Re: [PATCH] ALSA: hda - allow 40 bit DMA mask for NVidia devices

[Mike Travis]

On 10/18/2016 5:49 AM, Ard Biesheuvel wrote:
> On 18 October 2016 at 13:46, Mike Travis <travis@xxxxxxx> wrote:
>>
>> Have you tested this change on a system which has physical
>> addressing > 40 bits to insure that addresses > 40 bits are
>> not simply truncated to an incorrect (mod 40) address?
> 
> The device should never see addresses > 40 bits, given that the DMA
> mask is set to 40 bits not 64.

I agree that the device should never see that.  But how is that
restriction imposed?  If a user presents an buffer address > 40 bits
how would that be converted to useful buffer at <= 40 bits?

I believe I tried this (at the time the kernel was at version 2.6)
and it did not work.  Since audio I/O was such a low (relative)
bandwidth, it was not worth the effort to rework the audio code
to allow buffers > 32 bits but <= 40 bits.  It may have changed
since then?

> 
>> By not restricting the addresses to 32 bits you are not
>> restricting the device to the DMA32 zone.
>>
>> This is what caused the original panic on UV systems.
>>
>> Btw, the newest nVidia GPUs have > 40 bits of DMA address bits.
>>
> 
> Thanks,
> Ard.
>