Re: Problem with DMA on x86_64 with 3 GB RAM

[Larry Finger <Larry.Finger@lwfinger.net>]

Andi Kleen wrote:
> Larry Finger <Larry.Finger@lwfinger.net> writes:
> 
>> I am trying to debug a bcm43xx DMA problem on an x86_64 system with 3
>> GB RAM. Depending on the particular chip and its implementation, dma
>> transfers may use 64-, 32-, or 30-bit addressing, with the problem
>> interface using 30-bit addressing. From test prints, the correct mask
>> (0x3FFFFFFF) is supplied to pci_set_dma_mask and
>> pci_set_consistent_dma_mask. Neither call returns an error. In
>> addition, several x86_64 systems with more than 1 GB RAM have worked
>> with the current code.
> 
> 30bit DMA has be bounced through GFP_DMA. The driver needs special
> code for this. You can look at the b44 driver for a working reference.
> 
> The pci_dma_* interfaces on x86-64 only support masks >= 0xffffffff,
> anything smaller has to be handled manually.

Thanks for the info. I obviously get to devote some effort to seeing how the b44 code handles the 
same problem.

Shouldn't this problem be mentioned somewhere in the documentation, or did I miss something?

Larry