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

[Nicolai Buchwitz]

Hi Florian, hi James

On 9.10.2026 18:04, Florian Fainelli wrote:

> [...]

> > When there is no clock, unimac_mdio_clk_set() assumes a 250 MHz
> > reference rate, and no in-tree DT gives GENET or UniMAC a clock. For
> > BCM2711 I have no documentation of the reference rate or of the clock
> > that supplies it, but MDIO busy times measured at several MDC dividers
> > fit a rate of 200 MHz. This patch checks the parent's compatible 
> > string,
> > which is unsatisfactory. I would prefer to get the rate from DT, and
> > would welcome suggestions for the right DT description.
>
> You can, and should define a chip-specific compatible string for the 
> MDIO contorller node. We have one for 2711 already for GENET 
> (brcm,bcm2711-genet-v5), so you could define brcm,bcm2711-genet-mdio-v5
>
> The clock frequency is fixed, so you could also provide a fixed clock 
> to ensure that the clock frequency is derived correctly. I will check 
> the actual clocking because 200MHz sounds odd to me, since the RGMII 
> interface does require 125MHz and therefore a 250MHz source makes that 
> easy.

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.

Regards,
Nicolai