Messages in this thread | | | Date | Mon, 08 Jan 2007 08:40:39 -0500 | From | Jeff Garzik <> | Subject | Re: Initial Promise SX4 hw docs opened |
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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
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