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

On 7/17/2020 2:57 PM, Vladimir Oltean wrote:

On Sat, Jul 18, 2020 at 12:13:42AM +0300, Sergey Organov wrote:

quoted

Vladimir Oltean [off-list ref] writes:

quoted

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.

This approach doesn't seem bad to me. We then document that
NETIF_F_PHY_HWTSTAMP should only set of the correct conditions are met.

Thanks,
-Vladimir