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

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Message-ID: <20121121204045.GA17627@electric-eye.fr.zoreil.com>
Date:	Wed, 21 Nov 2012 21:40:45 +0100
From:	Francois Romieu <romieu@...zoreil.com>
To:	David Woodhouse <dwmw2@...radead.org>
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

David Woodhouse <dwmw2@...radead.org> :
> 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.

Straight to -stable ?

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.

-- 
Ueimor
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