Re: [PATCH v3 net-next 05/10] net: dsa: microchip: add DSA support for microchip lan937x

[Andrew Lunn]

On Mon, Aug 02, 2021 at 03:15:50PM +0300, Vladimir Oltean wrote:
> On Mon, Aug 02, 2021 at 04:15:08PM +0530, Prasanna Vengateshan wrote:
> > On Sat, 2021-07-31 at 18:04 +0300, Vladimir Oltean wrote:
> > > > +void lan937x_mac_config(struct ksz_device *dev, int port,
> > > > +                     phy_interface_t interface)
> > > > +{
> > > > +     u8 data8;
> > > > +
> > > > +     lan937x_pread8(dev, port, REG_PORT_XMII_CTRL_1, &data8);
> > > > +
> > > > +     /* clear MII selection & set it based on interface later */
> > > > +     data8 &= ~PORT_MII_SEL_M;
> > > > +
> > > > +     /* configure MAC based on interface */
> > > > +     switch (interface) {
> > > > +     case PHY_INTERFACE_MODE_MII:
> > > > +             lan937x_config_gbit(dev, false, &data8);
> > > > +             data8 |= PORT_MII_SEL;
> > > > +             break;
> > > > +     case PHY_INTERFACE_MODE_RMII:
> > > > +             lan937x_config_gbit(dev, false, &data8);
> > > > +             data8 |= PORT_RMII_SEL;
> > > > +             break;
> > > > +     case PHY_INTERFACE_MODE_RGMII:
> > > > +     case PHY_INTERFACE_MODE_RGMII_ID:
> > > > +     case PHY_INTERFACE_MODE_RGMII_TXID:
> > > > +     case PHY_INTERFACE_MODE_RGMII_RXID:
> > > > +             lan937x_config_gbit(dev, true, &data8);
> > > > +             data8 |= PORT_RGMII_SEL;
> > > > +
> > > > +             /* Add RGMII internal delay for cpu port*/
> > > > +             if (dsa_is_cpu_port(dev->ds, port)) {
> > >
> > > Why only for the CPU port? I would like Andrew/Florian to have a look
> > > here, I guess the assumption is that if the port has a phy-handle, the
> > > RGMII delays should be dealt with by the PHY, but the logic seems to be
> > > "is a CPU port <=> has a phy-handle / isn't a CPU port <=> doesn't have
> > > a phy-handle"? What if it's a fixed-link port connected to a downstream
> > > switch, for which this one is a DSA master?
> >
> > Thanks for reviewing the patches. My earlier proposal here was to check if there
> > is no phydev (dp->slave->phydev) or if PHY is genphy, then apply RGMII delays
> > assuming delays should be dealt with the phy driver if available. What do you
> > think of that?
> 
> I don't know what to suggest, this is a bit of a minefield.

In general, the MAC does nothing, and passes the value to the PHY. The
PHY inserts delays as requested. To address Vladimir point,
PHY_INTERFACE_MODE_RGMII_TXID would mean the PHY adds delay in the TX
direction, and assumes the RX delay comes from somewhere else,
probably the PCB.

I only recommend the MAC adds delays when the PHY cannot, or there is
no PHY, e.g. SoC to switch, or switch to switch link. There are a few
MAC drivers that do add delays, mostly because that is how the vendor
crap tree does it.

So as i said, what you propose is O.K, it follows this general rule of
thumb.

	Andrew