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This hook function is called when a program opens one of the devices supported by the driver. Status_t open_hook(const char * name, uint32 flags, void ** cookie )
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In all cases, return B_OK if the operation is successfully completed, or an appropriate error code if not.
The hook functions specified in the device_hooks function returned by the driver's find_device() function handle requests made by devfs (and through devfs, from user applications). The value of this variable will be changed with every revision to the driver API the value with which your driver was compiled will tell devfs how it can communicate with the driver. This variable defines the API version to which the driver was written, and should be set to B_CUR_DRIVER_API_VERSION at compile time. The device_hooks structure, and what each hook does, is described in the next section. The device_hooks structure returned lists out the hook functions.
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When a device published by the driver is accessed, devfs communicates with it through a series of hook functions that handle the requests.The find_device() function is called to obtain a list of these hook functions, so that devfs can call them. If the driver senses (and supports) two ethernet cards, it might return:ĭevice_hooks * find_device(const char * name ) Since only one instance of the driver will be loaded, if support for multiple devices of the same type is desired, the driver must be capable of supporting them. In this case, devfs will then create the pseudo-file /dev/net/ether, through which all user applications can access the driver. For example, an ethernet device driver might return: The driver should return a NULL-terminated array of strings indicating all the installed devices the driver supports. This lets the driver clean up after itself, freeing any resources it allocated.ĭevfs calls publish_devices() to learn the names, relative to /dev, of the devices the driver supports. This function is called by devfs just before the driver is unloaded from memory. Return B_OK if initialization succeeds, otherwise return an appropriate error code. When a driver is loaded by devfs, this function is called to let the driver allocate memory and other needed system resources. If this function returns an error, the driver won't be used.ĭrivers are loaded and unloaded on an as-needed basis. The function should return B_OK if the initialization is successful otherwise, an appropriate error code should be returned. This function is called when the system is booted, which lets the driver detect and reset the hardware it controls. This exported value tells the kernel what version of the driver API it was written to, and should always be set to B_CUR_DRIVER_API_VERSION in your source code.
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These entry points are:Ĭalled when the system is booted, to let the driver detect and reset the hardware.Ĭalled when the driver is loaded, so it can allocate needed system resources.Ĭalled just before the driver is unloaded, so it can free allocated resources.Ĭalled to obtain a list of device names supported by the driver.Ĭalled to obtain a list of pointers to the hook functions for a specified device.
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The kernel communicates with drivers by calling certain known entry points, which the driver must implement and export.
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This section covers the structure of device drivers, and provides some examples of how to write them. To reduce the risk of the system being adversely affected by a bug in your code, you should put as much of your code into user space as possible. Be very careful, and be sure to test your work well. We can't stress this enough: a bug in a device driver can bring down the entire system. Likewise, add-ons that know how to talk to a class such as SCSI disks, ATA devices, ATAPI disks, or USB input devices is also a device driver.Ī driver's job is to recognize the device and provide a means for applications to talk to it. For example, an add-on that specifically addresses an Ethernet card or graphics card is a device driver. Usually this communication involves some form of device-specific protocol. Drivers: Writing Drivers The Be Book Drivers Drivers IndexĪ device driver is an add-on that communicates with a specific device or type of device.
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