Re: [PATCH RFC net-next 0/9] net: stmmac: experimental PCS conversion

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

Hi Russell,

On 24/09/2025 23:47, Russell King (Oracle) wrote:
> This series is radical - it takes the brave step of ripping out much of
> the existing PCS support code and throwing it all away.
> 
> I have discussed the introduction of the STMMAC_FLAG_HAS_INTEGRATED_PCS
> flag with Bartosz Golaszewski, and the conclusion I came to is that
> this is to workaround the breakage that I've been going on about
> concerning the phylink conversion for the last five or six years.
> 
> The problem is that the stmmac PCS code manipulates the netif carrier
> state, which confuses phylink.
> 
> There is a way of testing this out on the Jetson Xavier NX platform as
> the "PCS" code paths can be exercised while in RGMII mode - because
> RGMII also has in-band status and the status register is shared with
> SGMII. Testing this out confirms my long held theory: the interrupt
> handler manipulates the netif carrier state before phylink gets a
> look-in, which means that the mac_link_up() and mac_link_down() methods
> are never called, resulting in the device being non-functional.
> 
> Moreover, on dwmac4 cores, ethtool reports incorrect information -
> despite having a full-duplex link, ethtool reports that it is
> half-dupex.
> 
> Thus, this code is completely broken - anyone using it will not have
> a functional platform, and thus it doesn't deserve to live any longer,
> especially as it's a thorn in phylink.
> 
> Rip all this out, leaving just the bare bones initialisation in place.
> 
> However, this is not the last of what's broken. We have this hw->ps
> integer which is really not descriptive, and the DT property from
> which it comes from does little to help understand what's going on.
> Putting all the clues together:
> 
> - early configuration of the GMAC configuration register for the
>    speed.
> - setting the SGMII rate adapter layer to take its speed from the
>    GMAC configuration register.
> 
> Lastly, setting the transmit enable (TE) bit, which is a typo that puts
> the nail in the coffin of this code. It should be the transmit
> configuration (TC) bit. Given that when the link comes up, phylink
> will call mac_link_up() which will overwrite the speed in the GMAC
> configuration register, the only part of this that is functional is
> changing where the SGMII rate adapter layer gets its speed from,
> which is a boolean.
> 
>  From what I've found so far, everyone who sets the snps,ps-speed
> property which configures this mode also configures a fixed link,
> so the pre-configuration is unnecessary - the link will come up
> anyway.
> 
> So, this series rips that out the preconfiguration as well, and
> replaces hw->ps with a boolean hw->reverse_sgmii_enable flag.
> 
> We then move the sole PCS configuration into a phylink_pcs instance,
> which configures the PCS control register in the same way as is done
> during the probe function.
> 
> Thus, we end up with much easier and simpler conversion to phylink PCS
> than previous attempts.
> 
> Even so, this still results in inband mode always being enabled at the
> moment in the new .pcs_config() method to reflect what the probe
> function was doing. The next stage will be to change that to allow
> phylink to correctly configure the PCS. This needs fixing to allow
> platform glue maintainers who are currently blocked to progress.
> 
> Please note, however, that this has not been tested with any SGMII
> platform.

Thanks for that.

I'll give this a test on socfpga next week, as I don't have access to 
the HW right now. It may not be the best platform to test this on, as it 
has a lynx PCS and no internal PCS :/

Maxime