Re: [PATCH 9/9] net: dsa: mt7530: use external PCS driver

["Russell King (Oracle)" <linux@armlinux.org.uk>]

Hi Daniel,

I haven't reviewed the patches yet, and probably won't for a while.
(I'm recovering from Covid).

On Sat, Feb 04, 2023 at 03:02:00PM +0000, Daniel Golle wrote:
> Hi Vladimir,
> 
> thank you for the review!
> 
> On Sat, Feb 04, 2023 at 12:19:15AM +0200, Vladimir Oltean wrote:
> > On Fri, Feb 03, 2023 at 07:06:53AM +0000, Daniel Golle wrote:
> > > @@ -2728,11 +2612,14 @@ mt753x_phylink_mac_select_pcs(struct dsa_switch *ds, int port,
> > >  
> > >  	switch (interface) {
> > >  	case PHY_INTERFACE_MODE_TRGMII:
> > > +		return &priv->pcs[port].pcs;
> > >  	case PHY_INTERFACE_MODE_SGMII:
> > >  	case PHY_INTERFACE_MODE_1000BASEX:
> > >  	case PHY_INTERFACE_MODE_2500BASEX:
> > > -		return &priv->pcs[port].pcs;
> > > +		if (!mt753x_is_mac_port(port))
> > > +			return ERR_PTR(-EINVAL);
> > 
> > What is the reason for returning ERR_PTR(-EINVAL) to mac_select_pcs()?
> 
> The SerDes PCS units are only available for port 5 and 6. The code
> should make sure that the corresponding interface modes are only used
> on these two ports, so a BUG_ON(!mt753x_is_mac_port(port)) would also
> do the trick, I guess. However, as dsa_port_phylink_mac_select_pcs may
> also return ERR_PTR(-EOPNOTSUPP), returning ERR_PTR(-EINVAL) felt like
> the right thing to do in that case.
> Are you suggesting to use BUG_ON() instead or rather return
> ERR_PTR(-EOPNOTSUPP)?

Returning ERR_PTR(-EOPNOTSUPP) from mac_select_pcs() must not be done
conditionally - it means that "this instance does not support the
mac_select_pcs() interface" and phylink will never call it again.

It was implemented to provide DSA a way to tell phylink that the DSA
driver doesn't implement this interface - phylink originally checked
whether mac_ops->mac_select_pcs was NULL, but with DSA's layering,
such a check can only be made by a runtime call.

Returning NULL means "there is no PCS for this interface mode", and
returning any other error code essentially signifies that the
interface mode is not supported (even e.g. GMII or INTERNAL)...
which really *should* be handled by setting supported_interfaces
correctly in the first place.

I would much prefer drivers to just return NULL if they have no PCS,
even for ports where it's not possible, or not implement the
interface over returning some kind of error code.

The only time I would expect mac_select_pcs() to return an error is
where it needed to do something in order to evaluate whether to
return a PCS or not, and it encountered an error while trying to
evaluate that or some truely bizarre situation. E.g. a failed
memory allocation, or "we know that a PCS is required for this but
we're missing it". Something of that ilk.

Anyway, more rest needed.

-- 
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