netdev - Re: [EXTERNAL] Re: [PATCH V2,net-next, 1/2] net: mana: Add support for coalesced RX packets on CQE

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Message-ID: <20260112172146.04b4a70f@kernel.org>
Date: Mon, 12 Jan 2026 17:21:46 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Haiyang Zhang <haiyangz@...rosoft.com>
Cc: Haiyang Zhang <haiyangz@...ux.microsoft.com>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>, KY Srinivasan
 <kys@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, Dexuan Cui
 <DECUI@...rosoft.com>, Long Li <longli@...rosoft.com>, Andrew Lunn
 <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, Eric
 Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Konstantin
 Taranov <kotaranov@...rosoft.com>, Simon Horman <horms@...nel.org>, Erni
 Sri Satya Vennela <ernis@...ux.microsoft.com>, Shradha Gupta
 <shradhagupta@...ux.microsoft.com>, Saurabh Sengar
 <ssengar@...ux.microsoft.com>, Aditya Garg
 <gargaditya@...ux.microsoft.com>, Dipayaan Roy
 <dipayanroy@...ux.microsoft.com>, Shiraz Saleem
 <shirazsaleem@...rosoft.com>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "linux-rdma@...r.kernel.org"
 <linux-rdma@...r.kernel.org>, Paul Rosswurm <paulros@...rosoft.com>
Subject: Re: [EXTERNAL] Re: [PATCH V2,net-next, 1/2] net: mana: Add support
 for coalesced RX packets on CQE

On Mon, 12 Jan 2026 21:01:59 +0000 Haiyang Zhang wrote:
> > > Our NIC can have up to 4 RX packets on 1 CQE. To support this feature,
> > > check and process the type CQE_RX_COALESCED_4. The default setting is
> > > disabled, to avoid possible regression on latency.
> > >
> > > And add ethtool handler to switch this feature. To turn it on, run:
> > >   ethtool -C <nic> rx-frames 4
> > > To turn it off:
> > >   ethtool -C <nic> rx-frames 1  
> > 
> > Exposing just rx frame count, and only two values is quite unusual.
> > Please explain in more detail the coalescing logic of the device.  
> Our NIC device only supports coalescing on RX. And when it's disabled each
> RX CQE indicates 1 RX packet; when enabled each RX CQE indicates up to 4 packets.

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?