Re: [net,PATCH v1] net: ks8851: Fix receiver error in 100BASE-TX mode following software power-down

[Jakub Kicinski]

This is an AI-generated review of your patch. The human sending this
email has considered the AI review valid, or at least plausible.
---
net: ks8851: Fix receiver error in 100BASE-TX mode following software power-down

Per the KSZ8851 errata (DS80000716D, Module 3), a software power-down
followed by a power-on can leave the receiver circuit non-functional. This
patch implements the documented workaround by dropping both transitions into
PMECR_PM_SOFTDOWN, in ks8851_write_mac_addr() and ks8851_net_stop().

Link: https://ww1.microchip.com/downloads/en/DeviceDoc/80000716D.pdf

> diff --git a/drivers/net/ethernet/micrel/ks8851_common.c b/drivers/net/ethernet/micrel/ks8851_common.c
> index 4afbb40bc0e4a..cd8dfca22720b 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;

[Severity: Low]
This isn't a bug, but should the two comments describing the software
power-down state be updated in the same patch?

After this hunk, ks8851_write_mac_addr() still carries this comment above
its remaining ks8851_set_powermode() call:

	/*
	 * Wake up chip in case it was powered off when stopped; otherwise,
	 * the first write to the MAC address does not take effect.
	 */
	ks8851_set_powermode(ks, PMECR_PM_NORMAL);

Nothing in the driver can put the chip into software power-down anymore, so
the "in case it was powered off when stopped" precondition can no longer
occur. A grep of the tree confirms PMECR_PM_SOFTDOWN has no users left,
only its definition in ks8851.h, and the two remaining
ks8851_set_powermode() call sites in ks8851_write_mac_addr() and
ks8851_net_start() both write PMECR_PM_NORMAL.

Since the commit message uses that very call site as the argument that
dropping the power-down here is safe, would it help to note in the comment
that the PM_NORMAL write is kept deliberately (e.g. referencing the errata),
so a later cleanup does not remove it or reintroduce the SOFTDOWN
transition?

> @@ -506,8 +503,6 @@ 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);

[Severity: Low]
Related to the above: the kernel-doc for ks8851_net_stop() just above this
function still promises the low power transition that is being removed here:

	/**
	 * 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.
	 */

The post-patch body only masks IER/ISR, zeroes RXCR1/TXCR, drains ks->txq
and calls free_irq(), with no PMECR access left. Should that last sentence
be dropped or reworded to mention the errata, so readers of
ks8851_suspend()->ks8851_net_stop() are not told a low power transition
happens?
-- 
pw-bot: cr