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

From: Andrew Lunn <[email-masked]>
Date: Wed, 2 Aug 2017 01:15:54 +0200

On Tue, Aug 01, 2017 at 04:37:39PM -0500, 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.
> 
> To avoid all these problems, we disable flow control autonegotiation
> by default, and only enable receiving pause frames.
> 
> Cc: [email-masked] # 4.11.x
> Signed-off-by: Timur Tabi <[email-masked]>
> ---
>  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;

Hi Timur

Pause frames are something you can auto-negotiate at the PHY
level. Should you also be clearing some bits in the phydev, so the
peer knows pause frames are not supported?

     Andrew