[Geekos-devel] new block device API

[David H. <da...@cs...>]

I commited some code to define an interface for block devices:
  include/sys/geekos/blockdev.h
  kern/blockdev.c

Basically, each request to read or write a block is represented by a
block_device_request object, which specifies the block number, memory
buffer, etc.  A condition variable (protected by the mutex in the block
device object) is used to signal the completion of the request.

I also wrote a ramdisk driver as an example of how the interface
works:
  include/sys/geekos/ramdisk.h
  drivers/ramdisk.c

It's not a very good model for real block devices, since it's 
completely synchronous.  A real block device driver would be
asynchronous, since things like disks are slow, the the requests
take a lot of time to complete.

I haven't actually tested any of the code, so there may be bugs.

The next thing I plan to work on is a file cache for filesystems
to use.  It would handle the tasks of calling the underlying
block device driver to schedule I/O and caching the buffers
in memory.  The pages of a file cache would be used by read() and
write() system calls, and also the VM system (for memory-mapped
file I/O.)

-Dave