netdev - Re: [PATCH net-next] vlan: Pass ethtool get_ts

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Message-ID: <1416763199.7215.53.camel@decadent.org.uk>
Date:	Sun, 23 Nov 2014 17:19:59 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	Stefan Sørensen 
	<stefan.sorensen@...ctralink.com>
Subject: Re: [PATCH net-next] vlan: Pass ethtool get_ts_info queries to real
 device.

On Sun, 2014-11-23 at 17:05 +0100, Richard Cochran wrote:
> On Sun, Nov 23, 2014 at 03:15:12AM +0000, Ben Hutchings wrote:
> > This assumes that the same PTP capabilities apply to VLAN-tagged frames.
> > I don't think it's at all safe to assume that RX filter modes other than
> > HWTSTAMP_FILTER_ALL will include VLAN-tagged frames.  I think it is
> > necessary to define additional modes that explicitly include VLAN-tagged
> > frames.
> 
> Unsafe? How?

I mean that the assumption is wrong in general.  So these changes will
result in silent failure to enable RX timestamps, on some devices.

> Do you mean that some HW cannot identify and time stamp PTP frames
> when VLAN tagged? That is surely disappointing for people who shell
> out money for such cards, but it is hardly unsafe.
>
> > I also disagree in general that reconfiguring a VLAN device should make
> > changes to the underlying device that affect more than just that VLAN,
> > i.e. SIOCSHWTSTAMP should not be passed through.  SIOCGHWTSTAMP could be
> > passed through, but rx_filter would need adjustment (VLAN-tagged modes
> > on the underlying devices become untagged modes on the VLAN device).
> 
> The whole filter list with every last combination (at least, the ones
> at the time) came directly from a early, limited HW design.  Sane,
> modern PTP hardware provides time stamps regardless of whether a frame
> is VLAN tagged or not.

I would hope so.  However, this is non-standard - IEEE802.1AS
*explicitly forbids* the use of VLAN tags.  Linux's software PTP
classifier also doesn't yet handle 802.1ad VLAN tags.

> I don't see any reason not to make our stack even more ugly just to cater
> to broken hardware.

So failure to implement a non-standard extension is now 'broken'?

> I have nothing against adding VLAN to the SIOCGHWTSTAMP list, because
> the hardware people *really* use all have:
> 
> 	HWTSTAMP_TX_ON, and
> 	HWTSTAMP_FILTER_ALL, or 
> 	HWTSTAMP_FILTER_PTP_V2_EVENT,
> 
> So adding more won't hurt. (But it won't help either. If your HW
> cannot time stamp Layer2 and you transmit Layer2, you simply never get
> a time stamp.)
>
> But please don't hold up progress just for this sort of thing.

This is progress for some devices, false advertising for others.

Ben.

-- 
Ben Hutchings
Never put off till tomorrow what you can avoid all together.

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