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:
 <BL3PR12MB6571D8D8CD1605B5FEE7915BC928A@BL3PR12MB6571.namprd12.prod.outlook.com>
Date: Mon, 11 Aug 2025 17:41:08 +0000
From: "Gupta, Suraj" <Suraj.Gupta2@....com>
To: Jakub Kicinski <kuba@...nel.org>, "Pandey, Radhey Shyam"
	<radhey.shyam.pandey@....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>,
	"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

[Public]

> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Monday, August 11, 2025 10:13 PM
> To: Pandey, Radhey Shyam <radhey.shyam.pandey@....com>
> Cc: Gupta, Suraj <Suraj.Gupta2@....com>; andrew+netdev@...n.ch;
> davem@...emloft.net; edumazet@...gle.com; pabeni@...hat.com; Simek,
> Michal <michal.simek@....com>; sean.anderson@...ux.dev;
> horms@...nel.org; netdev@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.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
>
> Caution: This message originated from an External Source. Use proper
> caution when opening attachments, clicking links, or responding.
>
>
> On Mon, 11 Aug 2025 15:55:02 +0000 Pandey, Radhey Shyam wrote:
> > > 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.
> >
> > Yes, IIRC there is 1:1 mapping for RX DMA callback and
> > axienet_rx_submit_desc(). In case there are failure in
> > axienet_rx_submit_desc() it is not able to reattempt in current
> > implementation. Theoretically there could be other error in
> > rx_submit_desc() (like dma_mapping/netdev
> > allocation)
> >
> > One thought is to have some flag/index to tell that it should be
> > reattempted in subsequent axienet_rx_submit_desc() ?
>
> Yes, some kind of counter of buffer that need to be allocated.
> The other problem to solve is when the buffers are completely depleted there
> will be no callback so no opportunity to refill.
> For drivers which refill from NAPI this is usually solved by periodically
> scheduling NAPI.
I think even after this same problem will exist in dmaengine flow because main problem here is axidma descriptor is freed after invoking callback. Callback (axienet_rx_cb()) will try to request new descriptor in rx_submit_desc(), but it won't be free till that time.
One possible solution I can think of is together with NAPI, having some DMAengine interface invoked by AXIDMA (similar to callback, say post_cb()) after freeing descriptor, which requests new descriptor from AXIDMA?