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After the discussion here: https://lore.kernel.org/netdev/20210603143453.if7hgifupx5k433b@pali/ which resulted in this patch: https://patchwork.kernel.org/project/netdevbpf/patch/20210704134325.24842-1-pali@kernel.org/ and many other discussions before it, notably: https://patchwork.kernel.org/project/linux-arm-kernel/patch/1512016235-15909-1-git-send-email-Bhaskar.Upadhaya@nxp.com/ it became apparent that nobody really knows what "SGMII 2500" is. Certainly, Freescale/NXP hardware engineers name this protocol "SGMII 2500" in the reference manuals, but the PCS devices do not support any "SGMII" specific features when operating at the speed of 2500 Mbps, no in-band autoneg and no speed change via symbol replication . So that leaves a fixed speed of 2500 Mbps using a coding of 8b/10b with a SERDES lane frequency of 3.125 GHz. In fact, "SGMII 2500 without in-band autoneg and at a fixed speed" is indistinguishable from "2500base-x without in-band autoneg", which is precisely what these NXP devices support. So it just appears that "SGMII 2500" is an unclear name with no clear definition that stuck. As such, in the Linux kernel, the drivers which use this SERDES protocol use the 2500base-x phy-mode. This patch converts U-Boot to use 2500base-x too, or at least, as much as it can. Note that I would have really liked to delete PHY_INTERFACE_MODE_SGMII_2500 completely, but the mvpp2 driver seems to even distinguish between SGMII 2500 and 2500base-X. Namely, it enables in-band autoneg for one but not the other, and forces flow control for one but not the other. This goes back to the idea that maybe 2500base-X is a fiber protocol and SGMII-2500 is an MII protocol (connects a MAC to a PHY such as Aquantia), but the two are practically indistinguishable through everything except use case. NXP devices can support both use cases through an identical configuration, for example RX flow control can be unconditionally enabled in order to support rate adaptation performed by an Aquantia PHY. At least I can find no indication in online documents published by Cisco which would point towards "SGMII-2500" being an actual standard with an actual definition, so I cannot say "yes, NXP devices support it". Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Reviewed-by: Ramon Fried <rfried.dev@gmail.com> |
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