Messages in this thread | | | Subject | Re: [PATCH net 1/3] net: bcmgenet: use RGMII loopback for MAC reset | From | Florian Fainelli <> | Date | Tue, 5 Nov 2019 19:45:02 -0800 |
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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@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com> -- Florian
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