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net: ks8851: Fix receiver error in 100BASE-TX mode following software power-down

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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: 3ba81f3ece ("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>
This commit is contained in:
Marek Vasut 2026-09-05 15:02:32 +02:00 committed by Jakub Kicinski
parent 59fb389ad6
commit 66ef5adb75
1 changed files with 1 additions and 7 deletions
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8
drivers/net/ethernet/micrel/ks8851_common.c
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@ -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 */