netdev - Re: [PATCH net-next v3 7/8] net: dsa: lantiq_gswip: store switch API version in priv

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Message-ID: <aKb59jMfDIJIK0KP@pidgin.makrotopia.org>
Date: Thu, 21 Aug 2025 11:50:30 +0100
From: Daniel Golle <daniel@...rotopia.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Hauke Mehrtens <hauke@...ke-m.de>, Vladimir Oltean <olteanv@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Russell King <linux@...linux.org.uk>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Andreas Schirm <andreas.schirm@...mens.com>,
	Lukas Stockmann <lukas.stockmann@...mens.com>,
	Alexander Sverdlin <alexander.sverdlin@...mens.com>,
	Peter Christen <peter.christen@...mens.com>,
	Avinash Jayaraman <ajayaraman@...linear.com>,
	Bing tao Xu <bxu@...linear.com>, Liang Xu <lxu@...linear.com>,
	Juraj Povazanec <jpovazanec@...linear.com>,
	"Fanni (Fang-Yi) Chan" <fchan@...linear.com>,
	"Benny (Ying-Tsan) Weng" <yweng@...linear.com>,
	"Livia M. Rosu" <lrosu@...linear.com>,
	John Crispin <john@...ozen.org>
Subject: Re: [PATCH net-next v3 7/8] net: dsa: lantiq_gswip: store switch API
 version in priv

On Thu, Aug 21, 2025 at 04:58:23AM +0200, Andrew Lunn wrote:
> > > +	priv->version = le16_to_cpu((__le16 __force)version);
> > 
> > I've researched this a bit more and came to the conclusion that while the
> > above works fine because all Lantiq SoCs with built-in switch are
> > big-endian machines it is still wrong.
> > I base this conclusion on the fact that when dealing with more recent
> > MDIO-connected switches (MaxLinear GSW1xx series) the host endian doesn't
> > play a role in the driver -- when dealing with 16-bit values on the MDIO
> > bus, the bus abstraction takes care of converting from/to host endianess.
> 
> I agree that all MDIO bus registers are host endian, 16 bit. The shift
> register in the hardware is responsible for putting the bits on the
> wire in the correct order for MDIO.
> 
> > Hence I believe this should simply be a swab16() which will always result
> > in the version being in the right byte order to use comparative operators
> > in a meaningful way.
> 
> How is this described in the datasheet? And is version special, or do
> all registers need swapping?

The (anyway public) datasheets I have access to don't describe the VERSION
register at all. In the existing precompiler macros, however, you can see
that most-significant and least-significant byte are swapped. REV is
more significant than MOD:

#define GSWIP_VERSION                   0x013
#define  GSWIP_VERSION_REV_SHIFT        0
#define  GSWIP_VERSION_REV_MASK         GENMASK(7, 0)
#define  GSWIP_VERSION_MOD_SHIFT        8
#define  GSWIP_VERSION_MOD_MASK         GENMASK(15, 8)
#define   GSWIP_VERSION_2_0             0x100
#define   GSWIP_VERSION_2_1             0x021
#define   GSWIP_VERSION_2_2             0x122
#define   GSWIP_VERSION_2_2_ETC         0x022

Now I'd like to add
#define   GSWIP_VERSION_2_3             0x023

and then have a simple way to make features available starting from a
GSWIP_VERSION. Now in order for GSWIP_VERSION_2_3 to be greater than
GSWIP_VERSION_2_2, and GSWIP_VERSION_2_1 to be greater than
GSWIP_VERSION_2_0, the bytes need to be swapped.

I don't think the vendor even considered any specific order of the two
bytes but just defines them as separate 8-bit fields, considering it a
16-bit unsigned integer is my interpretation which came from the need for
comparability.