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

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Message-ID: <aKZg3TviLUDgKgLz@pidgin.makrotopia.org>
Date: Thu, 21 Aug 2025 00:57:17 +0100
From: Daniel Golle <daniel@...rotopia.org>
To: Hauke Mehrtens <hauke@...ke-m.de>, Andrew Lunn <andrew@...n.ch>,
	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
Cc: 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 Wed, Aug 20, 2025 at 02:55:49AM +0100, Daniel Golle wrote:
> Store the switch API version in struct gswip_priv (in host endian) to
> prepare supporting newer features such as 4096 VLANs and per-port
> configurable learning.
> 
> Signed-off-by: Daniel Golle <daniel@...rotopia.org>
> ---
> v3: use __force for version field endian exception (__le16 __force) to
>     fix sparse warning.
> v2: no changes
> 
>  drivers/net/dsa/lantiq_gswip.c | 3 +++
>  drivers/net/dsa/lantiq_gswip.h | 1 +
>  2 files changed, 4 insertions(+)
> 
> diff --git a/drivers/net/dsa/lantiq_gswip.c b/drivers/net/dsa/lantiq_gswip.c
> index f8a43c351649..8999c3f2d290 100644
> --- a/drivers/net/dsa/lantiq_gswip.c
> +++ b/drivers/net/dsa/lantiq_gswip.c
> @@ -28,6 +28,7 @@
>  #include "lantiq_gswip.h"
>  #include "lantiq_pce.h"
>  
> +#include <linux/byteorder/generic.h>
>  #include <linux/delay.h>
>  #include <linux/etherdevice.h>
>  #include <linux/firmware.h>
> @@ -1936,6 +1937,8 @@ static int gswip_probe(struct platform_device *pdev)
>  					     "gphy fw probe failed\n");
>  	}
>  
> +	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.

The above statement will turn into a no-op on little-endian machines
(lets ignore the truncation to 16-bit for now). However, also on little-
endian machine the 'version' field is byte-swapped.

Hence I believe my original approach (using swab16) is better in my
opinion because rather than delcaring a specific endian, what needs to be
expressed is simply that this field is in opposite byte order than all the
other fields.

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.

Sorry for the confusion.

> +
>  	/* bring up the mdio bus */
>  	err = gswip_mdio(priv);
>  	if (err) {
> diff --git a/drivers/net/dsa/lantiq_gswip.h b/drivers/net/dsa/lantiq_gswip.h
> index 0b7b6db4eab9..077d1928149b 100644
> --- a/drivers/net/dsa/lantiq_gswip.h
> +++ b/drivers/net/dsa/lantiq_gswip.h
> @@ -258,6 +258,7 @@ struct gswip_priv {
>  	struct gswip_gphy_fw *gphy_fw;
>  	u32 port_vlan_filter;
>  	struct mutex pce_table_lock;
> +	u16 version;
>  };
>  
>  #endif /* __LANTIQ_GSWIP_H */
> -- 
> 2.50.1
>