netdev - Re: [PATCH RFC net-next 4/4] net: xpcs: allow 1000BASE-X to work with older XPCS IP

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Message-ID: <Z6nnwfPtm9LqK3rd@shell.armlinux.org.uk>
Date: Mon, 10 Feb 2025 11:49:21 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Vladimir Oltean <olteanv@...il.com>
Cc: Tristram.Ha@...rochip.com, UNGLinuxDriver@...rochip.com,
	Woojung Huh <woojung.huh@...rochip.com>,
	Andrew Lunn <andrew@...n.ch>,
	Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH RFC net-next 4/4] net: xpcs: allow 1000BASE-X to work
 with older XPCS IP

On Mon, Feb 10, 2025 at 01:05:55PM +0200, Vladimir Oltean wrote:
> On Wed, Feb 05, 2025 at 01:27:47PM +0000, Russell King (Oracle) wrote:
> > Older XPCS IP requires SGMII_LINK and PHY_SIDE_SGMII to be set when
> > operating in 1000BASE-X mode even though the XPCS is not configured for
> > SGMII. An example of a device with older XPCS IP is KSZ9477.
> > 
> > We already don't clear these bits if we switch from SGMII to 1000BASE-X
> > on TXGBE - which would result in 1000BASE-X with the PHY_SIDE_SGMII bit
> > left set.
> 
> Is there a confirmation written down somewhere that a transition from
> SGMII to 1000Base-X was explicitly tested? I have to remain a bit
> skeptical and say that although the code is indeed like this, it
> doesn't mean by itself there are no unintended side effects.
> 
> > It is currently believed to be safe to set both bits on newer IP
> > without side-effects.
> > 
> > Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
> > ---
> >  drivers/net/pcs/pcs-xpcs.c | 13 +++++++++++--
> >  drivers/net/pcs/pcs-xpcs.h |  1 +
> >  2 files changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/pcs/pcs-xpcs.c b/drivers/net/pcs/pcs-xpcs.c
> > index 1eba0c583f16..d522e4a5a138 100644
> > --- a/drivers/net/pcs/pcs-xpcs.c
> > +++ b/drivers/net/pcs/pcs-xpcs.c
> > @@ -774,9 +774,18 @@ static int xpcs_config_aneg_c37_1000basex(struct dw_xpcs *xpcs,
> >  			return ret;
> >  	}
> >  
> > -	mask = DW_VR_MII_PCS_MODE_MASK;
> > +	/* Older XPCS IP requires PHY_MODE (bit 3) and SGMII_LINK (but 4) to
>                                                                    ~~~
>                                                                    bit
> 
> > +	 * be set when operating in 1000BASE-X mode. See page 233
> > +	 * https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/KSZ9477S-Data-Sheet-DS00002392C.pdf
> > +	 * "5.5.9 SGMII AUTO-NEGOTIATION CONTROL REGISTER"
> > +	 */
> > +	mask = DW_VR_MII_PCS_MODE_MASK | DW_VR_MII_AN_CTRL_SGMII_LINK |
> > +	       DW_VR_MII_TX_CONFIG_MASK;
> >  	val = FIELD_PREP(DW_VR_MII_PCS_MODE_MASK,
> > -			 DW_VR_MII_PCS_MODE_C37_1000BASEX);
> > +			 DW_VR_MII_PCS_MODE_C37_1000BASEX) |
> > +	      FIELD_PREP(DW_VR_MII_TX_CONFIG_MASK,
> > +			 DW_VR_MII_TX_CONFIG_PHY_SIDE_SGMII) |
> > +	      DW_VR_MII_AN_CTRL_SGMII_LINK;
> >  
> >  	if (!xpcs->pcs.poll) {
> >  		mask |= DW_VR_MII_AN_INTR_EN;
> 
> I do believe that this is the kind of patch one would write when the
> hardware is completely a black box. But when we have Microchip engineers
> here with a channel open towards their hardware design who can help
> clarify where the requirement comes from, that just isn't the case.
> So I wouldn't rush with this.
> 
> Plus, it isn't even the most conservative way in which a (supposedly)
> integration-specific requirement is fulfilled in the common Synopsys
> driver. If one integration makes vendor-specific choices about these
> bits, I wouldn't assume that no other vendors made contradictory choices.
> 
> I don't want to say too much before Tristram comes with a statement from
> Microchip hardware design, but _if_ it turns out to be a KSZ9477
> specific requirement, it still seems safer to only enable this based
> (at least) on Tristram's MICROCHIP_KSZ9477_PMA_ID conditional from his
> other patch set, if not based on something stronger (a conditional
> describing some functional behavior, rather than a specific hardware IP).

So Jose's public reassurance means nothing?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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