A driver is just an ELF binary built in much the same way as a regular DLL. When a driver is loaded by the kernel it will search for a exported symbol named "device_init" and call it to allow the driver to initialize itself. The device_init() function is passed a unique device ID that is dynamically assigned to each driver when loaded and used by the kernel to identify the driver later.
Just before the driver is unloaded the kernel will look for a global function named "device_uninit()" and if found it will be called to allow the driver to clean up.
This device_init() and device_uninit() is the only functions that will be called directly by the kernel. For the driver to do any usefull work it must export one or more device-nodes through the AtheOS Device FS (ADFS). ADFS is a logical file system that is mounted at "/dev/" and controll all devices. Each device is present as a magic file located in "/dev/" or a sub-directory of "/dev/". Initially "/dev/" only contain "/dev/null" and "/dev/zero" which is controlled by ADFS itself. All other directory and device-nodes are created by device drivers. A device driver can create a device-node with create_device_node() and remove it with delete_device_node(). When creating a device node the driver must provide a function pointer table with entry points to the drivers functionality. The functions in the table will be called by the ADFS to controll the device. The most important functions is read(), write() and ioctl() but there is also functions to open/close the device aswell as functions called by select() to make it possible for one thread to wait for IO on multiple devices.
A typical device driver will create one node in device_init() and delete it again in device_uninit().
int g_nMyDeviceNode = -1;
status_t device_init( int nDeviceID ) { struct MyDeviceNode sNode; g_nMyDeviceNode = create_device_node( nDeviceID, "misc/mydevice", &g_sOperations, &sNode );
if ( g_nMyDeviceNode < 0 ) { return( g_nMyDeviceNode ); } else { return( EOK ); } }
status_t device_uninit( int nDeviceID ) { delete_device_node( g_nMyDeviceNode ); return( EOK ); }
Since the nodes inside /dev/ is created by the device drivers themself and not the kernel it is not directly obvious what driver should be loaded if a application tries to open for example "/dev/video/cpia".
If this is the first time someone attempts to open the CPiA device the driver is not loaded and "/dev/video/cpia" does not exists. If this is the first time anything inside /dev/ is touched neighter does the "/dev/video" directory.
To make it possible for the kernel to search for drivers in a efficient way the driver-binaries are located in a directory tree similar to the resulting tree inside /dev/. For example the CPiA driver from the above example whould be located in "/system/drivers/dev/video/cpia".
If the kernel is asked to open for example "/dev/video/cpia" it whould start by opening the "/dev" directory which whould cause the "/system/drivers/dev" directory to be iterated. During the iteration all drivers found will be attempted loaded and initiated and all directories will be replicated inside /dev/. Since "/system/drivers/dev" contains a directory named "video" this will cause "/dev/video" to be created. When "/dev/" is successfully opened it will attempt to open "/dev/video" which should now exist. Opening "/dev/video" will cause the "/system/drivers/dev/video" directory to be iterated and the "cpia" binary to be loaded. The CPiA driver will then probe for a CPiA device and if found it will create a device node named "/dev/video/cpia" which will then be found and opened when the kernel descend into the "/dev/video" directory.
In the trivial example above there was direct match between the name of the driver and the name of the device inside /dev. Since one driver might export more than one device this is not always the case. For example a IDE disk driver whould export one device for each disk connected to the bus and one device for each partitions found on those disks. The device-tree exported by a IDE driver might look something like this:
/dev/disk/ide/bus1/master/raw /dev/disk/ide/bus1/master/0 /dev/disk/ide/bus1/master/1 /dev/disk/ide/bus1/slave/raw /dev/disk/ide/bus2/master/raw /dev/disk/ide/bus2/master/0
In this case the ide driver should be located in "/system/drivers/dev/disk/ide/ide". If someone attempts to open the first partition on the master disk connected to the first controller it whould have to open "/dev/disk/ide/bus1/master/0".
When descending the path the kernel will first create the "/dev/disk" and the "/dev/disk/ide" directory. Then it will load the ide-driver which will detect that there is 3 disks connected to the two controllers before decoding the partition tables and add all the nodes listed above. At his point "/dev/disk/ide/bus1/master/0" already exists and no other drivers need to be loaded to fullfill the request.
1.2.5 written by Dimitri van Heesch,
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