RE: [PATCH net] net: phy: micrel: Fix MMD register access during SPD in ksz9131_resume()

[Biju Das]

> -----Original Message-----
> From: Ovidiu Panait <ovidiu.panait.rb@xxxxxxxxxxx>
> Sent: 03 April 2026 14:23
> To: Biju Das <biju.das.jz@xxxxxxxxxxxxxx>; andrew@xxxxxxx; hkallweit1@xxxxxxxxx; linux@xxxxxxxxxxxxxxx;
> davem@xxxxxxxxxxxxx; edumazet@xxxxxxxxxx; kuba@xxxxxxxxxx; pabeni@xxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-renesas-soc@xxxxxxxxxxxxxxx
> Subject: RE: [PATCH net] net: phy: micrel: Fix MMD register access during SPD in ksz9131_resume()
> 
> Hi,
> 
> >
> > Hi Ovidiu Panait,
> >
> > Thanks for the patch.
> >
> > > -----Original Message-----
> > > From: Ovidiu Panait <ovidiu.panait.rb@xxxxxxxxxxx>
> > > Sent: 03 April 2026 12:18
> > > Subject: [PATCH net] net: phy: micrel: Fix MMD register access
> > > during
> > SPD in ksz9131_resume()
> > >
> > > During system suspend, phy_suspend() puts the PHY into Software
> > > Power-
> > Down
> > > (SPD) by setting the BMCR_PDOWN bit in MII_BMCR. According to the
> > KSZ9131 datasheet, MMD register
> > > access is restricted during SPD:
> > >
> > >   - Only access to the standard registers (0 through 31) is supported.
> > >   - Access to MMD address spaces other than MMD address space 1 is
> > >     possible if the spd_clock_gate_override bit is set.
> > >   - Access to MMD address space 1 is not possible.
> > >
> > > However, ksz9131_resume() calls ksz9131_config_rgmii_delay() before
> > > kszphy_resume() clears BMCR_PDOWN. This means MMD registers are
> > > accessed
> > while the PHY is still in SPD,
> > > contrary to the datasheet.
> >
> > SPD mode: This mode is used to power down the device when it is not in
> > use after power-up.
> >           Previous register settings are maintained during and
> > following the removal of SPD.
> >
> > Suspend to Idle case, it is in SPD mode and the MMD register values
> > are retained.
> >
> 
> On resume from s2idle, ksz9131_resume() calls ksz9131_config_rgmii_delay() which does MMD accesses,
> while the PHY is in SPD. According to the datasheet, it shouldn't happen. See commit e398822c4751
> ("net: phy: micrel: populate .soft_reset for KSZ9131") which fixes the same issue.

On my board, while s2idle in SPD mode, it does not hang. The datasheet does not explain
the behaviour when it is SPD mode. But it states that it retains all previous register values
when it is out of SPD mode.

> 
> > But in Suspend to RAM, PHY loses power and the reset value of Power
> > Down bit 0 (ie normal mode), there is no restriction for accessing MMD
> > registers in this mode.
> >
> 
> If the PHY loses power, it loses all the configuration that was done by ksz9131_config_init(). Right
> now, only the RGMII delays are restored, which is a subset of the configurations done by
> ksz9131_config_init().

I agree, Only RZ/G3E reported this issue and with configuring only delays, it worked.

Calling ksz9131_config_init() restores more MMD registers or call phy_init() like this patch.

Cheers,
Biju