[RFC net PATCH v1] net: pcs: pcs-mtk-lynxi: fix bpi-r3 serdes configuration

[Daniel Golle]

On Thu, Apr 09, 2026 at 07:49:42PM +0300, Vladimir Oltean wrote:
> I notice Arınc, listed by ./scripts/get_maintainer.pl drivers/net/dsa/mt7530.c,
> and Felix, listed by ./scripts/get_maintainer.pl drivers/net/ethernet/mediatek/mtk_eth_soc.c,
> are not on CC. Maybe they have more info.
> 
> Only the switch port has a chance of having a non-zero default polarity
> setting? (coming from the efuse, if I understood this discussion properly)
> https://lore.kernel.org/netdev/C59EED96-3973-4074-A4D8-C264949D447E@linux.dev/
> The GMAC doesn't?

Yes, vendor SDK uses DT mediatek,pnswap{,-rx,-tx} properties only for the
SoC GMACs. For MT7531 there are **no** strap pins deciding the SerDes
polarity, and also no software-way to override the defaults in the vendor
SDK.

However, the MT7531 datasheet quite clearly states:
Register 000050EC QPHY_WRAP_CTRL -- QPHY wrapper control
Reset value: 0x00000501

BIT 1 RX_BIT_POLARITY -- RX bit polarity control
 1'b0: normal
 1'b1: inverted

BIT 0 TX_BIT_POLARITY -- TX bit polarity control (TX default inversed in MT7531)
 1'b0: normal
 1'b1: inverted

Hence the best would be to just assume the documented default in the driver
as well.

A quick register dump using the BPi-R3 confirms that this applies to *both*
SerDes PCS on MT7531A (port 5 and port 6) equally, both read 0x00000501
after reset.