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

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Message-ID: <20250811083738.04bf1e31@kernel.org>
Date: Mon, 11 Aug 2025 08:37:38 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: "Gupta, Suraj" <Suraj.Gupta2@....com>
Cc: "andrew+netdev@...n.ch" <andrew+netdev@...n.ch>, "davem@...emloft.net"
 <davem@...emloft.net>, "edumazet@...gle.com" <edumazet@...gle.com>,
 "pabeni@...hat.com" <pabeni@...hat.com>, "Simek, Michal"
 <michal.simek@....com>, "sean.anderson@...ux.dev"
 <sean.anderson@...ux.dev>, "Pandey, Radhey Shyam"
 <radhey.shyam.pandey@....com>, "horms@...nel.org" <horms@...nel.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "Katakam, Harini" <harini.katakam@....com>
Subject: 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:
> > 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.