netdev - Re: forcedeth: MAC-address reversed on resume from suspend

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Message-ID: <20080104224352.GA7853@rhlx01.hs-esslingen.de>
Date:	Fri, 4 Jan 2008 23:43:52 +0100
From:	Andreas Mohr <andi@...as.de>
To:	Björn Steinbrink <B.Steinbrink@....de>
Cc:	Andreas Mohr <andi@...as.de>, Adrian Bunk <bunk@...nel.org>,
	Richard Jonsson <richie@...erworld.net>,
	linux-kernel@...r.kernel.org, Ayaz Abdulla <aabdulla@...dia.com>,
	jgarzik@...ox.com, netdev@...r.kernel.org
Subject: Re: forcedeth: MAC-address reversed on resume from suspend

On Fri, Jan 04, 2008 at 11:17:40AM +0100, Björn Steinbrink wrote:
> On 2008.01.04 09:45:17 +0100, Andreas Mohr wrote:
> > And then it needs these card I/O functions wrapped into two functions which
> > interface with driver- and OS-related MAC variables
> > (struct variables ALWAYS stored in usual system order, NOT H/W order!!!!!!)
> > which optionally reverse the address (if needed for a particular card).
> 
> Hu? The MAC address is only ever reversed when the card is not in use,
> why the hell would you want to reverse anything when the rest of the OS
> is involved? This whole reversing stuff is purely before and after the
> card is actually used. It's not that you need to reverse the address to
> communicate with the card, it's just initially wrong on the card.

Huhrmm? OK, let me ask this then:
So what you're saying is that the address is only initially wrong
(e.g. due to card EEPROM content somehow initializing the registers
in wrong order on power-on), it's not _always_ supposed to be stored
in wrong order during operation.

IOW, the default card state after power-on is _unusable_ (due to
invalidly ordered MAC address) and has to be _corrected_ by the driver,
_initially_ only?

OTOH I know that at least acx100 has a _permanently_ wrong-ordered
MAC address setup, i.e. it's required to have it in "wrong" order
_during operation_. And I wouldn't be surprised to see several examples
of other hardware having a permanently wrongly-ordered address
requirement, given the amount of MAC reversal in Linux drivers.

Couldn't it be by chance that it's _believed_ to be reversed-on-power-on only,
whereas those cards should _actually_ have it reversed-during-lifetime
instead? Such a misunderstanding might explain a lot of trouble...

Obviously I was expecting the latter case, which my code layout proposal
was supposed to support in a clean way.

> > And then there will never be any confusion about any MAC address format
> > anywhere any more, right?
> 
> At probing time, you'll have to know whether the address you read from
> the hardware is reversed or not. Unless you write it back in reversed
> order when you release the card, you need a flag that at least survives
> until nv_probe is called the next time. kexec does not write it back, so
> you do need such a flag. But the flag apparently doesn't survive a
> suspend/resume cycle, so you need to write back the reversed address
> then. But the flag will survive a rmmod + modprobe cycle, so you need to
> reset it when writing back the reversed address.

If it's indeed reversed-on-power-on only, then one probably cannot do
much about it, thus I'd give up and simply check the MAC address for
validity (should be very easy to check for a simple reversal by
checking for the manufacturer-specific fingerprint in reverse order).
Keeping _any_ sort of state to record the fact that it used to be reversed
or now is reversed is futile (or rather: catastrophic) given the complexity of
suspend / virtualisation or whichever other nice operations Linux may invent
in the future ;)

> Well, let's see if my analysis was correct and the patch works, first. I
> saw the forcedeth code before, but kexec and suspend is totally new to
> me and I just made some assumptions based on the reported behaviour and
> the commit messages... ;-)

You most likely did more analysis than me, I just said
"this very obviously must be the problem" and replied ;)

Andreas Mohr
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