netdev - Re: [PATCH] net: macb: reject unsupported rgmii delays

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Date:   Wed, 17 Jun 2020 12:40:25 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Helmut Grohne <helmut.grohne@...enta.de>
Cc:     Nicolas Ferre <nicolas.ferre@...rochip.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: macb: reject unsupported rgmii delays

On Wed, Jun 17, 2020 at 01:21:53PM +0200, Helmut Grohne wrote:
> On Wed, Jun 17, 2020 at 12:55:18PM +0200, Russell King - ARM Linux admin wrote:
> > The individual RGMII delay modes are more about what the PHY itself is
> > asked to do with respect to inserting delays, so I don't think your
> > patch makes sense.
> 
> This seems to be the same aspect that Vladimir Oltean remarked. I agree
> that the relevant hunk should be dropped.
> 
> > > --- a/drivers/net/ethernet/cadence/macb_main.c
> > > +++ b/drivers/net/ethernet/cadence/macb_main.c
> > > @@ -514,7 +514,7 @@ static void macb_validate(struct phylink_config *config,
> > >  	    state->interface != PHY_INTERFACE_MODE_RMII &&
> > >  	    state->interface != PHY_INTERFACE_MODE_GMII &&
> > >  	    state->interface != PHY_INTERFACE_MODE_SGMII &&
> > > -	    !phy_interface_mode_is_rgmii(state->interface)) {
> > > +	    state->interface != PHY_INTERFACE_MODE_RGMII_ID) {
> > 
> > Here you reject everything except PHY_INTERFACE_MODE_RGMII_ID.
> > 
> > >  		bitmap_zero(supported, __ETHTOOL_LINK_MODE_MASK_NBITS);
> > >  		return;
> > >  	}
> > > @@ -694,6 +694,13 @@ static int macb_phylink_connect(struct macb *bp)
> > >  	struct phy_device *phydev;
> > >  	int ret;
> > >  
> > > +	if (of_phy_is_fixed_link(dn) &&
> > > +	    phy_interface_mode_is_rgmii(bp->phy_interface) &&
> > > +	    bp->phy_interface != PHY_INTERFACE_MODE_RGMII) {
> > 
> > but here you reject everything except PHY_INTERFACE_MODE_RGMII.  These
> > can't both be right.  If you start with PHY_INTERFACE_MODE_RGMII, and
> > have a fixed link, you'll have PHY_INTERFACE_MODE_RGMII passed into
> > the validate function, which will then fail.
> 
> For a fixed-link, the validation function is never called. Therefore, it
> cannot reject PHY_INTERFACE_MODE_RGMII. It works in practice.

Hmm, I'm not so sure, but then I don't know exactly what code you're
using.  Looking at mainline, even for a fixed link, you call
phylink_create().  phylink_create() will spot the fixed link, and
parse the description, calling the validation function.  If that
fails, it will generate a warning at that point:

  "fixed link %s duplex %dMbps not recognised"

It doesn't cause an operational failure, but it means that you end up
with a zero supported mask, which is likely not expected.

This is not an expected situation, so I'll modify your claim to "it
works but issues a warning" which still means that it's not correct.

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