netdev - Re: [RFC PATCH net-next 7/9] net: dsa: microchip: ksz9477: add hardware time stamping support

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Message-ID: <20201022113243.4shddtywgvpcqq6c@skbuf>
Date:   Thu, 22 Oct 2020 14:32:43 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Christian Eggers <ceggers@...i.de>
Cc:     Richard Cochran <richardcochran@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Helmut Grohne <helmut.grohne@...enta.de>,
        Paul Barker <pbarker@...sulko.com>,
        Codrin Ciubotariu <codrin.ciubotariu@...rochip.com>,
        George McCollister <george.mccollister@...il.com>,
        Marek Vasut <marex@...x.de>,
        Tristram Ha <Tristram.Ha@...rochip.com>,
        "David S . Miller" <davem@...emloft.net>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net-next 7/9] net: dsa: microchip: ksz9477: add
 hardware time stamping support

On Thu, Oct 22, 2020 at 01:11:40PM +0200, Christian Eggers wrote:
> Hi Vladimir,
> 
> On Thursday, 22 October 2020, 12:50:14 CEST, Vladimir Oltean wrote:
> after applying the RX timestamp correctly to the correction field (shifting
> the nanoseconds by 16),

That modification should have been done anyway, since the unit of
measurement for correctionField is scaled ppb (48 bits nanoseconds, 16
bits scaled nanoseconds), and not nanoseconds.

> it seems that "moving" the timestamp back to the tail tag on TX is not
> required anymore. Keeping the RX timestamp simply in the correction
> field (negative value), works fine now. So this halves the effort in
> the tag_ksz driver.

Ok, this makes sense.
Depending on what Richard responds, it now looks like the cleanest
approach would be to move your implementation that is currently in
ksz9477_update_ptp_correction_field() into a generic function called

static inline void ptp_onestep_p2p_move_t2_to_correction(struct sk_buff *skb,
							 unsigned int ptp_type,
							 struct ptp_header *ptp_header,
							 ktime_t t2)

You should then clearly document that this function is needed for
hardware capable of one-step P2P that does not already modify the
correctionField of Pdelay_Req event messages on ingress.