netdev - Re: [PATCH] 8139cp: set ring address after enabling C+ mode

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Date:	Wed, 21 Nov 2012 22:32:11 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Francois Romieu <romieu@...zoreil.com>
Cc:	Jeff Garzik <jgarzik@...ox.com>, Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Hayes Wang <hayeswang@...ltek.com>, gilboad@...il.com
Subject: Re: [PATCH] 8139cp: set ring address after enabling C+ mode

On Wed, 2012-11-21 at 21:40 +0100, Francois Romieu wrote:
> Straight to -stable ?

That's the way it works. You put the Cc: stable on the *original* commit
that goes upstream. There's no sane way to retroactively add that tag
after it's already been merged and tested.

Yes, you can bug Greg manually to 'please add this upstream commit which
we forgot to mark as Cc: stable' but that isn't the way it's usually
done.

> Afaik nobody complained from the original (pre b01af457) problem on
> real hardware.
>
> May be someone @realtek (hi Hayes) can give an explanation regarding
> the CpCmd, RingAddr, Cmd init sequence and the start of DMA.

That would be really useful; thanks. To recap for Hayes' benefit: the
concern is that if we follow the instructions in §6.33 of the data
sheet:

Recommendation to C+ mode programming: Enable C+ mode functions in C+CR
register first, => Enable transmit/receive in Command register (offset
37h), => Configure other related registers (ex. Descriptor start
address, TCR, RCR, ...).

... then we appear to be starting up the DMA before we actually tell it
the descriptor ring addresses, which will cause stray DMA to random
unconfigured addresses!

Is there some detail of the hardware which prevents this from actually
happening? Or if not, is my proposed workaround (enabling Tx/Rx in the
C+ Command Register *first*, then setting the descriptor addresses, and
enabling Tx/Rx in the old-style Command register last) OK?

It was observed that when setting the descriptor addresses *first*, the
Transmit Descriptor Start Address Register was getting overwritten with
garbage when we enabled Tx in the C+ Command Register.

I note that we're also setting a bunch of other things in the Rx and Tx
config registers *after* operation all seems to have started up... is
that OK too?

-- 
dwmw2

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