Messages in this thread | | | From | "Ismail, Mohammad Athari" <> | Subject | RE: [PATCH v2 net-next] net: pcs: Enable pre-emption packet for 10/100Mbps | Date | Fri, 23 Apr 2021 22:03:58 +0000 |
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Hi Vladimir,
> -----Original Message----- > From: Vladimir Oltean <olteanv@gmail.com> > Sent: Saturday, April 24, 2021 2:12 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 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. >
Thank you for the knowledge sharing. My guess, this "pre-emption packet" might be referring to "preamble" byte in Ethernet frame.
> > > > > > > > > > > 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?!
The databook states that the default value is 0. We don`t see any drawback of enabling it. As the databook mentions that, enabling the bit will allow SGMII 10/100M to receive/transmit preamble properly, so I think it is recommended to enable it for IP that support SGMII 10/100M speed.
> > > > > > > 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? :)
We are working with Vinicius to have ethtool support for frame pre-emption.
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