The same chapter you are referring to, has the answer to your question.
A definitive example of mmap usage can be seen by looking at a subset of the virtual memory areas for the X Window System server. Whenever the program reads or writes in the assigned address range, it is actually accessing the device. In the X server example, using mmap allows quick and easy access to the video card’s memory. For a performance-critical application like this, direct access makes a large difference.
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Another typical example is a program controlling a PCI device. Most PCI peripherals map their control registers to a memory address, and a high-performance application might prefer to have direct access to the registers instead of repeatedly having to call ioctl to get its work
done.
But you are correct that usually kernel drivers handle devices without revealing device memory to user space:
As you might suspect, not every device lends itself to the mmap abstraction; it makes no sense, for instance, for serial ports and other stream-oriented devices. Another limitation of mmap is that mapping is PAGE_SIZE grained.
In the end, it all depends on how you want your device to be used from user space:
- which interfaces you want to provide from driver to user space
- what are performance requirements
Usually you hide device memory from user, but sometimes it's needed to give user a direct access to device memory (when alternative is bad performance or ugly interface). Only you, as an engineer, can decide which way is the best, in each particular case.
