Re: [PATCH v2 net-next] net: pcs: Enable pre-emption packet for 10/100Mbps

[Vladimir Oltean <olteanv@gmail.com>]

On Fri, Apr 23, 2021 at 09:30:07AM +0000, Ismail, Mohammad Athari wrote:
> Hi Vladimir,
> 
> > -----Original Message-----
> > From: Vladimir Oltean <olteanv@gmail.com>
> > Sent: Friday, April 23, 2021 8:53 AM
> > To: Ismail, Mohammad Athari <mohammad.athari.ismail@intel.com>
> > Cc: Alexandre Torgue <alexandre.torgue@st.com>; Jose Abreu
> > <joabreu@synopsys.com>; David S . Miller <davem@davemloft.net>; Jakub
> > Kicinski <kuba@kernel.org>; Andrew Lunn <andrew@lunn.ch>; Heiner Kallweit
> > <hkallweit1@gmail.com>; Russell King <linux@armlinux.org.uk>; Ong, Boon
> > Leong <boon.leong.ong@intel.com>; Voon, Weifeng
> > <weifeng.voon@intel.com>; Wong, Vee Khee <vee.khee.wong@intel.com>;
> > netdev@vger.kernel.org; linux-kernel@vger.kernel.org
> > Subject: Re: [PATCH v2 net-next] net: pcs: Enable pre-emption packet for
> > 10/100Mbps
> > 
> > On Fri, Apr 23, 2021 at 12:45:25AM +0000, Ismail, Mohammad Athari wrote:
> > > Hi Vladimir,
> > >
> > > > -----Original Message-----
> > > > From: Vladimir Oltean <olteanv@gmail.com>
> > > > Sent: Friday, April 23, 2021 7:53 AM
> > > > To: Ismail, Mohammad Athari <mohammad.athari.ismail@intel.com>
> > > > Cc: Alexandre Torgue <alexandre.torgue@st.com>; Jose Abreu
> > > > <joabreu@synopsys.com>; David S . Miller <davem@davemloft.net>;
> > > > Jakub Kicinski <kuba@kernel.org>; Andrew Lunn <andrew@lunn.ch>;
> > > > Heiner Kallweit <hkallweit1@gmail.com>; Russell King
> > > > <linux@armlinux.org.uk>; Ong, Boon Leong <boon.leong.ong@intel.com>;
> > > > Voon, Weifeng <weifeng.voon@intel.com>; Wong, Vee Khee
> > > > <vee.khee.wong@intel.com>; netdev@vger.kernel.org;
> > > > linux-kernel@vger.kernel.org
> > > > Subject: Re: [PATCH v2 net-next] net: pcs: Enable pre-emption packet
> > > > for 10/100Mbps
> > > >
> > > > Hi Mohammad,
> > > >
> > > > On Fri, Apr 23, 2021 at 07:06:45AM +0800,
> > > > mohammad.athari.ismail@intel.com
> > > > wrote:
> > > > > From: Mohammad Athari Bin Ismail
> > > > > <mohammad.athari.ismail@intel.com>
> > > > >
> > > > > Set VR_MII_DIG_CTRL1 bit-6(PRE_EMP) to enable pre-emption packet
> > > > > for 10/100Mbps by default. This setting doesn`t impact pre-emption
> > > > > capability for other speeds.
> > > > >
> > > > > Signed-off-by: Mohammad Athari Bin Ismail
> > > > > <mohammad.athari.ismail@intel.com>
> > > > > ---
> > > >
> > > > What is a "pre-emption packet"?
> > >
> > > In IEEE 802.1 Qbu (Frame Preemption), pre-emption packet is used to
> > > differentiate between MAC Frame packet, Express Packet, Non-fragmented
> > > Normal Frame Packet, First Fragment of Preemptable Packet,
> > > Intermediate Fragment of Preemptable Packet and Last Fragment of
> > > Preemptable Packet.
> > 
> > Citation needed, which clause are you referring to?
> 
> Cited from IEEE802.3-2018 Clause 99.3.

Aha, you know that what you just said is not what's in the "MAC Merge
sublayer" clause, right? There is no such thing as "pre-emption packet"
in the standard, this is a made-up name, maybe preemptable packets, but
the definition of preemptable packets is not that, hence my question.

> > 
> > >
> > > This bit "VR_MII_DIG_CTRL1 bit-6(PRE_EMP)" defined in DesignWare Cores
> > > Ethernet PCS Databook is to allow the IP to properly receive/transmit
> > > pre-emption packets in SGMII 10M/100M Modes.
> > 
> > Shouldn't everything be handled at the MAC merge sublayer? What business
> > does the PCS have in frame preemption?
> 
> There is no further detail explained in the databook w.r.t to
> VR_MII_DIG_CTRL1 bit-6(PRE_EMP). The only statement it mentions is
> "This bit should be set to 1 to allow the DWC_xpcs to properly
> receive/transmit pre-emption packets in SGMII 10M/100M Modes".

Correct, I see this too. I asked our hardware design team, and at least
on NXP LS1028A (no Synopsys PCS), the PCS layer has nothing to do with
frame preemption, as mentioned.

But indeed, I do see this obscure bit in the Digital Control 1 register
too, I've no idea what it does. I'll ask around. Odd anyway. If you have
to set it, you have to set it, I guess. But it is interesting to see why
is it even a configurable bit, why it is not enabled by default, what is
the drawback of enabling it?!

> > 
> > Also, I know it's easy to forget, but Vinicius' patch series for supporting frame
> > preemption via ethtool wasn't accepted yet. How are you testing this?
> 
> For stmmac Kernel driver, frame pre-emption capability is already
> supported. For iproute2 (tc command), we are using custom patch based
> on Vinicius patch.

Don't you want to help contributing the ethtool netlink support to the
mainline kernel though? :)