Re: [PATCH net] net: xilinx: axienet: Increment Rx skb ring head pointer after BD is successfully allocated in dmaengine flow

[Jakub Kicinski <kuba@kernel.org>]

On Sat, 9 Aug 2025 20:31:40 +0000 Gupta, Suraj wrote:
> > The fix itself seems incomplete. Even if we correctly skip the increment we
> > will never try to catch up with the allocations, the ring will have fewer
> > outstanding Rx skbs until reset, right? Worst case we drop all the skbs and the
> > ring will be empty, no Rx will happen until reset.
> > The shutdown path seems to be checking for skb = NULL so I guess it's correct
> > but good to double check..
> 
> I agree that Rx ring will have fewer outstanding skbs. But I think
> that difference won't exceed one anytime as descriptors submission
> will fail only once due to insufficient space in AXIDMA BD ring. Rest
> of the time we already will have an extra entry in AXIDMA BD ring.
> Also, invoking callback (where Rx skb ring hp is filled in
> axienet)and freeing AXIDMA BD are part of same tasklet in AXIDMA
> driver so next callback will only be called after freeing a BD. I
> tested running stress tests (Both UPD and TCP netperf). Please let me
> know your thoughts if I'm missing something.

That wasn't my reading, maybe I misinterpreted the code.

From what I could tell the driver tries to give one new buffer for each
buffer completed. So it never tries to "catch up" on previously missed
allocations. IOW say we have a queue with 16 indexes, after 16 failures
(which may be spread out over time) the ring will be empty.