Messages in this thread | | | Date | Fri, 23 Apr 2021 21:11:33 +0300 | From | Vladimir Oltean <> | Subject | Re: [PATCH v2 net-next] net: pcs: Enable pre-emption packet for 10/100Mbps |
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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? :)
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