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

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Date:	Wed, 21 Nov 2012 22:47:39 -0500
From:	Jeff Garzik <jgarzik@...ox.com>
To:	David Woodhouse <dwmw2@...radead.org>
CC:	Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] 8139cp: set ring address after enabling C+ mode

On 11/21/2012 03:27 PM, David Woodhouse wrote:
> This fixes (for me) a regression introduced by commit b01af457 ("8139cp:
> set ring address before enabling receiver"). That commit configured the
> descriptor ring addresses earlier in the initialisation sequence, in
> order to avoid the possibility of triggering stray DMA before the
> correct address had been set up.
>
> Unfortunately, it seems that the hardware will scribble garbage into the
> TxRingAddr registers when we enable "plus mode" Tx in the CpCmd
> register. Observed on a Traverse Geos router board.
>
> To deal with this, while not reintroducing the problem which led to the
> original commit, we augment cp_start_hw() to write to the CpCmd register
> *first*, then set the descriptor ring addresses, and then finally to
> enable Rx and Tx in the original 8139 Cmd register. The datasheet
> actually indicates that we should enable Tx/Rx in the Cmd register
> *before* configuring the descriptor addresses, but that would appear to
> re-introduce the problem that the offending commit b01af457 was trying
> to solve. And this variant appears to work fine on real hardware.
>
> Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
> Cc: stable@...nel.org [3.5+]
>
> ---
> How about this? I'm still somewhat confused about when it actually
> *does* start doing DMA, given what the datasheet says.

Well, we have three logical code states:

State A:  pre-b01af457, known working
State B:  b01af457, known broken
State C:  dwmw2 proposed fix, tested on 1 hardware, new technique, query 
open with Realtek

State A seems safer for late -rc, which is where we are.  Fix the 
regression by reverting to well-tested, widely deployed state.

Then apply your patch here as an immediate candidate for net-next.

	Jeff






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