Message-ID: <1430834264.7191.9.camel@perches.com>
Subject: Re: [PATCH 02/12] net: axienet: Handle 0 packet receive gracefully
From: Joe Perches <joe@perches.com>
To: Michal Simek <michal.simek@xilinx.com>
Cc: netdev@vger.kernel.org, Peter Crosthwaite <peter.crosthwaite@xilinx.com>,
        =?ISO-8859-1?Q?S=F6ren?= Brinkmann <soren.brinkmann@xilinx.com>,
        monstr@monstr.eu, John Linn <John.Linn@xilinx.com>,
        Anirudha Sarangi <anirudh@xilinx.com>, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
Date: Tue, 05 May 2015 06:57:44 -0700
In-Reply-To: <55b3c89b3549c61d62b8440636516fd572870842.1430817941.git.michal.simek@xilinx.com>
References: <7fb84f65a61bbe0fdb4b61a871cf4d4f7910955d.1430817941.git.michal.simek@xilinx.com>
	 <55b3c89b3549c61d62b8440636516fd572870842.1430817941.git.michal.simek@xilinx.com>
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On Tue, 2015-05-05 at 11:25 +0200, Michal Simek wrote:
> From: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
> 
> The AXI-DMA rx-delay interrupt can sometimes be triggered
> when there are 0 outstanding packets received. This is due
> to the fact that the receive function will greedily consume
> as many packets as possible on interrupt. So if two packets
> (with a very particular timing) arrive in succession they
> will each cause the rx-delay interrupt, but the first interrupt
> will consume both packets.
> This means the second interrupt is a 0 packet receive.
> 
> This is mostly OK, except that the tail pointer register is
> updated unconditionally on receive. Currently the tail pointer
> is always set to the current bd-ring descriptor under
> the assumption that the hardware has moved onto the next
> descriptor. What this means for length 0 recv is the current
> descriptor that the hardware is potentially yet to use will
> be marked as the tail. This causes the hardware to think
> its run out of descriptors deadlocking the whole rx path.
> 
> Fixed by updating the tail pointer to the most recent
> successfully consumed descriptor.

I think some of this would be good to have as comments
in the code instead of just in the changelog.


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