netdev - Re: [PATCH net V2 1/1] net: phy: fix invalid phy id when probe using C22

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Date:   Thu, 18 Mar 2021 14:25:54 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Wong Vee Khee <vee.khee.wong@...el.com>,
        Andrew Lunn <andrew@...n.ch>,
        Russell King <linux@...linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Voon Weifeng <weifeng.voon@...el.com>,
        Ong Boon Leong <boon.leong.ong@...el.com>
Subject: Re: [PATCH net V2 1/1] net: phy: fix invalid phy id when probe using
 C22

On 18.03.2021 10:09, Wong Vee Khee wrote:
> When using Clause-22 to probe for PHY devices such as the Marvell
> 88E2110, PHY ID with value 0 is read from the MII PHYID registers
> which caused the PHY framework failed to attach the Marvell PHY
> driver.
> 
> Fixed this by adding a check of PHY ID equals to all zeroes.
> 

I was wondering whether we have, and may break, use cases where a PHY,
for whatever reason, reports PHY ID 0, but works with the genphy
driver. And indeed in swphy_read_reg() we return PHY ID 0, therefore
the patch may break the fixed phy.
Having said that I think your patch is ok, but we need a change of
the PHY ID reported by swphy_read_reg() first.
At a first glance changing the PHY ID to 0x00000001 in swphy_read_reg()
should be sufficient. This value shouldn't collide with any real world
PHY ID.

> Fixes: ee951005e95e ("net: phy: clean up get_phy_c22_id() invalid ID handling")
> Cc: stable@...r.kernel.org
> Reviewed-by: Voon Weifeng <voon.weifeng@...el.com>
> Signed-off-by: Wong Vee Khee <vee.khee.wong@...el.com>
> ---
> v2 changelog:
>  - added fixes tag
>  - marked for net instead of net-next
> ---
>  drivers/net/phy/phy_device.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
> index cc38e326405a..c12c30254c11 100644
> --- a/drivers/net/phy/phy_device.c
> +++ b/drivers/net/phy/phy_device.c
> @@ -809,8 +809,8 @@ static int get_phy_c22_id(struct mii_bus *bus, int addr, u32 *phy_id)
>  
>  	*phy_id |= phy_reg;
>  
> -	/* If the phy_id is mostly Fs, there is no device there */
> -	if ((*phy_id & 0x1fffffff) == 0x1fffffff)
> +	/* If the phy_id is mostly Fs or all zeroes, there is no device there */
> +	if (((*phy_id & 0x1fffffff) == 0x1fffffff) || (*phy_id == 0))
>  		return -ENODEV;
>  
>  	return 0;
>