Re: [PATCH net v1] net: dsa: mv88e6xxx: Make unsupported C45 reads return 0xffff

From: Vladimir Oltean <vladimir.oltean@nxp.com>
To: Andrew Lunn <andrew@lunn.ch>
Cc: netdev-maintainers <edumazet@google.com>,
	kuba@kernel.org, pabeni@redhat.com, davem@davemloft.net,
	netdev <netdev@vger.kernel.org>,
	stable@vger.kernel.org,
	Tim Menninger <tmenninger@purestorage.com>
Subject: Re: [PATCH net v1] net: dsa: mv88e6xxx: Make unsupported C45 reads return 0xffff
Date: Mon, 22 Jan 2024 14:24:57 +0200	[thread overview]
Message-ID: <20240122122457.jt6xgvbiffhmmksr@skbuf> (raw)
In-Reply-To: <20240120192125.1340857-1-andrew@lunn.ch>

Hi Andrew,

On Sat, Jan 20, 2024 at 08:21:25PM +0100, Andrew Lunn wrote:
> When there is no device on the bus for a given address, the pull up
> resistor on the data line results in the read returning 0xffff. The
> phylib core code understands this when scanning for devices on the
> bus, and a number of MDIO bus masters make use of this as a way to
> indicate they cannot perform the read.
> 
> Make us of this as a minimal fix for stable where the mv88e6xxx

s/us/use/

Also, what is the "proper" fix if this is the minimal one for stable?

> returns EOPNOTSUPP when the hardware does not support C45, but phylib
> interprets this as a fatal error, which it should not be.

I think the commit message is a bit backwards, it starts with an
explanation of the solution without ever clarifying exactly what is
the problem.

At least it could have referenced the old thread which explains that:
https://lore.kernel.org/netdev/CAO-L_44YVi0HDk4gC9QijMZrYNGoKtfH7qsXOwtDwM4VrFRDHw@mail.gmail.com/

> 
> Cc: stable@vger.kernel.org
> Reported-by: Tim Menninger <tmenninger@purestorage.com>
> Fixes: 1a136ca2e089 ("net: mdio: scan bus based on bus capabilities for C22 and C45")
> Fixes: da099a7fb13d ("net: phy: Remove probe_capabilities")
> Signed-off-by: Andrew Lunn <andrew@lunn.ch>
> ---
>  drivers/net/dsa/mv88e6xxx/chip.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
> index 383b3c4d6f59..614cabb5c1b0 100644
> --- a/drivers/net/dsa/mv88e6xxx/chip.c
> +++ b/drivers/net/dsa/mv88e6xxx/chip.c
> @@ -3659,7 +3659,7 @@ static int mv88e6xxx_mdio_read_c45(struct mii_bus *bus, int phy, int devad,
>  	int err;
>  
>  	if (!chip->info->ops->phy_read_c45)
> -		return -EOPNOTSUPP;
> +		return 0xffff;
>  
>  	mv88e6xxx_reg_lock(chip);
>  	err = chip->info->ops->phy_read_c45(chip, bus, phy, devad, reg, &val);
> -- 
> 2.43.0
>

Is this an RFC pending testing from Tim? Or have you reproduced the
problem and confirmed that this fixes it? It's not clear how the old
thread ended.