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

[Ismail, Mohammad Athari]

Hi Vladimir,

> -----Original Message-----
> From: Vladimir Oltean <olteanv@xxxxxxxxx>
> Sent: Friday, April 23, 2021 8:53 AM
> To: Ismail, Mohammad Athari <mohammad.athari.ismail@xxxxxxxxx>
> Cc: Alexandre Torgue <alexandre.torgue@xxxxxx>; Jose Abreu
> <joabreu@xxxxxxxxxxxx>; David S . Miller <davem@xxxxxxxxxxxxx>; Jakub
> Kicinski <kuba@xxxxxxxxxx>; Andrew Lunn <andrew@xxxxxxx>; Heiner Kallweit
> <hkallweit1@xxxxxxxxx>; Russell King <linux@xxxxxxxxxxxxxxx>; Ong, Boon
> Leong <boon.leong.ong@xxxxxxxxx>; Voon, Weifeng
> <weifeng.voon@xxxxxxxxx>; Wong, Vee Khee <vee.khee.wong@xxxxxxxxx>;
> netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxxxx>
> > > Sent: Friday, April 23, 2021 7:53 AM
> > > To: Ismail, Mohammad Athari <mohammad.athari.ismail@xxxxxxxxx>
> > > Cc: Alexandre Torgue <alexandre.torgue@xxxxxx>; Jose Abreu
> > > <joabreu@xxxxxxxxxxxx>; David S . Miller <davem@xxxxxxxxxxxxx>;
> > > Jakub Kicinski <kuba@xxxxxxxxxx>; Andrew Lunn <andrew@xxxxxxx>;
> > > Heiner Kallweit <hkallweit1@xxxxxxxxx>; Russell King
> > > <linux@xxxxxxxxxxxxxxx>; Ong, Boon Leong <boon.leong.ong@xxxxxxxxx>;
> > > Voon, Weifeng <weifeng.voon@xxxxxxxxx>; Wong, Vee Khee
> > > <vee.khee.wong@xxxxxxxxx>; netdev@xxxxxxxxxxxxxxx;
> > > linux-kernel@xxxxxxxxxxxxxxx
> > > 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@xxxxxxxxx
> > > wrote:
> > > > From: Mohammad Athari Bin Ismail
> > > > <mohammad.athari.ismail@xxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxx>
> > > > ---
> > >
> > > 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.