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SubjectInitial Promise SX4 hw docs opened
The first open hardware docs for the Promise SX4 (sata_sx4) series are 
now available:

These are only small, ancillary guides; the main hardware doc should be
opened soon.

However, I would like to take this opportunity to point hackers looking
for a project at this hardware. The Promise SX4 is pretty neat, and it
needs more attention than I can give, to reach its full potential.

Here's a braindump:

* It's an older chipset/board, probably not actively sold anymore

* ATA programming interface very close to sata_promise (PDC2037x)

* Contains on-board DIMM, to be used for any purpose the driver desires

* Contains on-board RAID5 XOR, also fully programmable

A key problem is that, under Linux, sata_sx4 cannot fully exploit the
RAID-centric power of this hardware by driving the hardware in "dumb ATA
mode" as it does.

A better driver would notice when a RAID1 or RAID5 array contains
multiple components attached to the SX4, and send only a single copy of
the data to the card (saving PCI bus bandwidth tremendously).
Similarly, a better driver would take advantage of the RAID5 XOR offload
capabilities, to offload the entire RAID5 read or write transaction to
the card.

All this is difficult within either the MD or DM RAID frameworks,
because optimizing each RAID transaction requires intimate knowledge of
the hardware. We have the knowledge... but I don't have good ideas --
aside from an SX4-specific RAID 0/1/5/6 driver -- on how to exploit this

Traditionally the vendor has distributed a SCSI driver that implements
the necessary RAID stack pieces entirely in the hardware driver itself.
That sort of approach definitely works, but is traditionally rejected
by upstream maintainers because it essentially requires a third (if h/w
specific) RAID stack.


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