linux-kernel - Re: Aquantia PHY in OCSGMII mode?

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Message-ID: <20250731171642.2jxmhvrlb554mejz@skbuf>
Date: Thu, 31 Jul 2025 20:16:42 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Alexander Wilhelm <alexander.wilhelm@...termo.com>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Russell King <linux@...linux.org.uk>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Aquantia PHY in OCSGMII mode?

Hi Alexander,

On Thu, Jul 31, 2025 at 04:59:09PM +0200, Alexander Wilhelm wrote:
> Hello devs,
> 
> I'm fairly new to Ethernet PHY drivers and would appreciate your help. I'm
> working with the Aquantia AQR115 PHY. The existing driver already supports the
> AQR115C, so I reused that code for the AQR115, assuming minimal differences. My
> goal is to enable 2.5G link speed. The PHY supports OCSGMII mode, which seems to
> be non-standard.
> 
> * Is it possible to use this mode with the current driver?
> * If yes, what would be the correct DTS entry?
> * If not, I’d be willing to implement support. Could you suggest a good starting point?
> 
> Any hints or guidance would be greatly appreciated.
> 
> 
> Best regards
> Alexander Wilhelm
> 

In addition to what Andrew and Russell said:

The Aquantia PHY driver is a bit unlike other PHY drivers, in that it
prefers not to change the hardware configuration, and work with the
provisioning of the firmware.

Do you know that the PHY firmware was built for OCSGMII, or do you just
intend to use OCSGMII knowing that the hardware capability is there?
Because the driver reads the VEND1_GLOBAL_CFG registers in
aqr107_fill_interface_modes(). These registers tell Linux what host
interface mode to use for each negotiated link speed on the media side.

If you haven't already,

[ and I guess you haven't, because you can find there this translation
  which clearly shows that OCSGMII corresponds to what Linux treats as
  2500base-x:

		case VEND1_GLOBAL_CFG_SERDES_MODE_OCSGMII:
			interface = PHY_INTERFACE_MODE_2500BASEX;
			break;

]

then you can instrument this function and see what host interface mode
it detects as configured for VEND1_GLOBAL_CFG_2_5G.