Introduced an API to support scenarios where FPU sharing across threads

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>

Author: msmttchr

arch/arm/core/cortex_m/fpu.c

@@ -16,7 +16,7 @@
 
 void z_arm_save_fp_context(struct fpu_ctx_full *buffer)
 {
-#if defined(CONFIG_FPU_SHARING)
+#if defined(CONFIG_FPU)
 	__ASSERT_NO_MSG(buffer != NULL);
 
 	uint32_t CONTROL = __get_CONTROL();
@@ -44,7 +44,7 @@ void z_arm_save_fp_context(struct fpu_ctx_full *buffer)
 
 void z_arm_restore_fp_context(const struct fpu_ctx_full *buffer)
 {
-#if defined(CONFIG_FPU_SHARING)
+#if defined(CONFIG_FPU)
 	if (buffer->ctx_saved) {
 		/* Set FPCA first so it is set even if an interrupt happens
 		 * during restoration.
@@ -61,3 +61,17 @@ void z_arm_restore_fp_context(const struct fpu_ctx_full *buffer)
 	}
 #endif
 }
+
+void z_arm_save_shared_fp_context(struct fpu_ctx_full *buffer)
+{
+#if defined(CONFIG_FPU_SHARING)
+	z_arm_save_fp_context(buffer);
+#endif
+}
+
+void z_arm_restore_shared_fp_context(struct fpu_ctx_full *buffer)
+{
+#if defined(CONFIG_FPU_SHARING)
+	z_arm_restore_fp_context(buffer);
+#endif
+}

include/zephyr/arch/arm/cortex_m/fpu.h

@@ -19,5 +19,7 @@ struct fpu_ctx_full {
 
 void z_arm_save_fp_context(struct fpu_ctx_full *buffer);
 void z_arm_restore_fp_context(const struct fpu_ctx_full *buffer);
+void z_arm_save_shared_fp_context(struct fpu_ctx_full *buffer)
+void z_arm_restore_shared_fp_context(const struct fpu_ctx_full *buffer);
 
 #endif /* ZEPHYR_INCLUDE_ARCH_ARM_CORTEX_M_FPU_H_ */

modules/trusted-firmware-m/interface/interface.c

@@ -55,11 +55,11 @@ int32_t tfm_ns_interface_dispatch(veneer_fn fn,
 
 	struct fpu_ctx_full context_buffer;
 
-	z_arm_save_fp_context(&context_buffer);
+	z_arm_save_shared_fp_context(&context_buffer);
 
 	result = fn(arg0, arg1, arg2, arg3);
 
-	z_arm_restore_fp_context(&context_buffer);
+	z_arm_restore_shared_fp_context(&context_buffer);
 
 	if (!isr_mode) {
 #if !defined(CONFIG_ARM_NONSECURE_PREEMPTIBLE_SECURE_CALLS)