^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) =============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) The Linux Kernel Device Model
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3) =============================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5) Patrick Mochel <mochel@digitalimplant.org>
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7) Drafted 26 August 2002
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) Updated 31 January 2006
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11) Overview
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12) ~~~~~~~~
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14) The Linux Kernel Driver Model is a unification of all the disparate driver
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15) models that were previously used in the kernel. It is intended to augment the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) bus-specific drivers for bridges and devices by consolidating a set of data
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) and operations into globally accessible data structures.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) Traditional driver models implemented some sort of tree-like structure
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20) (sometimes just a list) for the devices they control. There wasn't any
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) uniformity across the different bus types.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) The current driver model provides a common, uniform data model for describing
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) a bus and the devices that can appear under the bus. The unified bus
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25) model includes a set of common attributes which all busses carry, and a set
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) of common callbacks, such as device discovery during bus probing, bus
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) shutdown, bus power management, etc.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) The common device and bridge interface reflects the goals of the modern
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30) computer: namely the ability to do seamless device "plug and play", power
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) management, and hot plug. In particular, the model dictated by Intel and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) Microsoft (namely ACPI) ensures that almost every device on almost any bus
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) on an x86-compatible system can work within this paradigm. Of course,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34) not every bus is able to support all such operations, although most
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) buses support most of those operations.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) Downstream Access
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39) ~~~~~~~~~~~~~~~~~
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) Common data fields have been moved out of individual bus layers into a common
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) data structure. These fields must still be accessed by the bus layers,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43) and sometimes by the device-specific drivers.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45) Other bus layers are encouraged to do what has been done for the PCI layer.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46) struct pci_dev now looks like this::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48) struct pci_dev {
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49) ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51) struct device dev; /* Generic device interface */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52) ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53) };
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55) Note first that the struct device dev within the struct pci_dev is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) statically allocated. This means only one allocation on device discovery.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58) Note also that that struct device dev is not necessarily defined at the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59) front of the pci_dev structure. This is to make people think about what
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60) they're doing when switching between the bus driver and the global driver,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61) and to discourage meaningless and incorrect casts between the two.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63) The PCI bus layer freely accesses the fields of struct device. It knows about
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64) the structure of struct pci_dev, and it should know the structure of struct
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65) device. Individual PCI device drivers that have been converted to the current
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66) driver model generally do not and should not touch the fields of struct device,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67) unless there is a compelling reason to do so.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69) The above abstraction prevents unnecessary pain during transitional phases.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70) If it were not done this way, then when a field was renamed or removed, every
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71) downstream driver would break. On the other hand, if only the bus layer
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) (and not the device layer) accesses the struct device, it is only the bus
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73) layer that needs to change.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76) User Interface
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) ~~~~~~~~~~~~~~
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79) By virtue of having a complete hierarchical view of all the devices in the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80) system, exporting a complete hierarchical view to userspace becomes relatively
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81) easy. This has been accomplished by implementing a special purpose virtual
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82) file system named sysfs.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84) Almost all mainstream Linux distros mount this filesystem automatically; you
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85) can see some variation of the following in the output of the "mount" command::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87) $ mount
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89) none on /sys type sysfs (rw,noexec,nosuid,nodev)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90) ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91) $
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93) The auto-mounting of sysfs is typically accomplished by an entry similar to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94) the following in the /etc/fstab file::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) none /sys sysfs defaults 0 0
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98) or something similar in the /lib/init/fstab file on Debian-based systems::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100) none /sys sysfs nodev,noexec,nosuid 0 0
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) If sysfs is not automatically mounted, you can always do it manually with::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104) # mount -t sysfs sysfs /sys
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106) Whenever a device is inserted into the tree, a directory is created for it.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107) This directory may be populated at each layer of discovery - the global layer,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108) the bus layer, or the device layer.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110) The global layer currently creates two files - 'name' and 'power'. The
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) former only reports the name of the device. The latter reports the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112) current power state of the device. It will also be used to set the current
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113) power state.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) The bus layer may also create files for the devices it finds while probing the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116) bus. For example, the PCI layer currently creates 'irq' and 'resource' files
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 117) for each PCI device.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 118)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 119) A device-specific driver may also export files in its directory to expose
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 120) device-specific data or tunable interfaces.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 121)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 122) More information about the sysfs directory layout can be found in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 123) the other documents in this directory and in the file
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 124) Documentation/filesystems/sysfs.rst.
Orange Pi5 kernel
Deprecated Linux kernel 5.10.110 for OrangePi 5/5B/5+ boards
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