Re: [PATCH net-next 0/5] net: mdio: add timestamped MDIO writes for PHY gettimex64

[Andrew Lunn <andrew@lunn.ch>]

On Sun, Oct 11, 2026 at 08:07:13PM +0200, Nicolai Buchwitz wrote:
> Hi Andrew
> 
> On 11.10.2026 15:50, Andrew Lunn wrote:
> > > For example, using PTP_SYS_OFFSET_EXTENDED on the Raspberry Pi CM5 to
> > > synchronize the system clock from the BCM54210PE PHY PHC leads to the
> > > system clock being 32 µs ahead of the PHC. On the CM4, it is about 7
> > > µs.
> > > (The difference is because the MDIO clock frequency is higher on the
> > > CM4: 10 MHz as opposed to 2.08 MHz on the CM5.)
> > 
> > IEEE 802.3 says the clock should be 2.5Mhz. Does the CM4 use a
> > clock-frequency DT property to enable this faster clock?
> > 
> > 	Andrew
> 
> The Pi 400 is the only BCM2711 board with clock-frequency in DT
> (1950000, from a downstream "Limit MDIO clock speed" commit with no
> further explanation). There the driver's assumed 250 MHz reference
> gives divider 63, which with the real 200 MHz (see my comments on
> patch 5) is about 1.56 MHz.
> 
> Pi 4 and CM4 (the one with the PTP PHY) have no clock-frequency, so
> the driver keeps the firmware's divider 9, and 200 MHz / 20 is the
> 10 MHz James mentions.

For CM4, i think the PHY is on the board? Does the MDIO bus get
exported via a connector? If it is a closed system, then running the
bus faster by default is O.K, since it is impossible to connect a
device which is only standards compliant. But if the bus does get
connected to the carrier board, it would be better to stick to 2.5MHz.

	  Andrew