Re: [PATCH net v3 1/1] net: pcs: xpcs: fix incorrect CL37 AN sequence

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Thu, Sep 30, 2021 at 10:24:39AM +0100, Russell King (Oracle) wrote:
> On Thu, Sep 30, 2021 at 12:44:21PM +0800, Wong Vee Khee wrote:
> > According to Synopsys DesignWare Cores Ethernet PCS databook, it is
> > required to disable Clause 37 auto-negotiation by programming bit-12
> > (AN_ENABLE) to 0 if it is already enabled, before programming various
> > fields of VR_MII_AN_CTRL registers.
> > 
> > After all these programming are done, it is then required to enable
> > Clause 37 auto-negotiation by programming bit-12 (AN_ENABLE) to 1.
> > 
> > Fixes: b97b5331b8ab ("net: pcs: add C37 SGMII AN support for intel mGbE controller")
> > Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
> > Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
> > ---
> > v2 -> v3:
> >  - Added error handling after xpcs_write().
> >  - Added 'changed' flag.
> >  - Added fixes tag.
> > v1 -> v2:
> >  - Removed use of xpcs_modify() helper function.
> >  - Add conditional check on inband auto-negotiation.
> > ---
> >  drivers/net/pcs/pcs-xpcs.c | 41 +++++++++++++++++++++++++++++++++-----
> >  1 file changed, 36 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/net/pcs/pcs-xpcs.c b/drivers/net/pcs/pcs-xpcs.c
> > index fb0a83dc09ac..d2126f5d5016 100644
> > --- a/drivers/net/pcs/pcs-xpcs.c
> > +++ b/drivers/net/pcs/pcs-xpcs.c
> > @@ -697,14 +697,18 @@ EXPORT_SYMBOL_GPL(xpcs_config_eee);
> >  
> >  static int xpcs_config_aneg_c37_sgmii(struct dw_xpcs *xpcs, unsigned int mode)
> >  {
> > -	int ret;
> > +	int ret, reg_val;
> > +	int changed = 0;
> >  
> >  	/* For AN for C37 SGMII mode, the settings are :-
> > -	 * 1) VR_MII_AN_CTRL Bit(2:1)[PCS_MODE] = 10b (SGMII AN)
> > -	 * 2) VR_MII_AN_CTRL Bit(3) [TX_CONFIG] = 0b (MAC side SGMII)
> > +	 * 1) VR_MII_MMD_CTRL Bit(12) [AN_ENABLE] = 0b (Disable SGMII AN in case
> > +	      it is already enabled)
> > +	 * 2) VR_MII_AN_CTRL Bit(2:1)[PCS_MODE] = 10b (SGMII AN)
> > +	 * 3) VR_MII_AN_CTRL Bit(3) [TX_CONFIG] = 0b (MAC side SGMII)
> >  	 *    DW xPCS used with DW EQoS MAC is always MAC side SGMII.
> > -	 * 3) VR_MII_DIG_CTRL1 Bit(9) [MAC_AUTO_SW] = 1b (Automatic
> > +	 * 4) VR_MII_DIG_CTRL1 Bit(9) [MAC_AUTO_SW] = 1b (Automatic
> >  	 *    speed/duplex mode change by HW after SGMII AN complete)
> > +	 * 5) VR_MII_MMD_CTRL Bit(12) [AN_ENABLE] = 1b (Enable SGMII AN)
> >  	 *
> >  	 * Note: Since it is MAC side SGMII, there is no need to set
> >  	 *	 SR_MII_AN_ADV. MAC side SGMII receives AN Tx Config from
> > @@ -712,6 +716,19 @@ static int xpcs_config_aneg_c37_sgmii(struct dw_xpcs *xpcs, unsigned int mode)
> >  	 *	 between PHY and Link Partner. There is also no need to
> >  	 *	 trigger AN restart for MAC-side SGMII.
> >  	 */
> > +	ret = xpcs_read(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL);
> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	if (ret & AN_CL37_EN) {
> > +		changed = 1;
> > +		reg_val = ret & ~AN_CL37_EN;
> > +		ret = xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL,
> > +				 reg_val);
> > +		if (ret < 0)
> > +			return ret;
> > +	}
> 
> I don't think you need to record "changed" here - just maintain a
> local copy of the current state of the register, e.g:
> 
> +	ret = xpcs_read(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL);
> +	if (ret < 0)
> +		return ret;
> +
> +	reg_val = ret;
> +	if (reg_val & AN_CL37_EN) {
> +		reg_val &= ~AN_CL37_EN;
> ...
> 
> What you're wanting to do is ensure this bit is clear before you do the
> register changes, and once complete, set the bit back to one. If the bit
> was previously clear, you can omit this write, otherwise the write was
> needed - as you have the code. However, the point is that once you're
> past this code, the state of the register in the device will have the
> AN_CL37_EN clear. See below for the rest of my comments on this...

