netdev - Re: [PATCH net-next 0/3] Document more PTP timestamping known quirks

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Date:   Sat, 18 Jul 2020 00:57:19 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Sergey Organov <sorganov@...il.com>
Cc:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        richardcochran@...il.com, linux-doc@...r.kernel.org
Subject: Re: [PATCH net-next 0/3] Document more PTP timestamping known quirks

On Sat, Jul 18, 2020 at 12:13:42AM +0300, Sergey Organov wrote:
> Vladimir Oltean <olteanv@...il.com> writes:
> 
> > I've tried to collect and summarize the conclusions of these discussions:
> > https://patchwork.ozlabs.org/project/netdev/patch/20200711120842.2631-1-sorganov@gmail.com/
> > https://patchwork.ozlabs.org/project/netdev/patch/20200710113611.3398-5-kurt@linutronix.de/
> > which were a bit surprising to me. Make sure they are present in the
> > documentation.
> 
> As one of participants of these discussions, I'm afraid I incline to
> alternative approach to solving the issues current design has than the one
> you advocate in these patch series.
> 
> I believe its upper-level that should enforce common policies like
> handling hw time stamping at outermost capable device, not random MAC
> driver out there.
> 
> I'd argue that it's then upper-level that should check PHY features, and
> then do not bother MAC with ioctl() requests that MAC should not handle
> in given configuration. This way, the checks for phy_has_hwtstamp()
> won't be spread over multiple MAC drivers and will happily sit in the
> upper-level ioctl() handler.
> 
> In other words, I mean that it's approach taken in ethtool that I tend
> to consider being the right one.
> 
> Thanks,
> -- Sergey

Concretely speaking, what are you going to do for
skb_defer_tx_timestamp() and skb_defer_rx_timestamp()? Not to mention
subtle bugs like SKBTX_IN_PROGRESS. If you don't address those, it's
pointless to move the phy_has_hwtstamp() check to net/core/dev_ioctl.c.

The only way I see to fix the bug is to introduce a new netdev flag,
NETIF_F_PHY_HWTSTAMP or something like that. Then I'd grep for all
occurrences of phy_has_hwtstamp() in the kernel (which currently amount
to a whopping 2 users, 3 with your FEC "fix"), and declare this
netdevice flag in their list of features. Then, phy_has_hwtstamp() and
phy_has_tsinfo() and what not can be moved to generic places (or at
least, I think they can), and those places could proceed to advertise
and enable PHY timestamping only if the MAC declared itself ready. But,
it is a bit strange to introduce a netdev flag just to fix a bug, I
think.

Thanks,
-Vladimir