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

["Ismail, Mohammad Athari" <mohammad.athari.ismail@intel.com>]

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.

> 
> >
> > 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".

> 
> 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.