Re: [PATCH net-next] net: sfp: add quirk for XikeStor SKT-2.5G-100M

[Andrew Lunn <andrew@lunn.ch>]

> If bit 0 of register 0x75f3 on MMD 30 is set, reading any register on MMD 30
> except for the actual SerDes registers (and also registers 5/6) breaks the
> PHY.

[Goes and looks at 802.3, clause 45]

> I would really like to have a general fix for cases where neither of these
> two workarounds happen to already be in place. But I'm not sure how this
> could be done cleanly, as it requires special handling for these PHYs in the
> function that reads the PHY ID (or even before that).

I assume this PHY does have a valid ID in MMD 1-29?

> Downstream in OpenWrt, I added a patch for "get_phy_c45_ids" to avoid
> reading MMD 30 from RTL8221B PHYs based on the PHY ID in MMD 1:

This suggests it does.

I wounder if we can make use of:

	if ((devs_in_pkg & 0x1fffffff) == 0x1fffffff) {
		/* If mostly Fs, there is no device there, then let's probe
		 * MMD 0, as some 10G PHYs have zero Devices In package,
		 * e.g. Cortina CS4315/CS4340 PHY.
		 */
		phy_reg = get_phy_c45_devs_in_pkg(bus, addr, 0, &devs_in_pkg);
		if (phy_reg < 0)
			return -EIO;

		/* no device there, let's get out of here */
		if ((devs_in_pkg & 0x1fffffff) == 0x1fffffff)
			return -ENODEV;
	}

I assume this is not hit for this device?

I _guess_ there are ~0 PHYs which probe based on ID values in
MDIO_MMD_VEND1 or MDIO_MMD_VEND2. So maybe move the code looking for
device present in MDIO_MMD_VEND1 or MDIO_MMD_VEND2 inside this clause?
Then in the normal case we never look in these registers.

If we don't look to see if the MDIO_MMD_VEND1 or MDIO_MMD_VEND2
devices are present, i assume the next loop:

	/* Now probe Device Identifiers for each device present. */
	for (i = 1; i < num_ids; i++) {
		if (!(devs_in_pkg & (1 << i)))
			continue;

will also leave them alone?

But if there is an oddball PHY around which relies on MDIO_MMD_VEND1
or MDIO_MMD_VEND2 IDs, we still look there, if we failed to find
anything anywhere else, and so hopefully it does not cause a
regression?

	Andrew