linux-kernel - Re: [PATCH 2/6]: powerpc/cell spidernet low watermark patch.

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Message-Id: <200608201203.15645.arnd@arndb.de>
Date:	Sun, 20 Aug 2006 12:03:14 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linuxppc-dev@...abs.org, akpm@...l.org,
	James K Lewis <jklewis@...ibm.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Jeff Garzik <jgarzik@...ox.com>,
	ens Osterkamp <Jens.Osterkamp@...ibm.com>
Subject: Re: [PATCH 2/6]: powerpc/cell spidernet low watermark patch.

On Sunday 20 August 2006 08:31, Benjamin Herrenschmidt wrote:
> > card->low_watermark->next->dmac_cmd_status |= SPIDER_NET_DESCR_TXDESFLG;
> > mb();
> > card->low_watermark->dmac_cmd_status &= ~SPIDER_NET_DESCR_TXDESFLG;
> > card->low_watermark = card->low_watermark->next;
> > 
> > when we queue another frame for TX.
> 
> I would have expected those to be racy vs. the hardware... what if the
> hardware is updating dmac_cmd_status just as your are trying to and the
> bit out of it ?

Right, that doesn't work. It is the only bit we use in that byte though,
so maybe it can be done with a single byte write.

	Arnd <><
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