[mpf70-devel] info about the cache used by the kernel module

[Jeremy <jer...@m4...>]

The mpf70 kernel module has a cache whose size is a multiple
(MPF70_CACHE = 2 at present) of MPF70_BLOCK (=32) sectors. The reason
for this cache is that the internal memory of the player is a flash
device and whenever a write operation is requested to one or more
sectors, the full corresponding block first gets erased! I had
originally tried to set the block device's physical sector size to
match the block size, but this value is too large and the kernel
refuses it. Anyway, as the filesystem is FAT, the block device has to
support 512 byte writes. Alan Cox was the one who suggested a buffer
mechanism in response to a message I posted on the Linux kernel
mailing list.

The idea of the cache is that as we buffer sector write requests to
the player until the sector we are asked to write is outside of the
cache. A map of the sectors in the cache is maintained, so that when
it is time to flush the cache, we know if any sectors need to be read
off the player before we actually start writing.

In practice, when writing files to the player we always write full
blocks (except the very last sectors of the file) so no reading is
required. Updating the FAT is a different story, as we only get a
request to write a single 512 byte sector so here we do have to read
the missing sectors from the player before the write occurs. This does
not really cause a big performance hit as there a very few writes to
the FAT compared to actual data transfers.

Jeremy

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