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

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Message-ID: <40321a07-bff4-4293-d85b-bddba31d1c33@sgi.com>
Date:   Tue, 18 Oct 2016 06:04:31 -0700
From:   Mike Travis <travis@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:     Takashi Iwai <tiwai@...e.de>, <alsa-devel@...a-project.org>,
        <perex@...ex.cz>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ALSA: hda - allow 40 bit DMA mask for NVidia devices



On 10/18/2016 5:49 AM, Ard Biesheuvel wrote:
> On 18 October 2016 at 13:46, Mike Travis <travis@....com> 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.
>