GT Multicast/Replicated (MCR) Register Support

Some GT registers are designed as “multicast” or “replicated” registers: multiple instances of the same register share a single MMIO offset. MCR registers are generally used when the hardware needs to potentially track independent values of a register per hardware unit (e.g., per-subslice, per-L3bank, etc.). The specific types of replication that exist vary per-platform.

MMIO accesses to MCR registers are controlled according to the settings programmed in the platform’s MCR_SELECTOR register(s). MMIO writes to MCR registers can be done in either multicast (a single write updates all instances of the register to the same value) or unicast (a write updates only one specific instance) form. Reads of MCR registers always operate in a unicast manner regardless of how the multicast/unicast bit is set in MCR_SELECTOR. Selection of a specific MCR instance for unicast operations is referred to as “steering.”

If MCR register operations are steered toward a hardware unit that is fused off or currently powered down due to power gating, the MMIO operation is “terminated” by the hardware. Terminated read operations will return a value of zero and terminated unicast write operations will be silently ignored. During device initialization, the goal of the various init_steering_*() functions is to apply the platform-specific rules for each MCR register type to identify a steering target that will select a non-terminated instance.

MCR registers are not available on Virtual Function (VF).

Internal API