DMA Broken Up on MPC8248

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Bill 20 years ago

I am running Linux 2.4 on an MPC8248. In my code, I do many chaining DMAs with chains consisting of a single segment descriptor. Each segment descriptor has a valid source and destination physical address. The byte count = 0x60 and the next descriptor = 1. The DMAs seem to work as expected, i.e., the 0x60 bytes are successfully transferred from the given sources to the given destinations, but, when I look at the logic analyzer, rather than seeing single DMAs of 0x60 bytes, I see

6 separate DMAs of 0x10 bytes where there should be a single DMA of
0x60 bytes. Shouldn't it DMA 0x60 bytes in one single DMA transfer?

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Didi 20 years ago

I do not know the 8248, but I remember on the 8240 I tried to use the DMA to move memory to memory. It worked, only several times slower than when I did it using the CPU, so I abandoned it. I did not bother to look into why it was so slow, it obviously not going to get me anywhere so I did tweak the CPU move code as far as I could and am happy ever since :-).

Dimiter

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Bill wrote:

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dg 20 years ago

Play with the following parameters: BUF_INV,SS_MAX,STS,DTS.

Dimiter, Unlike the mpc8240, where the DMA was implemented in H/W, the mpc8248 IDMA is CPM microcode based. This suggests that the PQx-IDMA is more sophisticated, and can be fine tuned to specific requirements.

David Gabbay DoGav Systems

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dg 20 years ago

Play with the following parameters: BUF_INV,SS_MAX,STS,DTS.

Dimiter, Unlike the mpc8240, where the DMA was implemented in H/W, the mpc8248 IDMA is CPM microcode based. This suggests that the PQx-IDMA is more sophisticated, and can be fine tuned to specific requirements.

David Gabbay DoGav Systems

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DMA Broken Up on MPC8248

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