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

On Sat, 9 Aug 2025 20:31:40 +0000 Gupta, Suraj wrote:

quoted

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.