Message-ID: <b7fb3c8c-bfa6-4e46-b5ed-05e4752bbc00@intel.com>
Date: Wed, 24 Sep 2025 12:13:18 -0700
From: Jacob Keller <jacob.e.keller@...el.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>, Andrew Lunn
	<andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>
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Subject: Re: [PATCH RFC net-next 0/9] net: stmmac: experimental PCS conversion



On 9/24/2025 11:17 AM, 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.
> 
> I've tried to get as many people into the Cc list with get_maintainers,
> I hope that's sufficient to get enough eyeballs on this.
> 

I'm no expert with this hardware or driver, but all of your explanations
seem reasonable to me.

I'd guess the real step is to try and get this tested against the
variety of hardware supported by stmmac?


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