Re: [PATCH] ALSA: hda - allow 40 bit DMA mask for NVidia devices
From: Mike Travis
Date: Tue Oct 18 2016 - 09:05:04 EST
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
>> 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.