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

From: Michal Simek <[email-masked]>
Date: Tue, 5 May 2015 20:49:53 +0200

On 05/05/2015 03:57 PM, Joe Perches wrote:
> 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.

Is it really needed? If yes, no problem to add it but git blame can
point you to that.

Thanks,
Michal