User-level requests. The kernel calls device drivers to service requests from commands such as prtconf 1M. Device interrupts. The kernel calls a device driver to handle interrupts generated by a device. Bus reset. The kernel calls a device driver to re-initialize the driver, the device, or both when the bus is reset. The bus is the path from the CPU to the device. As a result, this function should not be called before the driver is fully initialized and ready to respond to requests for the registered disk.
It can be noticed that the basic structure in working with block devices disks is the struct gendisk structure. The struct gendisk structure stores information about a disk.
The struct gendisk structure has the following important fields:. An example of filling a struct gendisk structure is presented below:. As stated before, the kernel considers a disk as a vector of byte sectors. In reality, the devices may have a different size of the sector. To work with these devices, the kernel needs to be informed about the real size of a sector, and for all operations the necessary conversions must be made. All requests generated by the kernel will be multiple of this sector size and will be aligned accordingly.
The association of operations is done through the fops field in the struct gendisk structure. In a mount operation, the open function is called directly from the kernel space, the file descriptor being stored by the kernel.
A driver for a block device can not differentiate between open calls performed from user space and kernel space. Please notice that there are no read or write operations. These operations are performed by the request function associated with the request queue of the disk. The request queue is made up of a double-linked list of requests and their associated control information.
The requests are added to the queue by higher-level kernel code for example, file systems. The block device driver associates each queue with a handling function, which will be called for each request in the queue the struct request structure.
The problem with this approach is that it requires a per-queue lock, making it inefficient in distributed systems. The Multi-Queue Block Queing Mechanism solves this issue by splitting the device driver queue in two parts:.
The staging queues hold requests from the clients before sending them to the block device driver. To prevent the waiting for a per-queue lock, a staging queue is allocated for each CPU or node. A software queue is associated to only one hardware queue. This means that only the requests coming from the same CPU or node can be optimized.
Once in this queue, the requests can't be merged or reordered. Depending on the underlying hardware, a block device driver can create multiple hardware queues in order to improve parallelism and maximize performance. A block device driver can accept a request before the previous one is completed. As a consequence, the upper layers need a way to know when a request is completed.
For this, a "tag" is added to each request upon submission and sent back using a completion notification after the request is completed. The tag set structure is allocated and initialized before the request queues and also stores some of the queues properties. The first function creates both the hardware and the software queues and initializes their structures. This function is the equivalent of read and write functions encountered on character devices. The function receives the requests for the device as arguments and can use various functions for processing them.
A request for a block device is described by struct request structure. The fields of struct request structure include:. Typically, the subsystem that creates requests for block devices is the file management subsystem. As stated in the Create and delete a request queue section, this function is associated to the driver when creating the tag set structure.
The function must start processing the requests from the queue, but it is not mandatory to finish them, as requests may be finished by other parts of the driver. Such drivers include class and device-specific drivers from Microsoft or a third-party vendor. A client driver creates requests to communicate with the device by calling public interfaces exposed by the USB driver stack.
A client driver for a composite device is no different from a client driver for a non-composite device, except for its location in the driver stack. For a composite USB device that exposes multiple functions and does not have a parent class driver, Windows loads the USB generic parent driver Usbccgp. The parent driver creates a separate PDO for each function of a composite device.
Client drivers FDOs for functions are loaded above the generic parent driver. Vendors might choose to provide a separate client driver for each function. A USB client driver can run in either user mode or kernel mode, depending on the requirements of the driver. Microsoft provides the following helper libraries to help kernel-mode drivers and applications to communicate with the USB driver stack:.
Microsoft provides the Usbd. The helper routines simplify the operational tasks of a client driver. This helper library is new for Windows 8. The library exports routines primarily for allocating and building URBs.
Those routines replace some of the legacy routines exported by Usbd. The new routines require the client driver to register with the USB driver stack, which maintains the handle for registration. That handle is used for calls to other Usbdex.
Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services. Privacy policy. Each device is represented by a device node, and each device node has a device stack. For more information, see Device nodes and device stacks.
We also use the term driver stack to refer to the layered set of drivers for a particular technology. In some cases, more than one device stack is involved in processing an IRP. The following diagram illustrates a case where four device stacks are involved in processing a single IRP.
The IRP is created by Disk. Notice that Usbstor. The IRP is owned by the driver, Usbstor.
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