Re: [PATCH net-next v6 1/2] net: dsa: realtek: rtl8365mb: add SGMII support for RTL8367S

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi Johan,

On 7/12/26 06:31, Johan Alvarado via B4 Relay wrote:
> From: Johan Alvarado <contact@c127.dev>
> 
> The RTL8367S can mux its embedded SerDes to external interface 1,
> which is typically used to connect the switch to a CPU port. The chip
> info table already declares SGMII as a supported interface mode for
> this chip, but the driver only implements RGMII so far.
> 
> Implement SGMII support as a phylink PCS, with the configuration
> sequence derived from the GPL-licensed Realtek rtl8367c vendor driver
> as distributed in the Mercusys MR80X GPL code drop:
> 
>  - Add accessors for the SerDes indirect access registers (SDS_INDACS),
>    through which the SerDes internal registers are reached.
> 
>  - Register a phylink_pcs for the SerDes, selected from mac_select_pcs
>    for the SGMII interface, so the SerDes handling lives in the PCS
>    operations rather than in the MAC operations.
> 
>  - Probe the SerDes tuning variant from the chip option register once
>    at setup. The vendor driver keeps two sets of SerDes tuning
>    parameters and selects between them based on this option; only the
>    variant for a non-zero option (which all RTL8367S parts seen so far
>    report) has been validated on hardware, so the SerDes interface
>    modes are only advertised in that case. An unsupported variant thus
>    fails at phylink validation time instead of at link configuration
>    time.
> 
>  - Keep the embedded DW8051 microcontroller in reset and disabled. The
>    vendor driver loads firmware into it to manage the SerDes link, but
>    analysis of that firmware shows it only duplicates the link
>    management phylink already performs: it polls the port status and
>    writes the external interface force registers behind the driver's
>    back.
> 
>  - Clear the line rate bypass bit for the external interface, tune the
>    SerDes with the vendor-prescribed parameters, mux the SerDes to MAC8
>    in SGMII mode and only then take the SerDes out of reset, as the
>    vendor driver does.
> 
>  - After deasserting the SerDes reset, reset the SerDes data path via
>    the SerDes BMCR register to flush the FIFOs and resync the PLL.
>    This mirrors what the vendor firmware does right after deasserting
>    the SerDes reset, and ensures a clean link state from cold boot.
> 
>  - Force the SGMII link parameters (link, speed, duplex) in the SDS_MISC
>    register from pcs_link_up(). SGMII in-band autonegotiation is not
>    implemented, so only fixed-link and conventional PHY setups are
>    supported, just like RGMII. This is reported to phylink through
>    pcs_inband_caps() returning LINK_INBAND_DISABLE, so phylink never
>    selects an in-band-enabled negotiation mode for this PCS.
> 
>  - Program the SerDes pause enables in SDS_MISC from the resolved
>    pause modes when forcing the MAC external interface in mac_link_up,
>    as the vendor driver does, rather than leaving whatever state the
>    boot firmware left there. Flow control testing shows these bits,
>    not the MAC force pause bits, gate pause on the SerDes external
>    interface. This is done in the MAC layer because pcs_link_up()
>    carries no pause information.
> 
>  - Implement pcs_get_state() by reading the link status from the
>    SerDes, with the forced speed and duplex read back from SDS_MISC.
>    Although the supported fixed-link and conventional PHY setups do not
>    use it, the PCS owns the SerDes link state, and phylink consults
>    pcs_get_state() to track the physical link when operating in in-band
>    mode with autonegotiation disabled. The SerDes has no link interrupt
>    wired up, so the PCS sets its poll flag.
> 
> Tested on a Mercusys MR80X v2.20, where the RTL8367S is connected to
> the SoC over SGMII.
> 
> Suggested-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
> Suggested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
> Suggested-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
> Signed-off-by: Johan Alvarado <contact@c127.dev>

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Maxime