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Date:	Thu, 12 May 2016 16:09:20 +0200
From:	Francois Romieu <romieu@...zoreil.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	David Miller <davem@...emloft.net>, 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>,
	Chunhao Lin <hau@...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
[...]
> > 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 a really outdated reasoning behind the current default setting.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.

I doubt there will be much *reporting* from broken systems that
include plain old PCI realtek chipsets (r8169.c::RTL_CFG_0). Changing
the default for those is imnsho asking for troubles without clear
benefit (experimental evidence suggests that smsc etherpower II grows
older more easily than plain pci 8169 :o/ ).

I'd rather leave these alone and change the default for the PCI Express
chipsets. Btw, while it does not seem to hurt, they should not need any
CPlusCmd Dual Access Cycle tweak either. Realtek may establish it (Lin ?)

A few news from the "pathologically better safe than sorry" squad:
I have switched the default on a couple of non-critical production
servers that include 8168c (RTL_GIGA_MAC_VER_22). It should give an hint
for hardware from 2008 ~ 2009. I'll do some basic sanity testing with
different chipsets.

-- 
Ueimor

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