netdev - Re: [PATCH] r8169: default to 64-bit DMA on systems without memory below 4 GB

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Date:	Thu, 12 May 2016 09:58:04 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	David Miller <davem@...emloft.net>
Cc:	"<netdev@...r.kernel.org>" <netdev@...r.kernel.org>,
	Ricardo Salveti <ricardo.salveti@...aro.org>,
	Leo Duran <leo.duran@....com>,
	G Gregory <graeme.gregory@...aro.org>,
	Realtek linux nic maintainers <nic_swsd@...ltek.com>
Subject: Re: [PATCH] r8169: default to 64-bit DMA on systems without memory
 below 4 GB

On 12 May 2016 at 01:58, David Miller <davem@...emloft.net> wrote:
> From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> Date: Wed, 11 May 2016 09:47:49 +0200
>
>> The current logic around the 'use_dac' module parameter prevents the
>> r81969 driver from being loadable on 64-bit systems without any RAM
>> below 4 GB when the parameter is left at its default value.
>> So introduce a new default value -1 which indicates that 64-bit DMA
>> should be enabled implicitly, but only if setting a 32-bit DMA mask
>> has failed earlier. This should prevent any regressions like the ones
>> caused by previous attempts to change this code.
>>
>> Cc: Realtek linux nic maintainers <nic_swsd@...ltek.com>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>
> I think we should just seriously consider changing the default, it's
> a really outdated reasoning behind the current default setting.  Maybe
> relevant a decade ago, but probably not now.
>
> And if the card is completely disfunctional in said configuration, the
> default is definitely wrong.

The card is indeed completely disfunctional. So we could try to
resurrect 353176888386 ("r8169: enable 64-bit DMA by default for PCI
Express devices"), and instead of backing it out again if regressions
are reported, blacklist the particular chips. This is a much better
approach, since then we can also print some kind of diagnostic on
those arm64 systems why such a blacklisted NIC is not supported.