From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Marek Vasut <marex@nabladev.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.18 0900/1250] net: ks8851: Fix receiver error in 100BASE-TX mode following software power-down
Date: Thu, 17 Sep 2026 16:11:35 +0100
Message-ID: <20260917151616.383278342@linuxfoundation.org>
In-Reply-To: <20260917151551.901433442@linuxfoundation.org>
References: <20260917151551.901433442@linuxfoundation.org>
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6.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Marek Vasut <marex@nabladev.com>

[ Upstream commit 66ef5adb75446627f8b6c26cd04f2adc86d4de56 ]

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 <marex@nabladev.com>
Link: https://patch.msgid.link/20260905130327.203851-1-marex@nabladev.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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
@@ -143,9 +143,6 @@ static int ks8851_write_mac_addr(struct net_device *dev)
 		ks8851_wrreg16(ks, KS_MAR(i), val);
 	}
 
-	if (!netif_running(dev))
-		ks8851_set_powermode(ks, PMECR_PM_SOFTDOWN);
-
 	ks8851_unlock(ks);
 
 	return 0;
@@ -478,8 +475,7 @@ static int ks8851_net_open(struct net_device *dev)
  * @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)
 {
@@ -506,8 +502,6 @@ static int ks8851_net_stop(struct net_device *dev)
 	/* 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 */
-- 
2.53.0