netdev - Re: [PATCH 1/2] [for 4.13] net: qcom/emac: disable flow control autonegotiation by default

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Date:   Tue, 1 Aug 2017 14:55:19 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Timur Tabi <timur@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 1/2] [for 4.13] net: qcom/emac: disable flow control
 autonegotiation by default

On 08/01/2017 02:37 PM, Timur Tabi wrote:
> The EMAC has a curious qwirk when RX flow control is enabled and the
> kernel hangs.  With the kernel hung, the EMAC's RX queue soon fills.
> If RX flow control is enabled, the EMAC will then send a non-stop
> stream of pause frames until the system is reset.  The EMAC does not
> have a built-in watchdog.
> 
> In various tests, the pause frame stream sometimes overloads nearby
> switches, effectively disabling the network.  Since the RX queue is
> large and the host processor is more than capable of handling incoming
> packets quickly, the only time the EMAC will send any pause frames is
> when the kernel is hung and unrecoverable.

This is not specific to your EMAC, a lot of adapters have this problem
actually.

I wonder if it would make sense to reach for a broader solution where we
could have a networking stack panic/oops notifier which will actively
clean up the active network devices' RX queue(s) and if tx_pause was
enabled, disable it. We could have drivers announce themselves as
needing this either via NETIF_F_* feature bit or some other private flag.

> 
> To avoid all these problems, we disable flow control autonegotiation
> by default, and only enable receiving pause frames.
> 
> Cc: stable@...r.kernel.org # 4.11.x
> Signed-off-by: Timur Tabi <timur@...eaurora.org>
> ---
>  drivers/net/ethernet/qualcomm/emac/emac.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/qualcomm/emac/emac.c b/drivers/net/ethernet/qualcomm/emac/emac.c
> index 60850bfa3d32..475c0ea29235 100644
> --- a/drivers/net/ethernet/qualcomm/emac/emac.c
> +++ b/drivers/net/ethernet/qualcomm/emac/emac.c
> @@ -441,8 +441,13 @@ static void emac_init_adapter(struct emac_adapter *adpt)
>  	/* others */
>  	adpt->preamble = EMAC_PREAMBLE_DEF;
>  
> -	/* default to automatic flow control */
> -	adpt->automatic = true;
> +	/* Disable transmission of pause frames by default, to avoid the
> +	 * risk of a pause frame flood that can occur if the kernel hangs.
> +	 * We still want to be able to respond to them, however.
> +	 */
> +	adpt->automatic = false;
> +	adpt->tx_flow_control = false;
> +	adpt->rx_flow_control = true;
>  }
>  
>  /* Get the clock */
> 


-- 
Florian