Re: [PATCH net-next 3/5] net: dsa: mv88e6xxx: Let taggers specify a can_timestamp function

[Richard Cochran]

On Thu, May 30, 2019 at 10:42:41AM +0300, Vladimir Oltean wrote:
> The meta frames generated by the SJA1105 do not contain any seqid.

So this cannot ever work...

> They contain:
> * A globally programmable DMAC
> * A globally programmable SMAC

Don't know what these are, but doesn't sound like they uniquely
identify the original frame.

> * The 0x8 EtherType
> * A partial (24-bit or 32-bit) RX timestamp
> * Two bytes from the initial (pre follow-up) frame's DMAC, before the
> switch mangled those with the source port and switch id. The driver is
> supposed to patch these bytes from the follow-up back into the initial
> frame before passing them up the stack.
> * The source port that generated the meta frame
> * The switch id that generated the meta frame

None of these match to the original frame uniquely.  Looks like this
is a dead end.

I recommend forgetting about these meta frames.  Instead, read out the
time stamps over MDIO.

Thanks,
Richard