Re: Initial Promise SX4 hw docs opened

Jeff Garzik
2007-01-08T13:40:43Z

Dan Williams wrote:
> On 1/3/07, Jeff Garzik <jeff@garzik.org> wrote:
>> The first open hardware docs for the Promise SX4 (sata_sx4) series are
>> now available:
>> http://gkernel.sourceforge.net/specs/promise/pdc20621-pguide-dimm-1.6.pdf.bz2
>>
>> http://gkernel.sourceforge.net/specs/promise/pdc20621-pguide-pll-ata-timing-1.2.pdf.bz2
>>
>>
>> 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
>> knowledge.
>>
> Can the XOR engines on the card access host memory or do they only
> operate on the card's local memory? And are there DMA (copy) engines?

XOR only operates on the card's local memory.

There is one DMA-copy engine, for copying host<->card memory. It can be
polled or interrupt-driven. There are four ATA engines (one for each
port), each with DMA engines of their own that only talk to the card's
local memory.

>> 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.
>>
> [ brainstorming ]
> For the RAID5 case: With the hardware acceleration patches the part of
> MD that actually operates on the stripe cache is factored out of
> handle_stripe5. So MD can be used to handle all of the cache state
> information and then use a SX4 specific operations routine to handle
> the data transfers. However this assumes that all the members of the
> array are behind the SX4 (which is the same assumption the vendor
> driver makes so maybe its not so bad).
>
> The part I am lost at it is how do you tell libata and the lldd that
> data for a transaction is coming from or headed to device local memory
> and not host memory? It seems there would need to be changes to the
> dma api to handle this distinction?

That is indeed the tough part. Open questions. I'm not even convinced
that its a good idea to pretend that card local memory is accessible
through the existing DMA API... abstracting that may cost more than
inventing a new API for such a situation.

Jeff