DMA transfer speed issues on PCI card with PCI and PCI-X busses

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Hi all,

we're using a PCI card with large on-board memory and a PCI interface chip with DMA busmaster engine (PCI9656 from PLX but only using 32 bit width). Data transfer is done using Scatter Gather DMA and is running stable. The card is working in PCI and PCI-X busses with 33 MHz and 66 MHz.

When measuring the DMA transfer speed I get some strange results:

used in a standard 32 bit PCI slot:
read: card to PC memory: 80-100 MB/s
write: PC memory to card: 80-100 MB/s

used in a 64 bit PCI-X slot with 66 MHz:
read: card to PC memory: 200-220 MB/s
write: PC memory to card: 40-50 MB/s

using an older design with a pure 33 MHz 32 bit interface chip (PCI9080)in a PCI-X slot:
read: card to PC memory: 80-100 MB/s
write: PC memory to card: 40-50 MB/s

Concerning the read direction the speed is as expected but as soon as a PCI-X slot is involved the bus transfer speed in write direction drops a lot. The Scatter-Gather list is located in PC memory what should make access easier in write direction as the PCI bus need not to change direction.

We tested at least 20 different PCI-X systems and get these results with ±10%. It can't be a local bus issue as the card is at least able to stream data with 100 MB/s on a 32 bit PCI slot.

Anybody an idea where to start searching and which setup to change?
When monitoring PCI bus signal I see that in write direction the transfer drops and is restarted after only a few bytes of transfer. But why? We modified the design to be sure that the local bus is able to run with even 250 MB/s with no latency.
There're no other card accesses while DMA is running, the driver simply waits for the interrupt.

Any help is appreciated.

Best regards
Oliver
.

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