linux-kernel - Re: [PATCH net-next v5 08/16] net: ethtool: Add a command to expose current time stamping layer

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Date:   Fri, 13 Oct 2023 09:30:56 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     Köry Maincent <kory.maincent@...tlin.com>,
        Florian Fainelli <florian.fainelli@...adcom.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Jay Vosburgh <j.vosburgh@...il.com>,
        Andy Gospodarek <andy@...yhouse.net>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Claudiu Beznea <claudiu.beznea@...on.dev>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        UNGLinuxDriver@...rochip.com,
        Broadcom internal kernel review list 
        <bcm-kernel-feedback-list@...adcom.com>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Richard Cochran <richardcochran@...il.com>,
        Radu Pirea <radu-nicolae.pirea@....nxp.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Michael Walle <michael@...le.cc>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Maxime Chevallier <maxime.chevallier@...tlin.com>
Subject: Re: [PATCH net-next v5 08/16] net: ethtool: Add a command to expose
 current time stamping layer

On Fri, 13 Oct 2023 19:14:46 +0300 Vladimir Oltean wrote:
> > What is "PRECISION"? DMA is a separate block like MAC and PHY.  
> 
> If DMA is a separate block like MAC and PHY, can it have its own PHC
> device, and the ethtool UAPI only lists the timestamping-capable PHCs
> for one NIC, and is able to select between them? 

Possibly, I guess. There are some devices which use generic (i.e.
modeled by Linux as separate struct device) DMA controllers to read 
out packets from "MAC" FIFOs. In practice I'm not sure if any of those
DMA controllers has time stamping capabilities.

> Translation between the UAPI-visible PHC index and MAC, DMA, phylib
> PHY, other PHY etc can then be done by the kernel as needed.

Translation by the kernel at which point?

IMHO it'd indeed be clearer for the user to have an ability to read 
the PHC for SOF_..._DMA via ETHTOOL_MSG_TS_LIST_GET_REPLY as a separate
entry, rather than e.g. assume that DMA uses the same PHC as MAC.