arch: arm: cortex_m: Update APIs to save and restore FPU context #93057
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Signed-off-by: Michele Sardo <msmttchr@gmail.com>
is required. This API is a replacement of the original API whose purpose has been changed (see below). This API also allows applications to disable FP sharing mode and use unshared FP registers, assuming that FP instructions are only issued within a single, designated thread. Currently, there is no known usage of this API, as the FPU context is automatically preserved across threads by hardware on Cortex-M (via the FPCCR.ASPEN register field). The original API has been modified to always perform FPU context save/restore, as its name implies. Conditional compilation on CONFIG_FPU is necessary to avoid build errors when the FPU is not enabled in the configuration. The primary use case is preserving FPU context during suspend-to-RAM transitions. TFM code was updated to retain its original behavior. However, it is unclear whether FPU context save/restore should happen unconditionally, regardless of CONFIG_PM_SHARING. Signed-off-by: Michele Sardo <msmttchr@gmail.com>
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Update API to save and restore FPU context to address suspend to ram use case.
The API is updated as follow:
z_arm_save_fp_contextandz_arm_restore_fp_contexthave been changed to always save/restore FPU context regardless ofFPU_SHARINGsettingz_arm_save_shared_fp_contextandz_arm_restore_shared_fp_context) to addressFPU_SHARINGuse cases (See https://docs.zephyrproject.org/latest/kconfig.html#!%5ECONFIG_FPU_SHARING$)The existing API
z_arm_save_fp_contextandz_arm_restore_fp_contextwere used only in TFM code, but I believe that in this use case we need to save/restore FPU context regardless of FPU_SHARING setting. Please feedback on this point.I also noticed by reading code and ARM documentation that the FPU context is preserved across threads by hardware on Cortex-M devices (via the FPCCR.ASPEN register field), so it seems to me that this new API (which mimic existing API) is not needed (except for the doubtful TFM case above). If this is the case, this PR can be simplified.
See also #90001, #92847 (comment)