sparc: write through switched_from in arch_switch()

Write through switched_from in arch_switch() as required by the
switch protocol.

Also restructure the implementation to better match the template in
kernel_arch_interface.h, by removing a wrapper routine and instead
use CONTAINER_OF().

Fixes #32197

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>

arch/sparc/core/switch.S

@@ -13,12 +13,16 @@ GTEXT(z_sparc_arch_switch)
 GTEXT(z_sparc_context_switch)
 GTEXT(z_thread_entry_wrapper)
 
-/* In this implementation, switch_handle is the thread itself. */
-SECTION_FUNC(TEXT, z_sparc_arch_switch)
-	ba	z_sparc_context_switch
-	 sub	%o1, ___thread_t_switch_handle_OFFSET, %o1
-
 /*
+ * The routine z_sparc_context_switch() is called from arch_switch(), or from
+ * the interrupt trap handler in case of preemption. The subtraction to get the
+ * "old" thread from "switched_from" has already been performed and the "old"
+ * thread is now in register %o1. We can address old->switch_handle in assembly
+ * as: [%o1 + ___thread_t_switch_handle_OFFSET].
+ *
+ * The switch_handle is written in z_sparc_context_switch() after the old
+ * context has been saved.
+ *
  * This is a leaf function, so only out registers
  * can be used without saving their context first.
  *
@@ -101,6 +105,15 @@ SECTION_FUNC(TEXT, z_sparc_context_switch)
 	nop
 	nop
 
+	/*
+	 * We have finished saving the "old" context and are also back in the
+	 * register window for which z_sparc_context_switch() was called.
+	 *
+	 * Now write the old thread into switch handle.
+	 * "old->switch_handle = old".
+	 */
+	st	%o1, [%o1 + ___thread_t_switch_handle_OFFSET]
+
 	ldd	[%o0 + _thread_offset_to_y], %o4
 	mov	%o4, %y
 

arch/sparc/include/kernel_arch_func.h

@@ -26,11 +26,20 @@ static ALWAYS_INLINE void arch_kernel_init(void)
 {
 }
 
-void z_sparc_arch_switch(void *switch_to, void **switched_from);
+void z_sparc_context_switch(struct k_thread *newt, struct k_thread *oldt);
 
+/*
+ * In this implementation, the thread->switch_handle is the thread itself, so
+ * the parameter "switched_from" is assumed to be the address of
+ * thread->switch_handle.
+ */
 static inline void arch_switch(void *switch_to, void **switched_from)
 {
-	z_sparc_arch_switch(switch_to, switched_from);
+	struct k_thread *newt = switch_to;
+	struct k_thread *oldt = CONTAINER_OF(switched_from, struct k_thread,
+					     switch_handle);
+
+	z_sparc_context_switch(newt, oldt);
 }
 
 FUNC_NORETURN void z_sparc_fatal_error(unsigned int reason,