Object Locking

Before allocating the DMA resources for a memory object, the object must be prevented from moving. If it is not, the system may remove the object from memory while the device is writing to it, causing the data transfer to fail and possibly corrupting the system. The process of preventing memory objects from moving during a DMA transfer is known as locking down the object.

Locking objects in memory is not related to the type of locking used to protect data.

The following object types do not require explicit locking:

• Buffers coming from the file system through strategy(9E). These buffers are already locked by the file system.

• Kernel memory allocated within the device driver, such as that allocated by ddi_dma_mem_alloc(9F).

For other objects (such as buffers from user space), physio(9F) must be used to lock down the objects. This is usually performed in the read(9E) or write(9E) routines of a character device driver. See "Data Transfer Methods" for an example.