net: sfp: add workaround for Realtek RTL8672 and RTL9601C chips

net: sfp: add workaround for Realtek RTL8672 and RTL9601C chips

The workaround for VSOL V2801F brand based GPON SFP modules added in commit
0d035bed2a4a ("net: sfp: VSOL V2801F / CarlitoxxPro CPGOS03-0490 v2.0
workaround") works only for IDs added explicitly to the list. Since there
are rebranded modules where OEM vendors put different strings into the
vendor name field, we cannot base workaround on IDs only.

Moreover the issue which the above mentioned commit tried to work around is
generic not only to VSOL based modules, but rather to all GPON modules
based on Realtek RTL8672 and RTL9601C chips.

These include at least the following GPON modules:
* V-SOL V2801F
* C-Data FD511GX-RM0
* OPTON GP801R
* BAUDCOM BD-1234-SFM
* CPGOS03-0490 v2.0
* Ubiquiti U-Fiber Instant
* EXOT EGS1

These Realtek chips have broken EEPROM emulator which for N-byte read
operation returns just the first byte of EEPROM data, followed by N-1
zeros.

Introduce a new function, sfp_id_needs_byte_io(), which detects SFP modules
with broken EEPROM emulator based on N-1 zeros and switch to 1 byte EEPROM
reading operation.

Function sfp_i2c_read() now always uses single byte reading when it is
required and when function sfp_hwmon_probe() detects single byte access,
it disables registration of hwmon device, because in this case we cannot
reliably and atomically read 2 bytes as is required by the standard for
retrieving values from diagnostic area.

(These Realtek chips are broken in a way that violates SFP standards for
diagnostic interface. Kernel in this case simply cannot do anything less
of skipping registration of the hwmon interface.)

This patch fixes reading of EEPROM content from SFP modules based on
Realtek RTL8672 and RTL9601C chips. Diagnostic interface of EEPROM stays
broken and cannot be fixed.

Fixes: 0d035bed2a4a ("net: sfp: VSOL V2801F / CarlitoxxPro CPGOS03-0490 v2.0 workaround")
Co-developed-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>