Re: [PATCH v3 2/2] net: dsa: mxl-gsw1xx: Support R(G)MII slew rate configuration

[Sverdlin, Alexander]

Hello Vladimir, Rob!

On Mon, 2026-01-05 at 22:00 +0100, Alexander Sverdlin wrote:
> > > +	return regmap_update_bits(gsw1xx_priv->shell, GSW1XX_SHELL_RGMII_SLEW_CFG,
> > > +				  RGMII_SLEW_CFG_DRV_TXD | RGMII_SLEW_CFG_DRV_TXC,
> > > +				  (RGMII_SLEW_CFG_DRV_TXD | RGMII_SLEW_CFG_DRV_TXC) * rate);
> > 
> > I don't have a particularly strong EE background, but my understanding
> > is this:
> > 
> > RGMII MACs provide individual slew rate configuration for TXD[3:0] and
> > for TX_CLK because normally, you'd want to focus on the TX_CLK slew rate
> > (in the sense of reducing EMI) more than on the TXD[3:0] slew rate.
> > This is for 2 reasons:
> > (1) the EMI noise produced by TX_CLK is in a much narrower spectrum
> >     (runs at fixed 125/25/2.5 MHz) than TXD[3:0] (pseudo-random data).
> > (2) reducing the slew rate for TXD[3:0] risks introducing inter-symbol
> >     interference, risk which does not exist for TX_CLK
> > 
> > Your dt-binding does not permit configuring the slew rates separately,
> > even though the hardware permits that. Was it intentional?
> 
> thanks for the hint! This is definitely something I need to discuss with HW
> colleagues and get back to you!

Vladimir, according to the responsible HW colleague, it's OK and is desired
to have TXD in "slow" as long as Setup-/Hold-Timing is in spec.

I do understand, that this is board-specific. Do you propose to introduce
two separate properties?

Rob, in such case just "slew-rate" probably wouldn't fit any longer and
I'd need to go back to "maxlinear,slew-rate-txd" and "maxlinear,slew-rate-txc"
probably?

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com