Linux: create mmap()able virtual file (e.g. SPI memory)

Question

I have a char device which enables access to an external SPI memory, and I'd like to mmap() the external memory so that I can access it from a program as if it were normal memory.

If I use the usual mmap() page remapping implementation on the char device file, it just enables me to see a device memory region, not its virtual char file...

Is there a trick to allow me to do that?

TIA

Answer 1

If the character device driver provided an mmap implementation, it would work. There's probably a good reason it doesn't:

Memory access instructions create memory transactions on the bus. An SPI memory is not addressable that way (although the SPI controller may use memory-mapped I/O, that's for its own register-level interface, not the memory content). You could build an SPI memory controller with a memory bus interface, I suppose, but you'd lose the device-indepedence of the SPI standard.

Emulating a memory region is possible (grab a page of memory, mark it for no access, and deal with the access violations (SIGBUS and SIGSEGV), but that would be horribly inefficient. Sometimes you find virtual machines doing this, but performance is very bad.

Answer 2

It sounds like you would need some sort of driver that translates the memory region accesses to commands sent through the character-oriented interface. This would probably be a pretty straight-forward block device driver.