netdev - Re: [PATCH net-next v8 2/6] net: pcs: xpcs: re-initiate clause 37 Auto-negotiation

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Message-ID: <Z78141ZNWEYDSi5l@shell.armlinux.org.uk>
Date: Wed, 26 Feb 2025 15:40:19 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Choong Yong Liang <yong.liang.choong@...ux.intel.com>
Cc: Simon Horman <horms@...nel.org>, Jose Abreu <joabreu@...opsys.com>,
	Jose Abreu <Jose.Abreu@...opsys.com>,
	David E Box <david.e.box@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Rajneesh Bhardwaj <irenic.rajneesh@...il.com>,
	David E Box <david.e.box@...el.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jiawen Wu <jiawenwu@...stnetic.com>,
	Mengyuan Lou <mengyuanlou@...-swift.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Hans de Goede <hdegoede@...hat.com>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Richard Cochran <richardcochran@...il.com>,
	Serge Semin <fancer.lancer@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	platform-driver-x86@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next v8 2/6] net: pcs: xpcs: re-initiate clause 37
 Auto-negotiation

On Wed, Feb 26, 2025 at 03:48:33PM +0800, Choong Yong Liang wrote:
> diff --git a/drivers/net/pcs/pcs-xpcs-wx.c b/drivers/net/pcs/pcs-xpcs-wx.c
> index fc52f7aa5f59..f73ab04d09f0 100644
> --- a/drivers/net/pcs/pcs-xpcs-wx.c
> +++ b/drivers/net/pcs/pcs-xpcs-wx.c
> @@ -172,11 +172,9 @@ int txgbe_xpcs_switch_mode(struct dw_xpcs *xpcs, phy_interface_t interface)
>  		return 0;
>  	}
>  
> -	if (xpcs->interface == interface && !txgbe_xpcs_mode_quirk(xpcs))
> +	if (!txgbe_xpcs_mode_quirk(xpcs))
>  		return 0;
>  
> -	xpcs->interface = interface;
> -

...

> --- a/drivers/net/pcs/pcs-xpcs.c
> +++ b/drivers/net/pcs/pcs-xpcs.c
> @@ -602,12 +602,37 @@ static void xpcs_get_interfaces(struct dw_xpcs *xpcs, unsigned long *interfaces)
>  		__set_bit(compat->interface, interfaces);
>  }
>  
> +static int xpcs_switch_interface_mode(struct dw_xpcs *xpcs,
> +				      phy_interface_t interface)
> +{
> +	int ret = 0;
> +
> +	if (xpcs->interface != interface) {
> +		if (xpcs->info.pma == WX_TXGBE_XPCS_PMA_10G_ID) {
> +			ret = txgbe_xpcs_switch_mode(xpcs, interface);
> +			if (ret)
> +				return ret;

The above modification changes the functionality.

In the old code, txgbe_xpcs_switch_mode() does its work when
xpcs->interface is not the same as interface OR txgbe_xpcs_mode_quirk()
is true.

Your replacement code calls txgbe_xpcs_switch_mode() when 
xpcs->interface is not the same as interface, *and* it can do its
work when txgbe_xpcs_mode_quirk() returns true.

So, e.g. when txgbe_xpcs_mode_quirk() returns false, but the interface
changes, txgbe_xpcs_mode_quirk() used to do its work, but as a result
if your changes, it becomes a no-op.

The point of txgbe_xpcs_mode_quirk() is to always do the work if it
returns true, even if the interface mode doesn't change.

Therefore, this patch is logically incorrect, and likely breaks TXGBE.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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