PPC4XX DMA polarity bug in linuxppc-2.6.9

Colin Wernham CWernham at Airspan.com
Fri Dec 10 20:09:39 EST 2004

[a copy of this has also been posted on linuxppc-dev]

I believe that there is a PPC4xx DMA driver bug in the following kernel, 
file and function:

Symptoms are:
DMA does not work as the polarity bits (DMA Req, Ack, EOT) are not set 
correctly in the DMA controller using the ppc4xx_init_dma_channel() 
function call, but when this is called twice in succession, then it does 

Cause is (I believe):
On line 469 the old polarity bits in p_dma_ch are ORed in when it should 
be ORing in the new polarity bits from p_init. p_init is then saved into 
p_dma_ch. This means that it only gets correctly programmed when the 
function is called twice:

    467  /* clear all polarity signals and then "or" in new signal levels */
    468  polarity &= ~GET_DMA_POLARITY(dmanr);
    469  polarity |= p_dma_ch->polarity;

Fix should be:
    469  polarity |= p_init->polarity;

Is there anyone else using the PPC4XX DMA?

Colin Wernham

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