netdev - Re: [PATCH stable 4.9] net: dsa: bcm_sf2: force pause link settings

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Date:   Tue, 5 Jul 2022 02:34:57 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     stable@...r.kernel.org, Doug Berger <opendmb@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sasha Levin <sashal@...nel.org>, netdev@...r.kernel.org,
        andrew@...n.ch
Subject: Re: [PATCH stable 4.9] net: dsa: bcm_sf2: force pause link settings

Hi Florian,

On Mon, Jul 04, 2022 at 08:35:07AM -0700, Florian Fainelli wrote:
> From: Doug Berger <opendmb@...il.com>
> 
> commit 7c97bc0128b2eecc703106112679a69d446d1a12 upstream
> 
> The pause settings reported by the PHY should also be applied to the GMII port
> status override otherwise the switch will not generate pause frames towards the
> link partner despite the advertisement saying otherwise.
> 
> Fixes: 246d7f773c13 ("net: dsa: add Broadcom SF2 switch driver")
> Signed-off-by: Doug Berger <opendmb@...il.com>
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> ---
>  drivers/net/dsa/bcm_sf2.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/net/dsa/bcm_sf2.c b/drivers/net/dsa/bcm_sf2.c
> index 40b3adf7ad99..03f38c36e188 100644
> --- a/drivers/net/dsa/bcm_sf2.c
> +++ b/drivers/net/dsa/bcm_sf2.c
> @@ -671,6 +671,11 @@ static void bcm_sf2_sw_adjust_link(struct dsa_switch *ds, int port,
>  		reg |= LINK_STS;
>  	if (phydev->duplex == DUPLEX_FULL)
>  		reg |= DUPLX_MODE;
> +	if (phydev->pause) {
> +		if (phydev->asym_pause)
> +			reg |= TXFLOW_CNTL;
> +		reg |= RXFLOW_CNTL;
> +	}

Is this correct? phydev->pause and phydev->asym_pause keep the Pause and
Asym_Pause bits advertised by the link partner. In other words, in this
manual resolution you are ignoring what the local switch port has
advertised.

To give you an example (I'm looking at the infamous Table 28B–3—Pause
resolution from IEEE 802.3).

Your logic, simplified, says: enable TXFLOW_CNTL as local device
resolution iff the link partner advertised PAUSE=1 && ASM_DIR=1, and
RXFLOW_CNTL if the link partner advertised PAUSE=1 at all.

The most trivial counter-example is if we (local device) advertise
PAUSE=0 ASM_DIR=0 (this can be achieved with ethtool rx off tx off).
That is one way to rig the negotiated flow control to off/off regardless
of what the link partner has advertised. But your logic would enable
local capabilities just because the link partner said so, ignoring what
was advertised here.

Another example is if the local station advertises PAUSE=1, ASM_DIR=1,
and remote PAUSE=0, ASM_DIR=1. According to IEEE, the link partners
should agree on "rx on tx off" for the local system, and "tx on rx off"
for the link partner. But your logic does not even enter the first "if"
condition, because phydev->pause (link partner's PAUSE advertisement) is
0. So neither TXFLOW_CNTL nor RXFLOW_CNTL will be enabled locally,
despite the expectation and what is printed by phy_print_status().

>  
>  	core_writel(priv, reg, CORE_STS_OVERRIDE_GMIIP_PORT(port));
>  
> -- 
> 2.25.1
>