Re: [PATCH net-next v3] net: dsa: microchip: Add SGMII port support to KSZ9477 switch

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi Tristram,

On Tue, 13 May 2025 15:22:24 -0700
<Tristram.Ha@microchip.com> wrote:

> From: Tristram Ha <tristram.ha@microchip.com>
> 
> The KSZ9477 switch driver uses the XPCS driver to operate its SGMII
> port.  However there are some hardware bugs in the KSZ9477 SGMII
> module so workarounds are needed.  There was a proposal to update the
> XPCS driver to accommodate KSZ9477, but the new code is not generic
> enough to be used by other vendors.  It is better to do all these
> workarounds inside the KSZ9477 driver instead of modifying the XPCS
> driver.
> 
> There are 3 hardware issues.  The first is the MII_ADVERTISE register
> needs to be write once after reset for the correct code word to be
> sent.  The XPCS driver disables auto-negotiation first before
> configuring the SGMII/1000BASE-X mode and then enables it back.  The
> KSZ9477 driver then writes the MII_ADVERTISE register before enabling
> auto-negotiation.  In 1000BASE-X mode the MII_ADVERTISE register will
> be set, so KSZ9477 driver does not need to write it.
> 
> The second issue is the MII_BMCR register needs to set the exact speed
> and duplex mode when running in SGMII mode.  During link polling the
> KSZ9477 will check the speed and duplex mode are different from
> previous ones and update the MII_BMCR register accordingly.
> 
> The last issue is 1000BASE-X mode does not work with auto-negotiation
> on.  The cause is the local port hardware does not know the link is up
> and so network traffic is not forwarded.  The workaround is to write 2
> additional bits when 1000BASE-X mode is configured.
> 
> Note the SGMII interrupt in the port cannot be masked.  As that
> interrupt is not handled in the KSZ9477 driver the SGMII interrupt bit
> will not be set even when the XPCS driver sets it.
> 
> Signed-off-by: Tristram Ha <tristram.ha@microchip.com>

I was able to test this patch this morning, with both SGMII (copper
SFP) and 1000BaseX (Fibre SFP), hot-swapping and all, it works well :)

Thanks for that !

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>

Maxime