netdev - Re: [PATCH net 1/3] net: bcmgenet: use RGMII loopback for MAC reset

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Date:   Tue, 5 Nov 2019 19:45:02 -0800
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Doug Berger <opendmb@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     bcm-kernel-feedback-list@...adcom.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/3] net: bcmgenet: use RGMII loopback for MAC reset



On 11/5/2019 11:07 AM, Doug Berger wrote:
> As noted in commit 28c2d1a7a0bf ("net: bcmgenet: enable loopback
> during UniMAC sw_reset") the UniMAC must be clocked while sw_reset
> is asserted for its state machines to reset cleanly.
> 
> The transmit and receive clocks used by the UniMAC are derived from
> the signals used on its PHY interface. The bcmgenet MAC can be
> configured to work with different PHY interfaces including MII,
> GMII, RGMII, and Reverse MII on internal and external interfaces.
> Unfortunately for the UniMAC, when configured for MII the Tx clock
> is always driven from the PHY which places it outside of the direct
> control of the MAC.
> 
> The earlier commit enabled a local loopback mode within the UniMAC
> so that the receive clock would be derived from the transmit clock
> which addressed the observed issue with an external GPHY disabling
> it's Rx clock. However, when a Tx clock is not available this
> loopback is insufficient.
> 
> This commit implements a workaround that leverages the fact that
> the MAC can reliably generate all of its necessary clocking by
> enterring the external GPHY RGMII interface mode with the UniMAC in
> local loopback during the sw_reset interval. Unfortunately, this
> has the undesirable side efect of the RGMII GTXCLK signal being
> driven during the same window.
> 
> In most configurations this is a benign side effect as the signal
> is either not routed to a pin or is already expected to drive the
> pin. The one exception is when an external MII PHY is expected to
> drive the same pin with its TX_CLK output creating output driver
> contention.
> 
> This commit exploits the IEEE 802.3 clause 22 standard defined
> isolate mode to force an external MII PHY to present a high
> impedance on its TX_CLK output during the window to prevent any
> contention at the pin.
> 
> The MII interface is used internally with the 40nm internal EPHY
> which agressively disables its clocks for power savings leading to
> incomplete resets of the UniMAC and many instabilities observed
> over the years. The workaround of this commit is expected to put
> an end to those problems.
> 
> Fixes: 1c1008c793fa ("net: bcmgenet: add main driver file")
> Signed-off-by: Doug Berger <opendmb@...il.com>

Acked-by: Florian Fainelli <f.fainelli@...il.com>
-- 
Florian