netdev - RE: [PATCH v1 0/4] Add support for DMA timestamp for non-PTP packets

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Date:   Fri, 30 Sep 2022 08:52:38 +0000
From:   "Zulkifli, Muhammad Husaini" <muhammad.husaini.zulkifli@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     "intel-wired-lan@...osl.org" <intel-wired-lan@...osl.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "Gomes, Vinicius" <vinicius.gomes@...el.com>,
        "Gunasekaran, Aravindhan" <aravindhan.gunasekaran@...el.com>,
        "Ahmad Tarmizi, Noor Azura" <noor.azura.ahmad.tarmizi@...el.com>,
        Richard Cochran <richardcochran@...il.com>,
        Gal Pressman <gal@...dia.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "leon@...nel.org" <leon@...nel.org>,
        Michael Chan <michael.chan@...adcom.com>,
        Andy Gospodarek <andy@...yhouse.net>
Subject: RE: [PATCH v1 0/4] Add support for DMA timestamp for non-PTP packets



> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Thursday, 29 September, 2022 9:56 PM
> To: Zulkifli, Muhammad Husaini <muhammad.husaini.zulkifli@...el.com>
> Cc: intel-wired-lan@...osl.org; netdev@...r.kernel.org;
> davem@...emloft.net; edumazet@...gle.com; Gomes, Vinicius
> <vinicius.gomes@...el.com>; Gunasekaran, Aravindhan
> <aravindhan.gunasekaran@...el.com>; Ahmad Tarmizi, Noor Azura
> <noor.azura.ahmad.tarmizi@...el.com>; Richard Cochran
> <richardcochran@...il.com>; Gal Pressman <gal@...dia.com>; Saeed
> Mahameed <saeed@...nel.org>; leon@...nel.org; Michael Chan
> <michael.chan@...adcom.com>; Andy Gospodarek <andy@...yhouse.net>
> Subject: Re: [PATCH v1 0/4] Add support for DMA timestamp for non-PTP
> packets
> 
> On Thu, 29 Sep 2022 02:35:29 +0000 Zulkifli, Muhammad Husaini wrote:
> > > High level tho, are we assuming that the existing HW timestamps are
> > > always PTP-quality, i.e. captured when SFD crosses the RS layer, or
> > > whatnot? I'm afraid some NICs already report PCI stamps as the HW
> ones.
> >
> > Yes. HW timestamps always can be assume equivalent to PTP quality.
> > Could you provide additional information regarding SFD crosses the RS
> layer?
> 
> I mean true PTP timestamps, rather than captured somewhere in the NIC
> pipeline or at the DMA engine.

When SOF_TIMESTAMPING_TX_HARDWARE is been used, we guaranteed a PTP quality
Timestamps (timestamp capture when packet leave the wire upon sensing the SFD).
As of SOF_TIMESTAMPING_TX_HARDWARE_DMA_FETCH, it is not a PTP quality because
the HW timestamp reported in this case, is a time when the data is DMA'ed into the NIC packet buffer.

> 
> > According to what I observed, The HW Timestamps will be requested if
> > the application side specifies tx type = HWTSTAMP TX ON and
> timestamping flags = SOF TIMESTAMPING TX HARDWARE.
> > So it depends on how the application used it.
> >
> > > So the existing HW stamps are conceptually of "any" type, if we want
> > > to be 100% sure NIC actually stamps at the PHY we'd need another
> > > tx_type to express that.
> >
> > Yes, you're right. Are you suggesting that we add a new tx_type to
> > specify Only MAC/PHY timestamp ? Ex. HWTSTAMP_TX_PHY/MAC_ON.
> 
> Perhaps we can call them HWTSTAMP_TX_PTP_* ? Was the general time
> stamping requirement specified in IEEE 1588 or 802.1 (AS?)?
> 
> Both MAC and PHY can provide the time stamps IIUC, so picking one of those
> will not be entirely fortunate. In fact perhaps embedded folks will use this
> opportunity to disambiguate the two..

With the help of SOF_TIMESTAMPING_TX_HARDWARE, we will get the 
PHY level timestamp(PTP quality) while using SOF_TIMESTAMPING_TX_HARDWARE_DMA_FETCH,
we will get the timestamp at a point in the NIC pipeline.

Linuxptp application uses SOF_TIMESTAMPING_TX_HARDWARE for their socket option.
And this can guarantee a PTP quality timestamp. 

Can we just use a SOF_TIMESTAMPING to identify which timestamp that we want rather 
than creating a new tx_type?

> 
> > Sorry about the naming here. Just so you know, the DMA timestamp does
> > not quite match the PTP's level timestamping. The DMA timestamp will
> > be capture when DMA request to fetch the data from the memory.
> >
> > >
> > > Same story on the Rx - what do you plan to do there? We'll need to
> > > configure the filters per type, but that's likely to mean two new
> > > filters, because the current one gives no guarantee.
> >
> > Current I225 HW only allow to retrieve the dma time for TX packets only.
> > So as of now based on our HW, on RX side we just requesting rx filter to
> timestamps any incoming packets.
> > We always allocating additional bytes in the packet buffer for the receive
> packets for timestamp.
> > It is a 1588 PTP level kind of timestamping accuracy here.
> 
> I see. I think datacenter NICs can provide DMA stamps for Rx as well.
> Intel, Mellanox, Broadcom folks, could you confirm if your NIC can do Rx
> DMA stamps?