[net,PATCH v2] net: ks8851: Fix receiver error in 100BASE-TX mode following software power-down

KSZ8851 errata sheet DS80000716D-page 4 Module 3 [1] states that,
when issuing a software power-down (PMECR[1:0] = 10) followed by a
power-on (PMECR[1:0] = 00), the receiver circuit can fail to start
properly preventing communication. The Transmitter will still send
data, but no data will be received.

The errata sheet also includes a workaround, which states that,
it is recommended that the software power-down feature not be used.

Implement that workaround and drop the entry into software power-down
mode. The ks8851_write_mac_addr() calls entry into normal power-on
mode at the very beginning of the function, therefore dropping the
second call to enter software power-down mode is sufficient here.
The ks8851_net_stop() can only be called after ks8851_net_start()
was already called, and ks8851_net_start() also makes the MAC enter
normal power-on mode, therefore it is also fine to drop the call to
enter software power-down mode from ks8851_net_stop().

This will lead to a slight increase in power consumption, but it also
fixes a sporadic reliability problem on at least KSZ8851-16MLL, which
is where the problem was reported and this fix was tested.

[1] https://ww1.microchip.com/downloads/en/DeviceDoc/80000716D.pdf

Fixes: 3ba81f3ece3c ("net: Micrel KS8851 SPI network driver")
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Marek Vasut <redacted>
---
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Andrew Lunn <andrew+netdev@lunn.ch>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Ronald Wahl <redacted>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Yicong Hui <redacted>
Cc: kernel@dh-electronics.com
Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
---
V2: - Add RB from Sebastian
    - Update ks8851_net_stop() description based on input from Jakub
---
 drivers/net/ethernet/micrel/ks8851_common.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/micrel/ks8851_common.c b/drivers/net/ethernet/micrel/ks8851_common.c
index 4afbb40bc0e4a..d49f281c78676 100644
--- a/drivers/net/ethernet/micrel/ks8851_common.c
+++ b/drivers/net/ethernet/micrel/ks8851_common.c

@@ -139,17 +139,14 @@ static int ks8851_write_mac_addr(struct net_device *dev)
 	ks8851_set_powermode(ks, PMECR_PM_NORMAL);
 
 	for (i = 0; i < ETH_ALEN; i += 2) {
 		val = (dev->dev_addr[i] << 8) | dev->dev_addr[i + 1];
 		ks8851_wrreg16(ks, KS_MAR(i), val);
 	}
 
-	if (!netif_running(dev))
-		ks8851_set_powermode(ks, PMECR_PM_SOFTDOWN);
-
 	ks8851_unlock(ks);
 
 	return 0;
 }
 
 /**
  * ks8851_read_mac_addr - read mac address from device registers

@@ -474,16 +471,15 @@ static int ks8851_net_open(struct net_device *dev)
 }
 
 /**
  * ks8851_net_stop - close network device
  * @dev: The device being closed.
  *
  * Called to close down a network device which has been active. Cancel any
- * work, shutdown the RX and TX process and then place the chip into a low
- * power state whilst it is not being used.
+ * work and shutdown the RX and TX process.
  */
 static int ks8851_net_stop(struct net_device *dev)
 {
 	struct ks8851_net *ks = netdev_priv(dev);
 
 	netif_info(ks, ifdown, dev, "shutting down\n");
 

@@ -502,16 +498,14 @@ static int ks8851_net_stop(struct net_device *dev)
 	ks8851_lock(ks);
 	/* shutdown RX process */
 	ks8851_wrreg16(ks, KS_RXCR1, 0x0000);
 
 	/* shutdown TX process */
 	ks8851_wrreg16(ks, KS_TXCR, 0x0000);
 
-	/* set powermode to soft power down to save power */
-	ks8851_set_powermode(ks, PMECR_PM_SOFTDOWN);
 	ks8851_unlock(ks);
 
 	/* ensure any queued tx buffers are dumped */
 	while (!skb_queue_empty(&ks->txq)) {
 		struct sk_buff *txb = skb_dequeue(&ks->txq);
 
 		netif_dbg(ks, ifdown, ks->netdev,

-- 
2.53.0