linux-kernel - Re: [PATCH v2 2/5] net: macb: add support for sgmii MAC-PHY interface

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Date:   Wed, 19 Jun 2019 13:32:06 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Parshuram Raju Thombare <pthombar@...ence.com>
Cc:     "andrew@...n.ch" <andrew@...n.ch>,
        "nicolas.ferre@...rochip.com" <nicolas.ferre@...rochip.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "hkallweit1@...il.com" <hkallweit1@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rafal Ciepiela <rafalc@...ence.com>,
        Anil Joy Varughese <aniljoy@...ence.com>,
        Piotr Sroka <piotrs@...ence.com>
Subject: Re: [PATCH v2 2/5] net: macb: add support for sgmii MAC-PHY interface

On Wed, Jun 19, 2019 at 11:23:01AM +0000, Parshuram Raju Thombare wrote:
> >From: Russell King - ARM Linux admin <linux@...linux.org.uk>
> >
> >On Wed, Jun 19, 2019 at 09:40:46AM +0100, Parshuram Thombare wrote:
> >
> >> This patch add support for SGMII interface) and
> >
> >> 2.5Gbps MAC in Cadence ethernet controller driver.
> 
> >>  	switch (state->interface) {
> >
> >> +	case PHY_INTERFACE_MODE_SGMII:
> >
> >> +		if (bp->caps & MACB_CAPS_GIGABIT_MODE_AVAILABLE)
> >
> >> +			phylink_set(mask, 2500baseT_Full);
> >
> >
> >
> >This doesn't look correct to me.  SGMII as defined by Cisco only
> >supports 1G, 100M and 10M speeds, not 2.5G.
> 
> Cadence MAC support 2.5G SGMII by using higher clock frequency.

Ok, so why not set 2.5GBASE-X too?  Does the MAC handle auto-detecting
the SGMII/BASE-X speed itself or does it need to be programmed?  If it
needs to be programmed, you need additional handling in the validate
callback to deal with that.

> >> +	case PHY_INTERFACE_MODE_2500BASEX:
> >
> >> +		if (bp->caps & MACB_CAPS_GIGABIT_MODE_AVAILABLE)
> >
> >> +			phylink_set(mask, 2500baseX_Full);
> >
> >> +	/* fallthrough */
> >
> >> +	case PHY_INTERFACE_MODE_1000BASEX:
> >
> >> +		if (bp->caps & MACB_CAPS_GIGABIT_MODE_AVAILABLE)
> >
> >> +			phylink_set(mask, 1000baseX_Full);
> >
> >> +		break;
> >
> >
> >
> >Please see how other drivers which use phylink deal with the validate()
> >format, and please read the phylink documentation:
> >
> > * Note that the PHY may be able to transform from one connection
> > * technology to another, so, eg, don't clear 1000BaseX just
> > * because the MAC is unable to BaseX mode. This is more about
> > * clearing unsupported speeds and duplex settings.
> >
> 
> There are some configs used in this driver which limits MAC speed.
> Above checks just to make sure this use case does not break.

That's not what I'm saying.

By way of example, you're offering 1000BASE-T just because the MAC
connection supports it.  However, the MAC doesn't _actually_ support
1000BASE-T, it supports a connection to a PHY that _happens_ to
convert the MAC connection to 1000BASE-T.  It could equally well
convert the MAC connection to 1000BASE-X.

So, only setting 1000BASE-X when you have a PHY connection using
1000BASE-X is fundamentally incorrect.

For example, you could have a MAC <-> PHY link using standard 1.25Gbps
SGMII, and the PHY offers 1000BASE-T _and_ 1000BASE-X connections on
a first-link-up basis.  An example of a PHY that does this are the
Marvell 1G PHYs (eg, 88E151x).

This point is detailed in the PHYLINK documentation, which I quoted
above.

> >> @@ -506,18 +563,26 @@ static void gem_mac_config(struct phylink_config
> >*pl_config, unsigned int mode,
> >>  		switch (state->speed) {
> >> +		case SPEED_2500:
> >> +			gem_writel(bp, NCFGR, GEM_BIT(GBE) |
> >> +				   gem_readl(bp, NCFGR));
> >>  		}
> >> -		macb_or_gem_writel(bp, NCFGR, reg);
> >>
> >>  		bp->speed = state->speed;
> >>  		bp->duplex = state->duplex;
> >
> >
> >
> >This is not going to work for 802.3z nor SGMII properly when in-band
> >negotiation is used.  We don't know ahead of time what the speed and
> >duplex will be.  Please see existing drivers for examples showing
> >how mac_config() should be implemented (there's good reason why its
> >laid out as it is in those drivers.)
> >
> Ok, Here I will configure MAC only for FIXED and PHY mode.

As you are not the only one who has made this error, I'm considering
splitting mac_config() into mac_config_fixed() and mac_config_inband()
so that it's clearer what is required.  Maybe even taking separate
structures so that it's impossible to access members that should not
be used.

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