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Writing Device Drivers for HelenOS
What is a device driver?
The hardware an OS runs on can be modelled as a hierarchy of (peripheral) interconnects (buses) to which devices are attached. Each bus provides connectivity to devices or other buses. This hiearchy can be often represented as a tree (or directed acyclic graph), where inner (nexus) nodes correspond to buses and leaf nodes correspond to devices.
Needless to say this is just a model, meaning nothing is given a priori, the model can look differently depending on our choice. It is often not clear what is a device and what is not, where are the boundaries of a particular device, etc. Consequently, there is also no clear line between what still is a device driver and what is not.
Usually devices (or their drivers) have one or more of the following traits. When a system is coming up, it attempts to transitively discover all buses and devices connected to it. For each bus or device it selects the appropriate driver. The driver takes (complete) control of the bus or device and makes its services available to the system (to other drivers in case of a bus, to applications in case of a device). The driver abstracts away the details of a particular device model and provides an interface common to a class of devices (e.g. Ethernet adapter). The system often also virtualizes the device, providing concurrent access to multiple clients, but this can be done at a higher level, rather than in the driver itself.
Overview of Drivers in HelenOS
Device drivers in HelenOS come in two flavors, plain drivers and DDF drivers. Plain drivers originated before the Device Driver Framework (DDF) was created. They are simple servers which are started manually (from command line or from init task) and they reside in
/srv. DDF drivers make use of the Device Driver Framework which takes care of starting them, attaching them to devices, etc. They reside in
Both kinds of drivers export their services to clients in the same way, as services registered with the Location Service. Each service has a unique name and it can be added to one or more categories. For a client it does not matter how a service is implemented, whether in a plain driver or in a DDF driver. This is an important design point.
DDF drivers are most useful for drivers that reside on busses that support discovery and hotplug (PCI, USB). Plain drivers are useful for implementing pseudo-devices. file_bd is an example of a plain driver. It implements a file-backed block device and it is started from the command line.
DDF (Device Driver Framework)
The Device Driver Framework (DDF) implements common functionality which is useful for most device drivers. It manages the device topology graph (device tree), coordinates enumeration, automatically starts drivers and allows communication between drivers of parent and child devices. DDF imposes certain structure of the driver and defines calls and entry points by which DDF and the driver communicate.
Internally DDF consists of a server, the Device Manager (devman) and the Device Driver Library (libdrv). Every driver is linked against libdrv, which internally communicates with the Device Manager via IPC — this is hidden from the driver. Device Manager holds information about drivers and device topology and coordinates operation of the drivers.
There is an administration tool associated with the Device Manager, devctl. devctl can be used to control operation of the Device Manager. It can display the device tree, offline and online devices (i.e. perform anticipated unplug operations).
DDF Driver Structure
When HelenOS runs the driver is an executable stored in
/drv/foo). The driver is normally started automatically by the Device Manager. When the driver is needed, the Device Manager will start it and the driver connects to the Device Manager.
In HelenOS source tree drivers are located under
uspace/drv. To add a new driver foo, you need to:
- create a directory
- create a makefile
- create at least one source file
- add directory
uspace/drv/a/b/footo the DIRS definition in
You can use an existing driver as a starting point, for example
uspace/drv/test/test1. A driver is a C program similar to a HelenOS server or application. It is linked with
libdrv (and the makefile should add libdrv's include path to the header search paths).
An example driver makefile (with license stripped):
USPACE_PREFIX = ../../.. LIBS = $(LIBDRV_PREFIX)/libdrv.a EXTRA_CFLAGS += -I$(LIBDRV_PREFIX)/include BINARY = foo SOURCES = \ foo.c include $(USPACE_PREFIX)/Makefile.common
The most basic DDF interfaces are defined in the header
- #include <ddf/driver.h>
- add_device (rename to dev_add)
Device and Function Life Cycle
Soft State Management
A driver can associate driver-specific data with its devices, functions or both. The framework provides a way to allocate a block of memory (soft state) associated with a device or function and allows the driver to obtain pointer to this memory block. The framework frees the soft state automatically (and in such a way that it does not happen while the driver is still accessing it).
The driver is expected to synchronize access to this soft-state structure on its own in order to achieve mutual exclusion. Since the framework ensures that soft-state is not deallocated while the driver is accessing it, the synchronization can be as simple as a mutex embedded in the structure.
The soft-state management functions are:
- void *ddf_dev_data_alloc(ddf_dev_t *dev, size_t size) - allocate driver-specific device data
- void *ddf_fun_data_alloc(ddf_fun_t *fun, size_t size) - allocate driver-specific function data
The function ddf_dev_data_alloc allocates a driver-specific data strutcure size bytes large, associated with the device dev. The structure will not be deallocated by the framework until the dev_remove or dev_gone entry point returns and control exits all driver entry points that are invoked with dev as a parameter. (Internally, this is achieved using reference counting).
The function ddf_fun_data_alloc allocates a driver-specific data structure size bytes large, associated with the function fun. The structure will not be deallocated by the framework until ddf_fun_destroy() is called and control exits all driver entry points that are invoked with fun as parameter.
In practice, as long as you don't copy the pointer to device or function soft-state to a global/heap structure or pass it to another thread, it can never become dangling. As long as you have the reference, it is valid (i.e. points to an allocated block).
Exposing Driver Services to Clients
All driver services are provided to clients (applications) via the location service. For each bound function node the device framework creates a service whose name is based on the physical path of that function. The services are created in the devices namespace. This is mapped to the
/loc/devices directory via locfs. For example:
/# ls /loc/devices \hw\pci0\00:01.0\com1\a \hw\pci0\00:01.0\ctl \hw\pci0\ctl \virt\lo\port0
Often the driver will want to add the function service to one or more service categories. Service categories imply the protocol spoken by a service (and possibly also the intended use). This can be done using the DDF function:
int ddf_fun_add_to_category(ddf_fun_t *fun, const char *cat_name)
For example, the input server automatically picks up and opens any device in the categories keyboard and mouse.