *phy_provider);
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void of_phy_provider_unregister(struct phy_provider *phy_provider);
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devm_of_phy_provider_unregister and of_phy_provider_unregister can be used to
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unregister the PHY.
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Creating the PHY
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================
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The PHY driver should create the PHY in order for other peripheral controllers
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to make use of it. The PHY framework provides 2 APIs to create the PHY.
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::
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struct phy *phy_create(struct device *dev, struct device_node *node,
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const struct phy_ops *ops);
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struct phy *devm_phy_create(struct device *dev,
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struct device_node *node,
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const struct phy_ops *ops);
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The PHY drivers can use one of the above 2 APIs to create the PHY by passing
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the device pointer and phy ops.
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phy_ops is a set of function pointers for performing PHY operations such as
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init, exit, power_on and power_off.
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Inorder to dereference the private data (in phy_ops), the phy provider driver
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can use phy_set_drvdata() after creating the PHY and use phy_get_drvdata() in
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phy_ops to get back the private data.
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4. Getting a reference to the PHY
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Before the controller can make use of the PHY, it has to get a reference to
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it. This framework provides the following APIs to get a reference to the PHY.
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::
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struct phy *phy_get(struct device *dev, const char *string);
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struct phy *phy_optional_get(struct device *dev, const char *string);
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struct phy *devm_phy_get(struct device *dev, const char *string);
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struct phy *devm_phy_optional_get(struct device *dev,
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const char *string);
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struct phy *devm_of_phy_get_by_index(struct device *dev,
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struct device_node *np,
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int index);
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phy_get, phy_optional_get, devm_phy_get and devm_phy_optional_get can
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be used to get the PHY. In the case of dt boot, the string arguments
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should contain the phy name as given in the dt data and in the case of
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non-dt boot, it should contain the label of the PHY. The two
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devm_phy_get associates the device with the PHY using devres on
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successful PHY get. On driver detach, release function is invoked on
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the devres data and devres data is freed. phy_optional_get and
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devm_phy_optional_get should be used when the phy is optional. These
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two functions will never return -ENODEV, but instead returns NULL when
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the phy cannot be found.Some generic drivers, such as ehci, may use multiple
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phys and for such drivers referencing phy(s) by name(s) does not make sense. In
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this case, devm_of_phy_get_by_index can be used to get a phy reference based on
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the index.
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It should be noted that NULL is a valid phy reference. All phy
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consumer calls on the NULL phy become NOPs. That is the release calls,
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the phy_init() and phy_exit() calls, and phy_power_on() and
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phy_power_off() calls are all NOP when applied to a NULL phy. The NULL
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phy is useful in devices for handling optional phy devices.
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Releasing a reference to the PHY
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================================
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When the controller no longer needs the PHY, it has to release the reference
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to the PHY it has obtained using the APIs mentioned in the above section. The
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PHY framework provides 2 APIs to release a reference to the PHY.
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::
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void phy_put(struct phy *phy);
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void devm_phy_put(struct device *dev, struct phy *phy);
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Both these APIs are used to release a reference to the PHY and devm_phy_put
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destroys the devres associated with this PHY.
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Destroying the PHY
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==================
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When the driver that created the PHY is unloaded, it should destroy the PHY it
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created using one of the following 2 APIs::
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void phy_destroy(struct phy *phy);
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void devm_phy_destroy(struct device *dev, struct phy *phy);
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Both these APIs destroy the PHY and devm_phy_destroy destroys the devres
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associated with this PHY.
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PM Runtime
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==========
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This subsystem is pm runtime enabled. So while creating the PHY,
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pm_runtime_enable of the phy device created by this subsystem is called and
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while destroying the PHY, pm_runtime_disable is called. Note that the phy
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device created by this subsystem will be a child of the device that calls
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phy_create (PHY provider device).
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So pm_runtime_get_sync of the phy_device created by this subsystem will invoke
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pm_runtime_get_sync of PHY provider device because of parent-child relationship.
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It should also be noted that phy_power_on and phy_power_off performs
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phy_pm_runtime_get_sync and phy_pm_runtime_put respectively.
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There are exported APIs like phy_pm_runtime_get, phy_pm_runtime_get_sync,
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phy_pm_runtime_put, phy_pm_runtime_put_sync, phy_pm_runtime_allow and
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phy_pm_runtime_forbid for performing PM operations.
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PHY Mappings
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============
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In order to get reference to a PHY without help from DeviceTree, the framework
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offers lookups which can be compared to clkdev that allow clk structures to be
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bound to devices. A lookup can be made be made during runtime when a handle to
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the struct phy already exists.
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The framework offers the following API for registering and unregistering the
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lookups::
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int phy_create_lookup(struct phy *phy, const char *con_id,
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const char *dev_id);
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void phy_remove_lookup(struct phy *phy, const char *con_id,
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const char *dev_id);
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DeviceTree Binding
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==================
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The documentation for PHY dt binding can be found @
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Documentation/devicetree/bindings/phy/phy-bindings.txt
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