On Thu, Jun 18, 2020 at 11:06:43AM -0700, Florian Fainelli wrote: > > > On 6/18/2020 1:45 AM, Russell King - ARM Linux admin wrote: > > On Thu, Jun 18, 2020 at 10:14:33AM +0200, Helmut Grohne wrote: > >> On Wed, Jun 17, 2020 at 02:08:09PM +0200, Russell King - ARM Linux admin > >> wrote: > >>> With a fixed link, we could be in either a MAC-to-PHY or MAC-to-MAC > >>> setup; we just don't know. However, we don't have is access to the > >>> PHY (if it exists) in the fixed link case to configure it for the > >>> delay. > >> > >> Let me twist that a little: We may have access to the PHY, but we don't > >> always have access. When we do have access, we have a separate device > >> tree node with another fixed-link and another phy-mode. For fixed-links, > >> we specify the phy-mode for each end. > > > > If you have access to the PHY, you shouldn't be using fixed-link. In > > any case, there is no support for a fixed-link specification at the > > PHY end in the kernel. The PHY binding doc does not allow for this > > either. > > > >>> In the MAC-to-MAC RGMII setup, where neither MAC can insert the > >>> necessary delay, the only way to have a RGMII conformant link is to > >>> have the PCB traces induce the necessary delay. That errs towards > >>> PHY_INTERFACE_MODE_RGMII for this case. > >> > >> Yes. > >> > >>> However, considering the MAC-to-PHY RGMII fixed link case, where the > >>> PHY may not be accessible, and may be configured with the necessary > >>> delay, should that case also use PHY_INTERFACE_MODE_RGMII - clearly > >>> that would be as wrong as using PHY_INTERFACE_MODE_RGMII_ID would > >>> be for the MAC-to-MAC RGMII with PCB-delays case. > >> > >> If you take into account that the PHY has a separate node with phy-mode > >> being rgmii-id, it makes a lot more sense to use rgmii for the phy-mode > >> of the MAC. So I don't think it is that clear that doing so is wrong. > > > > The PHY binding document does not allow for this, neither does the > > kernel. > > > >> In an earlier discussion Florian Fainelli said: > >> https://patchwork.ozlabs.org/project/netdev/patch/20190410005700.31582-19-olte...@gmail.com/#2149183 > >> | fixed-link really should denote a MAC to MAC connection so if you have > >> | "rgmii-id" on one side, you would expect "rgmii" on the other side > >> | (unless PCB traces account for delays, grrr). > >> > >> For these reasons, I still think that rgmii would be a useful > >> description for the fixed-link to PHY connection where the PHY adds the > >> delay. > > > > I think Florian is wrong; consider what it means when you have a > > fixed-link connection between a MAC and an inaccessible PHY and > > the link as a whole is operating in what we would call "rgmii-id" > > mode if the PHY were accessible. > > > > Taking Florian's stance, it basically means that DT no longer > > describes the hardware, but how we have chosen to interpret the > > properties in _one_ specific case in a completely different way > > to all the other cases. > > How do you ensure that a MAC to MAC connection using RGMII actually > works if you do not provide a 'phy-mode' for both sides right now? > > Yes this is more of a DT now describes a configuration choice rather > than the hardware but given that Ethernet MACs are usually capable of > supporting all 4 possible RGMII modes, how do you propose to solve the > negotiation of the appropriate RGMII mode here?
This is actually answered by yourself below. > >>> So, I think a MAC driver should not care about the specific RGMII > >>> mode being asked for in any case, and just accept them all. > >> > >> I would like to agree to this. However, the implication is that when you > >> get your delays wrong, your driver silently ignores you and you never > >> notice your mistake until you see no traffic passing and wonder why. > > > > So explain to me this aspect of your reasoning: > > > > - If the link is operating in non-fixed-link mode, the rgmii* PHY modes > > describe the delay to be applied at the PHY end. > > - If the link is operating in fixed-link mode, the rgmii* PHY modes > > describe the delay to be applied at the MAC end. > > > > That doesn't make sense, and excludes properly describing a MAC-to- > > inaccessible-PHY setup. > > The point with a fixed link is that it does not matter what is the other > end, it could be a MAC, it could be a PHY, it could go nowhere, it just > does not matter, the only thing that you can know is you configure your > side of the fixed link. That is *exactly* my point. Today, with a PHY on the other end, the four RGMII modes tell you how the PHY is to be configured, not the MAC. The documentation states this. So, why should a fixed-link be any different? -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