VK, I don't want to force you towards a certain implementation, but I
just want to throw this out there, this works for me, and doesn't do
more reads or writes than strictly necessary, and it also breaks up the
wall-of-text comment into more digestible pieces:

static int xpcs_config_aneg_c37_sgmii(struct dw_xpcs *xpcs, unsigned int mode)
{
	int ret, mdio_ctrl1, old_an_ctrl, an_ctrl, old_dig_ctrl1, dig_ctrl1;

	/* Disable SGMII AN in case it is already enabled */
	mdio_ctrl1 = xpcs_read(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL);
	if (mdio_ctrl1 < 0)
		return mdio_ctrl1;

	if (mdio_ctrl1 & AN_CL37_EN) {
		ret = xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL,
				 mdio_ctrl1 & ~AN_CL37_EN);
		if (ret < 0)
			return ret;
	}

	/* Set VR_MII_AN_CTRL[PCS_MODE] to SGMII AN, and
	 * VR_MII_AN_CTRL[TX_CONFIG] to MAC side SGMII.
	 */
	old_an_ctrl = xpcs_read(xpcs, MDIO_MMD_VEND2, DW_VR_MII_AN_CTRL);
	if (old_an_ctrl < 0)
		return old_an_ctrl;

	an_ctrl = old_an_ctrl & ~(DW_VR_MII_PCS_MODE_MASK | DW_VR_MII_TX_CONFIG_MASK);
	an_ctrl |= (DW_VR_MII_PCS_MODE_C37_SGMII << DW_VR_MII_AN_CTRL_PCS_MODE_SHIFT) &
		   DW_VR_MII_PCS_MODE_MASK;
	an_ctrl |= (DW_VR_MII_TX_CONFIG_MAC_SIDE_SGMII << DW_VR_MII_AN_CTRL_TX_CONFIG_SHIFT) &
		   DW_VR_MII_TX_CONFIG_MASK;

	if (an_ctrl != old_an_ctrl) {
		ret = xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_AN_CTRL, an_ctrl);
		if (ret < 0)
			return ret;
	}

	/* If using in-band autoneg, enable automatic speed/duplex mode change
	 * by HW after SGMII AN complete.
	 * 5) VR_MII_MMD_CTRL Bit(12) [AN_ENABLE] = 1b (Enable SGMII AN)
	 */
	old_dig_ctrl1 = xpcs_read(xpcs, MDIO_MMD_VEND2, DW_VR_MII_DIG_CTRL1);
	if (old_dig_ctrl1 < 0)
		return old_dig_ctrl1;

	if (phylink_autoneg_inband(mode))
		dig_ctrl1 = old_dig_ctrl1 | DW_VR_MII_DIG_CTRL1_MAC_AUTO_SW;
	else
		dig_ctrl1 = old_dig_ctrl1 & ~DW_VR_MII_DIG_CTRL1_MAC_AUTO_SW;

	if (dig_ctrl1 != old_dig_ctrl1) {
		ret = xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_DIG_CTRL1, dig_ctrl1);
		if (ret < 0)
			return ret;
	}

	if (!(mdio_ctrl1 & AN_CL37_EN) && phylink_autoneg_inband(mode)) {
		ret = xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL,
				 mdio_ctrl1 | AN_CL37_EN);
		if (ret)
			return ret;
	}

