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1=========2Bus Types3=========4 5Definition6~~~~~~~~~~7See the kerneldoc for the struct bus_type.8 9int bus_register(struct bus_type * bus);10 11 12Declaration13~~~~~~~~~~~14 15Each bus type in the kernel (PCI, USB, etc) should declare one static16object of this type. They must initialize the name field, and may17optionally initialize the match callback::18 19 struct bus_type pci_bus_type = {20 .name = "pci",21 .match = pci_bus_match,22 };23 24The structure should be exported to drivers in a header file:25 26extern struct bus_type pci_bus_type;27 28 29Registration30~~~~~~~~~~~~31 32When a bus driver is initialized, it calls bus_register. This33initializes the rest of the fields in the bus object and inserts it34into a global list of bus types. Once the bus object is registered,35the fields in it are usable by the bus driver.36 37 38Callbacks39~~~~~~~~~40 41match(): Attaching Drivers to Devices42~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~43 44The format of device ID structures and the semantics for comparing45them are inherently bus-specific. Drivers typically declare an array46of device IDs of devices they support that reside in a bus-specific47driver structure.48 49The purpose of the match callback is to give the bus an opportunity to50determine if a particular driver supports a particular device by51comparing the device IDs the driver supports with the device ID of a52particular device, without sacrificing bus-specific functionality or53type-safety.54 55When a driver is registered with the bus, the bus's list of devices is56iterated over, and the match callback is called for each device that57does not have a driver associated with it.58 59 60 61Device and Driver Lists62~~~~~~~~~~~~~~~~~~~~~~~63 64The lists of devices and drivers are intended to replace the local65lists that many buses keep. They are lists of struct devices and66struct device_drivers, respectively. Bus drivers are free to use the67lists as they please, but conversion to the bus-specific type may be68necessary.69 70The LDM core provides helper functions for iterating over each list::71 72 int bus_for_each_dev(struct bus_type * bus, struct device * start,73 void * data,74 int (*fn)(struct device *, void *));75 76 int bus_for_each_drv(struct bus_type * bus, struct device_driver * start,77 void * data, int (*fn)(struct device_driver *, void *));78 79These helpers iterate over the respective list, and call the callback80for each device or driver in the list. All list accesses are81synchronized by taking the bus's lock (read currently). The reference82count on each object in the list is incremented before the callback is83called; it is decremented after the next object has been obtained. The84lock is not held when calling the callback.85 86 87sysfs88~~~~~~~~89There is a top-level directory named 'bus'.90 91Each bus gets a directory in the bus directory, along with two default92directories::93 94 /sys/bus/pci/95 |-- devices96 `-- drivers97 98Drivers registered with the bus get a directory in the bus's drivers99directory::100 101 /sys/bus/pci/102 |-- devices103 `-- drivers104 |-- Intel ICH105 |-- Intel ICH Joystick106 |-- agpgart107 `-- e100108 109Each device that is discovered on a bus of that type gets a symlink in110the bus's devices directory to the device's directory in the physical111hierarchy::112 113 /sys/bus/pci/114 |-- devices115 | |-- 00:00.0 -> ../../../root/pci0/00:00.0116 | |-- 00:01.0 -> ../../../root/pci0/00:01.0117 | `-- 00:02.0 -> ../../../root/pci0/00:02.0118 `-- drivers119 120 121Exporting Attributes122~~~~~~~~~~~~~~~~~~~~123 124::125 126 struct bus_attribute {127 struct attribute attr;128 ssize_t (*show)(const struct bus_type *, char * buf);129 ssize_t (*store)(const struct bus_type *, const char * buf, size_t count);130 };131 132Bus drivers can export attributes using the BUS_ATTR_RW macro that works133similarly to the DEVICE_ATTR_RW macro for devices. For example, a134definition like this::135 136 static BUS_ATTR_RW(debug);137 138is equivalent to declaring::139 140 static bus_attribute bus_attr_debug;141 142This can then be used to add and remove the attribute from the bus's143sysfs directory using::144 145 int bus_create_file(struct bus_type *, struct bus_attribute *);146 void bus_remove_file(struct bus_type *, struct bus_attribute *);147