linux-kernel - Re: [PATCH net 1/1] net: fec: Fix temporary RMII clock reset on link up

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Date:   Fri, 22 Jan 2021 17:36:32 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Laurent Badel <laurentbadel@...on.com>
Cc:     Fugang Duan <fugang.duan@....com>,
        "David S . Miller" <davem@...emloft.net>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net 1/1] net: fec: Fix temporary RMII clock reset on
 link up

On Fri, 22 Jan 2021 16:13:47 +0100 Laurent Badel wrote:
> fec_restart() does a hard reset of the MAC module when the link status
> changes to up. This temporarily resets the R_CNTRL register which controls
> the MII mode of the ENET_OUT clock. In the case of RMII, the clock
> frequency momentarily drops from 50MHz to 25MHz until the register is
> reconfigured. Some link partners do not tolerate this glitch and
> invalidate the link causing failure to establish a stable link when using
> PHY polling mode. Since as per IEEE802.11 the criteria for link validity 
> are PHY-specific, what the partner should tolerate cannot be assumed, so 
> avoid resetting the MII clock by using software reset instead of hardware 
> reset when the link is up. This is generally relevant only if the SoC 
> provides the clock to an external PHY and the PHY is configured for RMII.

>  static const struct fec_devinfo fec_imx6q_info = {
> @@ -953,7 +954,8 @@ fec_restart(struct net_device *ndev)
>  	 * For i.MX6SX SOC, enet use AXI bus, we use disable MAC
>  	 * instead of reset MAC itself.
>  	 */
> -	if (fep->quirks & FEC_QUIRK_HAS_AVB) {
> +	if (fep->quirks & FEC_QUIRK_HAS_AVB ||
> +	    (fep->quirks & FEC_QUIRK_NO_HARD_RESET) && fep->link) {
>  		writel(0, fep->hwp + FEC_ECNTRL);
>  	} else {
>  		writel(1, fep->hwp + FEC_ECNTRL);

drivers/net/ethernet/freescale/fec_main.c: In function ‘fec_restart’:
drivers/net/ethernet/freescale/fec_main.c:958:46: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
  958 |      (fep->quirks & FEC_QUIRK_NO_HARD_RESET) && fep->link) {
      |      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~