> -----Original Message----- > From: Russell King <linux@xxxxxxxxxxxxxxx> > Sent: 04 June 2021 23:00 > To: Madalin Bucur <madalin.bucur@xxxxxxx> > Cc: Pali Rohár <pali@xxxxxxxxxx>; Andrew Lunn <andrew@xxxxxxx>; Igal > Liberman <Igal.Liberman@xxxxxxxxxxxxx>; Shruti Kanetkar > <Shruti@xxxxxxxxxxxxx>; Emil Medve <Emilian.Medve@xxxxxxxxxxxxx>; Scott > Wood <oss@xxxxxxxxxxxx>; Rob Herring <robh+dt@xxxxxxxxxx>; Michael > Ellerman <mpe@xxxxxxxxxxxxxx>; Benjamin Herrenschmidt > <benh@xxxxxxxxxxxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; > devicetree@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Camelia > Alexandra Groza (OSS) <camelia.groza@xxxxxxxxxxx> > Subject: Re: Unsupported phy-connection-type sgmii-2500 in > arch/powerpc/boot/dts/fsl/t1023rdb.dts > > On Fri, Jun 04, 2021 at 07:39:10PM +0000, Madalin Bucur wrote: > > > -----Original Message----- > > > From: Russell King <linux@xxxxxxxxxxxxxxx> > > > Sent: 04 June 2021 22:28 > > > To: Madalin Bucur <madalin.bucur@xxxxxxx> > > > Cc: Pali Rohár <pali@xxxxxxxxxx>; Andrew Lunn <andrew@xxxxxxx>; Igal > > > Liberman <Igal.Liberman@xxxxxxxxxxxxx>; Shruti Kanetkar > > > <Shruti@xxxxxxxxxxxxx>; Emil Medve <Emilian.Medve@xxxxxxxxxxxxx>; > Scott > > > Wood <oss@xxxxxxxxxxxx>; Rob Herring <robh+dt@xxxxxxxxxx>; Michael > > > Ellerman <mpe@xxxxxxxxxxxxxx>; Benjamin Herrenschmidt > > > <benh@xxxxxxxxxxxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; > > > devicetree@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Camelia > > > Alexandra Groza (OSS) <camelia.groza@xxxxxxxxxxx> > > > Subject: Re: Unsupported phy-connection-type sgmii-2500 in > > > arch/powerpc/boot/dts/fsl/t1023rdb.dts > > > > > > On Fri, Jun 04, 2021 at 07:35:33AM +0000, Madalin Bucur wrote: > > > > Hi, the Freescale emails no longer work, years after Freescale > joined > > > NXP. > > > > Also, the first four recipients no longer work for NXP. > > > > > > > > In regards to the sgmii-2500 you see in the device tree, it > describes > > > SGMII > > > > overclocked to 2.5Gbps, with autonegotiation disabled. > > > > > > > > A quote from a long time ago, from someone from the HW team on this: > > > > > > > > The industry consensus is that 2.5G SGMII is overclocked 1G > SGMII > > > > using XAUI electricals. For the PCS and MAC layers, it looks > exactly > > > > like 1G SGMII, just with a faster clock. > > > > > > > > The statement that it does not exist is not accurate, it exists in > HW, > > > and > > > > it is described as such in the device tree. Whether or not it is > > > properly > > > > treated in SW it's another discussion. > > > > > > Here's the issue though: > > > > > > 802.3 defined 1000base-X which is a fixed 1G speed interface using a > > > 16-bit control word. Implementations of this exist where the control > > > word can be disabled. > > > > > > Cisco came along, took 1000base-X and augmented it to allow speeds of > > > 10M and 100M by symbol repetition, and changing the format of the > > > 16-bit control word. Otherwise, it is functionally compatible - indeed > > > SGMII with the control word disabled will connect with 1000base-X with > > > the control word disabled. I've done it several times. > > > > > > There exists 2500base-X, which is 1000base-X clocked faster, and it > > > seems the concensus is that it has the AN disabled - in other words, > > > no control word. > > > > > > Now you're saying that SGMII at 2.5G speed exists, which is 1G SGMII > > > fixed at 1G speed, without a control word, upclocked by 2.5x. > > > > > > My question to you is how is how is this SGMII 2.5G different from > > > 2500base-X? > > > > > > > In 2015, when this was submitted, > > > > there were no other 2.5G compatibles in use, if I'm not mistaken. > > > > 2500Base-X started to be added to device trees four years later, it > > > should > > > > be compatible/interworking but it is less specific on the actual > > > implementation > > > > details (denotes 2.5G speed, 8b/10b coding, which is true for this > > > overclocked > > > > SGMII). If they are compatible, SW should probably treat them in the > > > same manner. > > > > > > My argument has been (since I've had experience of SGMII talking to > > > 1000base-X, and have also accidentally clocked such a scenario at > > > 2.5G speeds) that there is in fact no functional difference between > > > SGMII and base-X when they are running at identical speeds with the > > > control word disabled. > > > > > > Given that we well know that industry likes to use the term "SGMII" > > > very loosely to mean <whatever>base-X as well as SGMII, it becomes > > > a very bad term to use when we wish to differentiate between a > > > base-X and a real Cisco SGMII link with their different control word > > > formats. > > > > > > And this has always been my point - industry has created confusion > > > over these terms, but as software programmers, we need to know the > > > difference. So, SGMII should _only_ be used to identify the Cisco > > > SGMII modified version of 802.3 base-X _with_ the modified control > > > word or with the capability of symbol repetition. In other words, > > > the very features that make it SGMII as opposed to 802.3 base-X. > > > Everything else should not use the term SGMII. > > > > > > > There were some discussions a while ago about the mix or even > confusion > > > between > > > > the actual HW description (that's what the dts is supposed to do) > and > > > the settings > > > > one wants to represent in SW (i.e. speed) denoted loosely by > > > denominations like > > > > 10G Base-R. > > > > > > The "confusion" comes from an abuse of terms. Abused terms really > > > can't adequately be used to describe hardware properties. > > > > > > As I say above, we _know_ that some manufacturers state that their > > > single lane serdes is "SGMII" when it is in fact 1000base-X. That > > > doesn't mean we stuff "sgmii" into device tree because that's what > > > the vendor decided to call it. > > > > > > "sgmii" in the device tree means Cisco's well defined SGMII and > > > does not mean 1000base-X. > > > > The "sgmii-2500" compatible in that device tree describes an SGMII HW > > block, overclocked at 2.5G. Without that overclocking, it's a plain > > Cisco (like) SGMII HW block. That's the reason you need to disable it's > > AN setting when overclocked. With the proper Reset Configuration Word, > > you could remove the overclocking and transform that into a plain > "sgmii". > > Thus, the dts compatible describes the HW, as it is. > > I have given you a detailed explanation of my view on this, which is > based on reading the 802.3 and Cisco SGMII specifications and various > device datasheets from multiple different manufacturers. > > I find your argument which seems to be based on what your hardware > in front of you "does" and the actual explanation of it to be an > extremely weak response. > > Please provide a robust argument for your position. Thanks. By lacking AN abilities, this HW fits neither SGMII nor 1000Base-X, leave aside the speed increment. Other than that, I do not feel I have to justify someone's decision to not align in 2015 to something that became a habit in 2019, I just provided an explanation for the information they acted upon. Regards, Madalin
