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Enable the bi-directional EC channel for slow-control communication with the sub lpGBT

Laurent Petre requested to merge feature/lpgbt-ec into devel

From the commit message:

This commit adds an option to select whether the sub lpGBT IC
communication RX should go through the sub lpGBT IC channel or the boss
lpGBT EC channel. The former option involves less the boss, but requires
(partial) blind configuration of the sub; the latter option involves
more the boss, but allows fully checked configuration of the sub.

Note how the boss lpGBT configuration requires an update. Indeed, the
ROM configuration sets the EPTXEcTriState bit in simplex mode (bit 1 of
EPTXEcChnCntr [0x0ac]). The usage of a tri-state driver requires to
enable the internal pull-up on the receiver side (EPRXECPullUpEnable
[bit 0] of EPRXEcChnCntr [0x0ec]).

Considering the first tests as well as the strict behavior of the online
software on error checking, the option is enabled by default. No change
in case of blind unchecked transactions is expected. The old behavior
can be restored by setting the BEFE.GEM.GEM_SYSTEM.CTRL.GBT_IC_RX_USE_EC
register to 0.

Tests on different setups and with the 0xBEFE Python scripts are welcome!

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