Notes on Writing Stream Class Minidrivers

  • Minidrivers must run on both Intel and non-Intel x86 processors, and therefore must be written in C (or other high-level language). Minidrivers should not contain assembly language source code.

  • Some hardware requires firmware to be loaded before it will function. Minidrivers should embed firmware in the data segment of the minidriver. Optionally, the minidriver can dynamically load its firmware by calling the appropriate WDM file I/O functions, if, for example, the minidriver uses several versions of firmware that would make embedding it impractical.

  • Turn off synchronization (by setting the TurnOffSynchronization member in the HW_INITIALIZATION_DATA structure to TRUE) if the minidriver needs to go to PASSIVE_LEVEL (low priority) for most requests. See the Minidriver Synchronization section For more information about synchronization.

  • Hardware resources (port or memory addresses) should be accessed through the macros found in wdm.h. For example, to write to a word port, the macro WRITE_PORT_USHORT should be used. To write a buffer to a port, WRITE_PORT_BUFFER_USHORT should be used.

  • Minidrivers should not have unprotected spin loops. If the minidriver needs to spin while waiting for the value of a port to change, the loop must be protected by a loop counter.

  • Minidrivers that need to synchronously wait for a small period of time should use KeStallExecutionProcessor. This routine should be used sparingly so system performance is not degraded.

  • Minidrivers that need to wait a long period of time (more than a few microseconds) for an operation to complete should either be interrupt-driven or use the StreamClassScheduleTimer function to schedule a timed callback. Minidrivers should not spin for more than a few microseconds waiting for a status change, since it degrades overall system performance.

  • Devices that use bus-master DMA need only to use the scatter/gather DMA list supplied in the stream request block structure (HW_STREAM_REQUEST_BLOCK). No locking or mapping of the DMA buffers is necessary. See the HW_STREAM_REQUEST_BLOCK structure for more information. Additionally, if minidrivers need to break up DMA requests, the StreamClassGetPhysicalAddress function can be used to get the physical address of the offset within the virtual buffer pointers.

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