Merge branch 'net-sfp-add-support-for-control-of-rate-selection'

Russell King says:

====================
net: sfp: add support for control of rate selection

This series introduces control of the rate selection SFP pins (or
their soft state in the I2C diagnostics EEPROM). Several SNIA documents
(referenced in the commits) describe the various different modes for
these, and we implement them all for maximum compatibility, but as
we know, SFP modules tend to do their own thing, so that may not be
sufficient.

In order to implement this, we need to change the locking arrangement
in the SFP layer - we need to make st_mutex (state mutex) able to be
taken from within the rtnl lock and sm_mutex (state machine mutex).
Essentially, st_mutex protects the hard (gpio) and soft state signals.

So, patches 2 through 5 rejig the locking so that st_mutex is only
ever taken when we want to fiddle with the signal state variables,
read or write the GPIOs, or read or write the soft state.

Patch 1 adds a helper that makes the locking rejig a little easier
as it combines the update of sfp->state with setting the updated
control state to the module.

Patch 6 adds code to phylink to give the signalling rate for various
PHY interface modes that are relevant to SFPs - this is the baud rate
of the encoded signal, not the data rate, which is what matters for
SFPs. This rate is passed through the SFP bus layer into the SFP
socket driver, which initially has a stub sfp_set_signal_rate().

Patch 7 adds the code to the SFP socket driver to parse the rate
selection data in the EEPROM, configure which RS signals need to be
driven, and the signalling rate threshold. We fill in
sfp_set_signal_rate() to set the rate select pins as appropriate.
====================

Link: https://lore.kernel.org/r/ZGSuTY8GqjM+sqta@shell.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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