[PATCH net-next v4 06/10] net: pcs: xpcs: add ANRESTART support for SGMII link recovery

[Jakub Kicinski]

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---
net: pcs: xpcs: add ANRESTART support for SGMII link recovery

The patch sets BMCR_ANRESTART together with BMCR_ANENABLE when configuring
C37 SGMII in-band negotiation, and clears the latched CL37 AN complete
status before doing so.  It also restarts negotiation from
xpcs_get_state_c37_sgmii() when AN completion is latched but the link is
reported down.

>     On some hardware using the DesignWare XPCS IP (e.g., RK3568 MAC side
>     SGMII), the PCS does not automatically restart auto-negotiation when the
>     link goes down and comes back up. Without an explicit ANRESTART, the link
>     stays down forever.

[Severity: Low]
The changelog describes a user visible failure ("the link stays down
forever") in long standing shared code, but there is no Fixes: tag and no
statement about which histories are affected.

The only hardware named here, RK3568 XPCS, does not exist in the baseline
tree: searching for xpcs_rk, rk3568-xpcs or pcs-xpcs-rk under drivers/net at
879e280b8486 returns nothing, and the glue driver arrives later in this
series ("net: pcs: xpcs: add Rockchip RK3568 platform glue driver").

Could the changelog either name the pre-existing commit being fixed, or say
explicitly that the affected platform is introduced later in the same
series, so that stable tooling does not have to guess?

> diff --git a/drivers/net/pcs/pcs-xpcs.c b/drivers/net/pcs/pcs-xpcs.c
> index 0337e2bcc0125..50b9cdf94fdaa 100644
> --- a/drivers/net/pcs/pcs-xpcs.c
> +++ b/drivers/net/pcs/pcs-xpcs.c
> @@ -816,9 +816,14 @@ static int xpcs_config_aneg_c37_sgmii(struct dw_xpcs *xpcs,
>  	if (ret < 0)
>  		return ret;
>  
> +	/* Clear CL37 AN complete status */
> +	ret = xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_AN_INTR_STS, 0);
> +	if (ret < 0)
> +		return ret;
> +
>  	if (neg_mode == PHYLINK_PCS_NEG_INBAND_ENABLED)
>  		ret = xpcs_write(xpcs, MDIO_MMD_VEND2, MII_BMCR,
> -				 mdio_ctrl | BMCR_ANENABLE);
> +				 mdio_ctrl | BMCR_ANENABLE | BMCR_ANRESTART);

[Severity: Medium]
Should the comment at the top of xpcs_config_aneg_c37_sgmii() be updated
along with this change?  It still says the opposite of what the code now
does:

drivers/net/pcs/pcs-xpcs.c:xpcs_config_aneg_c37_sgmii() {
	/* For AN for C37 SGMII mode, the settings are :-
	 ...
	 * 5) VR_MII_MMD_CTRL Bit(12) [AN_ENABLE] = 1b (Enable SGMII AN)
	 *
	 * Note that VR_MII_MMD_CTRL is MII_BMCR.
	 *
	 * Note: Since it is MAC side SGMII, there is no need to set
	 *	 SR_MII_AN_ADV. ... There is also no need to
	 *	 trigger AN restart for MAC-side SGMII.
	 */

On the default path this same function selects MAC side SGMII:

	} else {
		tx_conf = DW_VR_MII_TX_CONFIG_MAC_SIDE_SGMII;
	}

so the restart is now issued for exactly the case the comment says does not
need it.  The new DW_VR_MII_AN_INTR_STS write is also a sixth step that the
enumerated sequence does not mention.

If the databook statement no longer holds for these parts, could the
comment be corrected, so that a later reader does not remove the ANRESTART
bit as a mistake?

> @@ -1093,10 +1098,15 @@ static int xpcs_get_state_c37_sgmii(struct dw_xpcs *xpcs,
>  		return 0;
>  	}
>  
> -	/* Clear AN complete status or interrupt */
> -	if (state->an_complete)
> +	if (state->an_complete) {
> +		/* Clear AN complete status or interrupt */
>  		xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_AN_INTR_STS, 0);
>  
> +		/* Initiate the next round of AN */
> +		xpcs_modify(xpcs, MDIO_MMD_VEND2, MII_BMCR, BMCR_ANRESTART,
> +			    BMCR_ANRESTART);
> +	}
> +
>  	return 0;
>  }

[Severity: Medium]
Can this sequence get stuck if the xpcs_modify() fails?

The latched status is cleared first, and DW_VR_MII_AN_STS_C37_ANCMPLT_INTR
is the only thing that brings execution back into this branch:

	state->an_complete = ret & DW_VR_MII_AN_STS_C37_ANCMPLT_INTR;

So if the clear succeeds but the restart does not, every later poll reads
ANCMPLT as 0, state->an_complete stays false, and the restart is never
retried, leaving the port down with AN idle until something calls
pcs_config again.

mdiodev_c45_modify() does a read followed by a write, so there are two
places it can return an error, for example a bus -ETIMEDOUT, or with the
Rockchip glue added later in this series the pm_runtime_resume_and_get()
error path in xpcs_rk_read_reg() / xpcs_rk_write_reg().

Both return values are also discarded and the function still ends with
return 0, so the diagnostic in the caller can never fire for them:

	case DW_AN_C37_SGMII:
		ret = xpcs_get_state_c37_sgmii(xpcs, state);
		if (ret)
			dev_err(&xpcs->mdiodev->dev, "%s returned %pe\n",
				"xpcs_get_state_c37_sgmii", ERR_PTR(ret));

Would it be better to assert BMCR_ANRESTART before clearing the latched
status, and to propagate both return codes to xpcs_get_state()?