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

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Date:	Wed, 11 May 2016 19:58:53 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	ard.biesheuvel@...aro.org
Cc:	netdev@...r.kernel.org, ricardo.salveti@...aro.org,
	leo.duran@....com, graeme.gregory@...aro.org, nic_swsd@...ltek.com
Subject: Re: [PATCH] r8169: default to 64-bit DMA on systems without memory
 below 4 GB

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a really outdated reasoning behind the current default setting.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.