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Diffstat (limited to 'Documentation/driver-model/overview.txt')
| -rw-r--r-- | Documentation/driver-model/overview.txt | 103 |
1 files changed, 56 insertions, 47 deletions
diff --git a/Documentation/driver-model/overview.txt b/Documentation/driver-model/overview.txt index 44662735cf8..6a8f9a8075d 100644 --- a/Documentation/driver-model/overview.txt +++ b/Documentation/driver-model/overview.txt @@ -1,50 +1,43 @@ The Linux Kernel Device Model -Patrick Mochel <mochel@osdl.org> +Patrick Mochel <mochel@digitalimplant.org> -26 August 2002 +Drafted 26 August 2002 +Updated 31 January 2006 Overview ~~~~~~~~ -This driver model is a unification of all the current, disparate driver models -that are currently in the kernel. It is intended to augment the +The Linux Kernel Driver Model is a unification of all the disparate driver +models that were previously used in the kernel. It is intended to augment the bus-specific drivers for bridges and devices by consolidating a set of data and operations into globally accessible data structures. -Current driver models implement some sort of tree-like structure (sometimes -just a list) for the devices they control. But, there is no linkage between -the different bus types. +Traditional driver models implemented some sort of tree-like structure +(sometimes just a list) for the devices they control. There wasn't any +uniformity across the different bus types. -A common data structure can provide this linkage with little overhead: when a -bus driver discovers a particular device, it can insert it into the global -tree as well as its local tree. In fact, the local tree becomes just a subset -of the global tree. - -Common data fields can also be moved out of the local bus models into the -global model. Some of the manipulations of these fields can also be -consolidated. Most likely, manipulation functions will become a set -of helper functions, which the bus drivers wrap around to include any -bus-specific items. - -The common device and bridge interface currently reflects the goals of the -modern PC: namely the ability to do seamless Plug and Play, power management, -and hot plug. (The model dictated by Intel and Microsoft (read: ACPI) ensures -us that any device in the system may fit any of these criteria.) - -In reality, not every bus will be able to support such operations. But, most -buses will support a majority of those operations, and all future buses will. -In other words, a bus that doesn't support an operation is the exception, -instead of the other way around. +The current driver model provides a common, uniform data model for describing +a bus and the devices that can appear under the bus. The unified bus +model includes a set of common attributes which all busses carry, and a set +of common callbacks, such as device discovery during bus probing, bus +shutdown, bus power management, etc. +The common device and bridge interface reflects the goals of the modern +computer: namely the ability to do seamless device "plug and play", power +management, and hot plug. In particular, the model dictated by Intel and +Microsoft (namely ACPI) ensures that almost every device on almost any bus +on an x86-compatible system can work within this paradigm. Of course, +not every bus is able to support all such operations, although most +buses support most of those operations. Downstream Access ~~~~~~~~~~~~~~~~~ Common data fields have been moved out of individual bus layers into a common -data structure. But, these fields must still be accessed by the bus layers, +data structure. These fields must still be accessed by the bus layers, and sometimes by the device-specific drivers. Other bus layers are encouraged to do what has been done for the PCI layer. @@ -53,25 +46,29 @@ struct pci_dev now looks like this: struct pci_dev { ... - struct device device; + struct device dev; /* Generic device interface */ + ... }; -Note first that it is statically allocated. This means only one allocation on -device discovery. Note also that it is at the _end_ of struct pci_dev. This is -to make people think about what they're doing when switching between the bus -driver and the global driver; and to prevent against mindless casts between -the two. +Note first that the struct device dev within the struct pci_dev is +statically allocated. This means only one allocation on device discovery. + +Note also that that struct device dev is not necessarily defined at the +front of the pci_dev structure. This is to make people think about what +they're doing when switching between the bus driver and the global driver, +and to discourage meaningless and incorrect casts between the two. The PCI bus layer freely accesses the fields of struct device. It knows about the structure of struct pci_dev, and it should know the structure of struct -device. PCI devices that have been converted generally do not touch the fields -of struct device. More precisely, device-specific drivers should not touch -fields of struct device unless there is a strong compelling reason to do so. +device. Individual PCI device drivers that have been converted to the current +driver model generally do not and should not touch the fields of struct device, +unless there is a compelling reason to do so. -This abstraction is prevention of unnecessary pain during transitional phases. -If the name of the field changes or is removed, then every downstream driver -will break. On the other hand, if only the bus layer (and not the device -layer) accesses struct device, it is only that layer that needs to change. +The above abstraction prevents unnecessary pain during transitional phases. +If it were not done this way, then when a field was renamed or removed, every +downstream driver would break. On the other hand, if only the bus layer +(and not the device layer) accesses the struct device, it is only the bus +layer that needs to change. User Interface @@ -80,15 +77,27 @@ User Interface By virtue of having a complete hierarchical view of all the devices in the system, exporting a complete hierarchical view to userspace becomes relatively easy. This has been accomplished by implementing a special purpose virtual -file system named sysfs. It is hence possible for the user to mount the -whole sysfs filesystem anywhere in userspace. +file system named sysfs. + +Almost all mainstream Linux distros mount this filesystem automatically; you +can see some variation of the following in the output of the "mount" command: + +$ mount +... +none on /sys type sysfs (rw,noexec,nosuid,nodev) +... +$ + +The auto-mounting of sysfs is typically accomplished by an entry similar to +the following in the /etc/fstab file: + +none /sys sysfs defaults 0 0 -This can be done permanently by providing the following entry into the -/etc/fstab (under the provision that the mount point does exist, of course): +or something similar in the /lib/init/fstab file on Debian-based systems: -none /sys sysfs defaults 0 0 +none /sys sysfs nodev,noexec,nosuid 0 0 -Or by hand on the command line: +If sysfs is not automatically mounted, you can always do it manually with: # mount -t sysfs sysfs /sys |