netdev - Re: [PATCH] net/davinci: do not use all descriptors for tx packets

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Message-ID: <20120104085001.GM5446@pengutronix.de>
Date:	Wed, 4 Jan 2012 09:50:01 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, anantgole@...com, srk@...com
Subject: Re: [PATCH] net/davinci: do not use all descriptors for tx packets

On Tue, Jan 03, 2012 at 01:51:41PM -0500, David Miller wrote:
> From: Sascha Hauer <s.hauer@...gutronix.de>
> Date: Tue,  3 Jan 2012 16:27:47 +0100
> 
> > The driver uses a shared pool for both rx and tx descriptors.
> > During open it queues fixed number of 128 descriptors for receive
> > packets. For each received packet it tries to queue another
> > descriptor. If this fails the descriptor is lost for rx.
> > The driver has no limitation on tx descriptors to use, so it
> > can happen during a nmap / ping -f attack that the driver
> > allocates all descriptors for tx and looses all rx descriptors.
> > The driver stops working then.
> > To fix this limit the number of tx descriptors used to half of
> > the descriptors available, the rx path uses the other half.
> > 
> > Tested on a custom board using nmap / ping -f to the board from
> > two different hosts.
> > 
> > Signed-off-by: Sascha Hauer <s.hauer@...gutronix.de>
> 
> Applied to net-next, thanks.
> 
> Well, at this point there is not logical reason to have a shared
> descriptor pool unless the hardware requires it.  Does it?

I don't have enough knowledge of the hardware to give an answer, but to
me it would appear like a very strange piece of hardware if it can
only receive *or* transmit depending on the type of the next descriptor.
I think the driver needs some cleanup in this area, but this is out of
skope for me at the moment.

Sascha

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