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

Hi Vladimir,

On Mon, 2026-01-05 at 21:30 +0200, Vladimir Oltean wrote:

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!

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com