> -----Original Message-----
> From: Jakub Kicinski <[email protected]>
> Sent: Tuesday, January 13, 2026 8:10 PM
> To: Haiyang Zhang <[email protected]>
> Cc: Haiyang Zhang <[email protected]>; linux-
> [email protected]; [email protected]; KY Srinivasan
> <[email protected]>; Wei Liu <[email protected]>; Dexuan Cui
> <[email protected]>; Long Li <[email protected]>; Andrew Lunn
> <[email protected]>; David S. Miller <[email protected]>; Eric
> Dumazet <[email protected]>; Paolo Abeni <[email protected]>; Konstantin
> Taranov <[email protected]>; Simon Horman <[email protected]>; Erni
> Sri Satya Vennela <[email protected]>; Shradha Gupta
> <[email protected]>; Saurabh Sengar
> <[email protected]>; Aditya Garg
> <[email protected]>; Dipayaan Roy
> <[email protected]>; Shiraz Saleem
> <[email protected]>; [email protected]; linux-
> [email protected]; Paul Rosswurm <[email protected]>
> Subject: Re: [EXTERNAL] Re: [PATCH V2,net-next, 1/2] net: mana: Add
> support for coalesced RX packets on CQE
>
> On Tue, 13 Jan 2026 15:13:24 +0000 Haiyang Zhang wrote:
> > > > I get that. What is the logic for combining 4 packets into a single
> > > > completion? How does it work? Your commit message mentions
> "regression
> > > > on latency" - what is the bound on that regression?
> > >
> > > When we received CQE type CQE_RX_COALESCED_4, it's a coalesced CQE.
> And in
> > > the CQE OOB, there is an array with 4 PPI elements, with each pkt's
> length:
> > > oob->ppi[i].pkt_len.
> > >
> > > So we read the related WQE and the DMA buffers for the RX pkt
> payloads, up
> > > to 4.
> > > But, if the coalesced pkts <4, the pkt_len will be 0 after the last
> pkt,
> > > so we know when to stop reading the WQEs.
> >
> > And, the coalescing can add up to 2 microseconds into one-way latency.
>
> I am asking you how the _device_ (hypervisor?) decides when to coalesce
> and when to send a partial CQE (<4 packets in 4 pkt CQE). You are using
> the coalescing uAPI, so I'm trying to make sure this is the correct API.
> CQE configuration can also be done via ringparam.
When coalescing is enabled, the device waits for packets which can
have the CQE coalesced with previous packet(s). That coalescing process
is finished (and a CQE written to the appropriate CQ) when the CQE is
filled with 4 pkts, or time expired, or other device specific logic is
satisfied.
Thanks,
- Haiyang
