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2cd14f5da6
Improve the wording by changing it from "is provide the" to "is to give the". Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
144 lines
4.0 KiB
Plaintext
144 lines
4.0 KiB
Plaintext
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Bus Types
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Definition
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~~~~~~~~~~
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See the kerneldoc for the struct bus_type.
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int bus_register(struct bus_type * bus);
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Declaration
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~~~~~~~~~~~
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Each bus type in the kernel (PCI, USB, etc) should declare one static
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object of this type. They must initialize the name field, and may
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optionally initialize the match callback.
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struct bus_type pci_bus_type = {
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.name = "pci",
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.match = pci_bus_match,
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};
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The structure should be exported to drivers in a header file:
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extern struct bus_type pci_bus_type;
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Registration
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~~~~~~~~~~~~
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When a bus driver is initialized, it calls bus_register. This
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initializes the rest of the fields in the bus object and inserts it
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into a global list of bus types. Once the bus object is registered,
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the fields in it are usable by the bus driver.
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Callbacks
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~~~~~~~~~
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match(): Attaching Drivers to Devices
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The format of device ID structures and the semantics for comparing
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them are inherently bus-specific. Drivers typically declare an array
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of device IDs of devices they support that reside in a bus-specific
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driver structure.
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The purpose of the match callback is to give the bus an opportunity to
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determine if a particular driver supports a particular device by
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comparing the device IDs the driver supports with the device ID of a
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particular device, without sacrificing bus-specific functionality or
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type-safety.
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When a driver is registered with the bus, the bus's list of devices is
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iterated over, and the match callback is called for each device that
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does not have a driver associated with it.
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Device and Driver Lists
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~~~~~~~~~~~~~~~~~~~~~~~
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The lists of devices and drivers are intended to replace the local
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lists that many buses keep. They are lists of struct devices and
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struct device_drivers, respectively. Bus drivers are free to use the
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lists as they please, but conversion to the bus-specific type may be
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necessary.
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The LDM core provides helper functions for iterating over each list.
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int bus_for_each_dev(struct bus_type * bus, struct device * start, void * data,
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int (*fn)(struct device *, void *));
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int bus_for_each_drv(struct bus_type * bus, struct device_driver * start,
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void * data, int (*fn)(struct device_driver *, void *));
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These helpers iterate over the respective list, and call the callback
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for each device or driver in the list. All list accesses are
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synchronized by taking the bus's lock (read currently). The reference
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count on each object in the list is incremented before the callback is
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called; it is decremented after the next object has been obtained. The
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lock is not held when calling the callback.
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sysfs
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~~~~~~~~
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There is a top-level directory named 'bus'.
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Each bus gets a directory in the bus directory, along with two default
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directories:
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/sys/bus/pci/
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|-- devices
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`-- drivers
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Drivers registered with the bus get a directory in the bus's drivers
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directory:
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/sys/bus/pci/
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|-- devices
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`-- drivers
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|-- Intel ICH
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|-- Intel ICH Joystick
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|-- agpgart
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`-- e100
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Each device that is discovered on a bus of that type gets a symlink in
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the bus's devices directory to the device's directory in the physical
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hierarchy:
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/sys/bus/pci/
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|-- devices
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| |-- 00:00.0 -> ../../../root/pci0/00:00.0
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| |-- 00:01.0 -> ../../../root/pci0/00:01.0
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| `-- 00:02.0 -> ../../../root/pci0/00:02.0
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`-- drivers
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Exporting Attributes
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~~~~~~~~~~~~~~~~~~~~
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struct bus_attribute {
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struct attribute attr;
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ssize_t (*show)(struct bus_type *, char * buf);
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ssize_t (*store)(struct bus_type *, const char * buf, size_t count);
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};
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Bus drivers can export attributes using the BUS_ATTR macro that works
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similarly to the DEVICE_ATTR macro for devices. For example, a definition
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like this:
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static BUS_ATTR(debug,0644,show_debug,store_debug);
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is equivalent to declaring:
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static bus_attribute bus_attr_debug;
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This can then be used to add and remove the attribute from the bus's
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sysfs directory using:
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int bus_create_file(struct bus_type *, struct bus_attribute *);
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void bus_remove_file(struct bus_type *, struct bus_attribute *);
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