	/* Note: Since it is MAC side SGMII, there is no need to set
	 * SR_MII_AN_ADV. MAC side SGMII receives AN Tx Config from PHY about
	 * the link state change after C28 AN is completed between PHY and Link
	 * Partner. There is also no need to trigger AN restart for MAC-side
	 * SGMII.
	 */

	return 0;
}

> 
> > @@ -736,7 +753,21 @@ static int xpcs_config_aneg_c37_sgmii(struct dw_xpcs *xpcs, unsigned int mode)
> >  	else
> >  		ret &= ~DW_VR_MII_DIG_CTRL1_MAC_AUTO_SW;
> >  
> > -	return xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_DIG_CTRL1, ret);
> > +	ret = xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_DIG_CTRL1, ret);
> > +	if (ret < 0)
> > +		return ret;
> > +
> > +	if (changed) {
> > +		if (phylink_autoneg_inband(mode))
> > +			reg_val |= AN_CL37_EN;
> > +		else
> > +			reg_val &= ~AN_CL37_EN;
> > +
> > +		return xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL,
> > +				  reg_val);
> > +	}
> 
> As I say above, here, you are _guaranteed_ that the AN_CL27_EN bit is
> clear in the register due to the effects of your change above. You say
> in the commit text:
> 
>   After all these programming are done, it is then required to enable
>   Clause 37 auto-negotiation by programming bit-12 (AN_ENABLE) to 1.
> 
> So that makes me think that you _always_ need to write back a value
> with AN_CL27_EN set. So:
> 
> 	reg_val |= AN_CL37_EN;
> 	return xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL, reg_val);

I will only say this: modifying the last part of the function I posted
above like this:

//	if (!(mdio_ctrl1 & AN_CL37_EN) && phylink_autoneg_inband(mode)) {
		ret = xpcs_write(xpcs, MDIO_MMD_VEND2, DW_VR_MII_MMD_CTRL,
				 mdio_ctrl1 | AN_CL37_EN);
		if (ret)
			return ret;
//	}

aka unconditionally writing an mdio_ctrl1 value with the AN_CL37_EN bit set,
will still end up with a functional link. But the only reason for that
is this:

static void xpcs_link_up_sgmii(struct dw_xpcs *xpcs, unsigned int mode,
			       int speed, int duplex)
{
	int val, ret;

	if (phylink_autoneg_inband(mode))
		return;

	switch (speed) {
	case SPEED_1000:
		val = BMCR_SPEED1000;
		break;
	case SPEED_100:
		val = BMCR_SPEED100;
		break;
	case SPEED_10:
		val = BMCR_SPEED10;
		break;
	default:
		return;
	}

	if (duplex == DUPLEX_FULL)
		val |= BMCR_FULLDPLX;

	ret = xpcs_write(xpcs, MDIO_MMD_VEND2, MDIO_CTRL1, val);   <- this clears the AN_CL37_EN bit again
	if (ret)
		pr_err("%s: xpcs_write returned %pe\n", __func__, ERR_PTR(ret));
}

If I add "val |= AN_CL37_EN;" before the xpcs_write in xpcs_link_up_sgmii(),
my SGMII link with in-band autoneg turned off breaks.

> If that is not correct, the commit message is misleading and needs
> fixing.
> 
> Lastly, I would recommend a much better name for this variable rather
> than the bland "reg_val" since you're needing the value preserved over
> a chunk of code. "ctrl" maybe?