Re: [PATCH net-next 5/5] net: mdio: bcm-unimac: implement timestamped MDIO writes

[Andrew Lunn <andrew@lunn.ch>]

On Sun, Oct 11, 2026 at 03:22:04PM +0200, Nicolai Buchwitz wrote:
> Hi James
> 
> On 11.10.2026 09:20, James Clark wrote:
> > On Sat, Oct 10, 2026 at 10:18 PM Nicolai Buchwitz <nb@tipi-net.de>
> > wrote:
> > > I dug a bit deeper on a CM4: AFAIU the MDC runs off the GISB clock,
> > > which the firmware sets to 200 MHz (CPRMAN 0x1d8, PLLD_PER 750 MHz /
> > > 3.75). Bumping that divider to 4.0 at runtime moves the MDIO busy time
> > > by exactly the same ratio, so that's the one. The 250 MHz you have in
> > > mind is genet250 / genet125, the RGMII side, a different generator.
> > > 
> > > So instead of a fixed clock or a compatible match this could just be
> > > BCM2711_CLOCK_GISB in clk-bcm2835 (critical, firmware owns it) and
> > > clocks = <&clocks BCM2711_CLOCK_GISB> on the mdio node. I can send
> > > that
> > > if it helps.
> 
> I've put together 3 patches, which do the clock plumbing:
> https://github.com/nbuchwitz/linux/commits/devel/bcm2711-gisb-clk/

It would be nice to extend this and set the divider based on this to
give 2.5Mhz, as required by 802.3, as the default. Allow boards to
override this using the clock-frequency property.

	Andrew