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SMCCC_ARCH_FEATURE_AVAILABILITY [1] is a call to query firmware about the features it is aware of and enables. This is useful when a feature is not enabled at EL3, eg due to an older FW image, but it is present in hardware. In those cases, the EL1 ID registers do not reflect the usable feature set and this call should provide the necessary information to remedy that. The call itself is very lightweight - effectively a sanitised read of the relevant system register. Bits that are not relevant to feature enablement are masked out and active low bits are converted to active high. The implementation is also very simple. All relevant, irrelevant, and inverted bits combined into bitmasks at build time. Then at runtime the masks are unconditionally applied to produce the right result. This assumes that context managers will make sure that disabled features do not have their bits set and the registers are context switched if any fields in them make enablement ambiguous. Features that are not yet supported in TF-A have not been added. On debug builds, calling this function will fail an assert if any bits that are not expected are set. In combination with CI this should allow for this feature to to stay up to date as new architectural features are added. If a call for MPAM3_EL3 is made when MPAM is not enabled, the call will return INVALID_PARAM, while if it is FEAT_STATE_CHECK, it will return zero. This should be fairly consistent with feature detection. The bitmask is meant to be interpreted as the logical AND of the relevant ID registers. It would be permissible for this to return 1 while the ID returns 0. Despite this, this implementation takes steps not to. In the general case, the two should match exactly. Finally, it is not entirely clear whether this call replies to SMC32 requests. However, it will not, as the return values are all 64 bits. [1]: https://developer.arm.com/documentation/den0028/galp1/?lang=en Co-developed-by: Charlie Bareham <charlie.bareham@arm.com> Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com> Change-Id: I1a74e7d0b3459b1396961b8fa27f84e3f0ad6a6f |
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