linux-kernel - Re: [PATCH net-next v3 09/12] net: dsa: lantiq_gswip: add vendor property to setup MII refclk output

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Message-ID: <20251028014407.x5jrwpw6f5rsfi5f@skbuf>
Date: Tue, 28 Oct 2025 03:44:07 +0200
From: Vladimir Oltean <olteanv@...il.com>
To: Daniel Golle <daniel@...rotopia.org>
Cc: Hauke Mehrtens <hauke@...ke-m.de>, Andrew Lunn <andrew@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Simon Horman <horms@...nel.org>,
	Russell King <linux@...linux.org.uk>, netdev@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andreas Schirm <andreas.schirm@...mens.com>,
	Lukas Stockmann <lukas.stockmann@...mens.com>,
	Alexander Sverdlin <alexander.sverdlin@...mens.com>,
	Peter Christen <peter.christen@...mens.com>,
	Avinash Jayaraman <ajayaraman@...linear.com>,
	Bing tao Xu <bxu@...linear.com>, Liang Xu <lxu@...linear.com>,
	Juraj Povazanec <jpovazanec@...linear.com>,
	"Fanni (Fang-Yi) Chan" <fchan@...linear.com>,
	"Benny (Ying-Tsan) Weng" <yweng@...linear.com>,
	"Livia M. Rosu" <lrosu@...linear.com>,
	John Crispin <john@...ozen.org>
Subject: Re: [PATCH net-next v3 09/12] net: dsa: lantiq_gswip: add vendor
 property to setup MII refclk output

On Mon, Oct 27, 2025 at 11:48:26PM +0000, Daniel Golle wrote:
> On Tue, Oct 28, 2025 at 01:36:26AM +0200, Vladimir Oltean wrote:
> > On Sun, Oct 26, 2025 at 11:47:21PM +0000, Daniel Golle wrote:
> > > Read boolean Device Tree property "maxlinear,rmii-refclk-out" and switch
> > > the RMII reference clock to be a clock output rather than an input if it
> > > is set.
> > > 
> > > Signed-off-by: Daniel Golle <daniel@...rotopia.org>
> > > ---
> > >  drivers/net/dsa/lantiq/lantiq_gswip_common.c | 4 ++++
> > >  1 file changed, 4 insertions(+)
> > > 
> > > diff --git a/drivers/net/dsa/lantiq/lantiq_gswip_common.c b/drivers/net/dsa/lantiq/lantiq_gswip_common.c
> > > index 60a83093cd10..bf38ecc13f76 100644
> > > --- a/drivers/net/dsa/lantiq/lantiq_gswip_common.c
> > > +++ b/drivers/net/dsa/lantiq/lantiq_gswip_common.c
> > > @@ -1442,6 +1442,10 @@ static void gswip_phylink_mac_config(struct phylink_config *config,
> > >  		return;
> > >  	}
> > >  
> > > +	if (of_property_read_bool(dp->dn, "maxlinear,rmii-refclk-out") &&
> > > +	    !(miicfg & GSWIP_MII_CFG_MODE_RGMII))
> > > +		miicfg |= GSWIP_MII_CFG_RMII_CLK;
> > > +
> > 
> > What did you mean with the !(miicfg & GSWIP_MII_CFG_MODE_RGMII) test?
> > If the schema says "Only applicable for RMII mode.", what's the purpose
> > of this extra condition? For example, GSWIP_MII_CFG_MODE_GMII also has
> > the "GSWIP_MII_CFG_MODE_RGMII" bit (0x4) unset. Does this have any significance?
> 
> You are right, probably the best would be to test (if at all) that
> (miicfg == GSWIP_MII_CFG_MODE_RMIIM || miicfg ==
> GSWIP_MII_CFG_MODE_RMIIP) and only in this case allow setting the
> GSWIP_MII_CFG_RMII_CLK bit.
> 
> I forgot that there is older hardware which supports "full" MII, and MII
> MAC as well as MII PHY modes also shouldn't allow to set the
> GSWIP_MII_CFG_RMII_CLK bit to not end up with undefined behavior.

Yeah, actually you'd be looking at FIELD_GET(GSWIP_MII_CFG_MODE_MASK, miicfg)
rather than miicfg directly.

If the schema restricted "maxlinear,rmii-refclk-out" to be used only in
combination with phy-mode = "rmii" and "rev-rmii", in theory that should
be sufficient with no further driver checks. But some checks that at
least make sense don't seem to hurt.