Subject: [PATCH 02/12] net: axienet: Handle 0 packet receive gracefully

From: [email-masked] (Joe Perches)
Date: Tue, 05 May 2015 06:57:44 -0700

On Tue, 2015-05-05 at 11:25 +0200, Michal Simek wrote:
> From: Peter Crosthwaite <[email-masked]>
> 
> 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.