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

Hello Vladimir, Rob!

On Mon, 2026-01-05 at 22:00 +0100, Alexander Sverdlin wrote:

quoted

quoted

+	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