Re: [PATCH net] net: phy: phy_caps: Don't skip better duplex macth on non-exact match

[Paolo Abeni <pabeni@redhat.com>]

On 6/3/25 10:35 AM, Maxime Chevallier wrote:
> When performing a non-exact phy_caps lookup, we are looking for a
> supported mode that matches as closely as possible the passed speed/duplex.
> 
> Blamed patch broke that logic by returning a match too early in case
> the caller asks for half-duplex, as a full-duplex linkmode may match
> first, and returned as a non-exact match without even trying to mach on
> half-duplex modes.
> 
> Reported-by: Jijie Shao <shaojijie@huawei.com>
> Closes: https://lore.kernel.org/netdev/20250603102500.4ec743cf@fedora/T/#m22ed60ca635c67dc7d9cbb47e8995b2beb5c1576
> Fixes: fc81e257d19f ("net: phy: phy_caps: Allow looking-up link caps based on speed and duplex")
> Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
> ---
>  drivers/net/phy/phy_caps.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/phy/phy_caps.c b/drivers/net/phy/phy_caps.c
> index 703321689726..d80f6a37edf1 100644
> --- a/drivers/net/phy/phy_caps.c
> +++ b/drivers/net/phy/phy_caps.c
> @@ -195,7 +195,7 @@ const struct link_capabilities *
>  phy_caps_lookup(int speed, unsigned int duplex, const unsigned long *supported,
>  		bool exact)
>  {
	> -	const struct link_capabilities *lcap, *last = NULL;
> +	const struct link_capabilities *lcap, *match = NULL, *last = NULL;
>  
>  	for_each_link_caps_desc_speed(lcap) {
>  		if (linkmode_intersects(lcap->linkmodes, supported)) {
> @@ -204,16 +204,19 @@ phy_caps_lookup(int speed, unsigned int duplex, const unsigned long *supported,
>  			if (lcap->speed == speed && lcap->duplex == duplex) {
>  				return lcap;
>  			} else if (!exact) {
> -				if (lcap->speed <= speed)
> -					return lcap;
> +				if (!match && lcap->speed <= speed)
> +					match = lcap;
> +
> +				if (lcap->speed < speed)
> +					break;
>  			}
>  		}
>  	}
>  
> -	if (!exact)
> -		return last;
> +	if (!match && !exact)
> +		match = last;

If I read correctly, when user asks for half-duplex, this can still
return a non exact matching full duplex cap, even when there is non
exact matching half-duplex cap available.

I'm wondering if the latter would be preferable, or at least if the
current behaviour should be explicitly called out in the function
documentation.

/P