Re: [PATCH net-next v4 3/3] net: phy: add a PHY write barrier when disabling interrupts

[Charles Perry]

On Tue, Apr 07, 2026 at 06:59:00PM +0200, Andrew Lunn wrote:
> > static void phy_write_barrier(struct phy_device *phydev)
> > {
> >        int err;
> >  
> >        err = mdiobus_read(phydev->mdio.bus, phydev->mdio.addr, MII_PHYSID1);
> >        if (err == -EOPNOTSUPP)
> >                mdiobus_c45_read(phydev->mdio.bus, phydev->mdio.addr,
> >                                 __ffs(phydev->c45_ids.mmds_present),
> >                                 MII_PHYSID1);
> > }
> 
> Using __ffs() is maybe more complex than needed.
> 
> All you are trying to do is ensure the last write happened, by doing a
> read. Any read should work, even if the device does not respond. We
> just need to be careful not to read a register which might clear on
> read, such as an interrupt status register, or the link status, which
> latches. MII_PHYID1 is safe. Since we throw away the value, we don't
> care if the MMD is not present, the read will still flush the previous
> write. So i would replace the _ffs() with a hard coded value. KISS.

Ok, I'll use device 1 (MDIO_MMD_PMAPMD).

Thanks for the explanations.

> 
> > Do you think there's any way I can test this on my VSC8574 or VSC8541? It
> > supports some C45 registers for EEE but not the device discovery part.
> 
> It is not so easy to do. You need to hack the C22 read so that is
> returns EOPNOTSUPP, but you also need the first few reads to return a
> valid value otherwise the probe will fail.
> 
> I think this is one of the cases that if the reviewers thinks its
> looks O.K, we can accept it without extensive testing.
> 

Ok

Thanks,
Charles