arm64: simplify the code around the call to z_get_next_switch_handle()

Remove the special SMP workaround and the extra wrapper.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

arch/arm64/core/isr_wrapper.S

@@ -117,49 +117,28 @@ z_arm64_irq_done:
 	bne	exit
 
 	/* No more nested: retrieve the task's stack. */
-	ldr	x0, [sp]
-	mov	sp, x0
+	ldr	x1, [sp]
+	mov	sp, x1
 
-	/*
-	 * z_arch_get_next_switch_handle() is returning:
-	 *
-	 * - The next thread to schedule in x0
-	 * - The current thread in x1. This value is returned using the
-	 *   **old_thread parameter, so we need to make space on the stack for
-	 *   that.
-	 */
-	sub	sp, sp, #16
-	mov	x0, sp
-	bl	z_arch_get_next_switch_handle
-	ldp	x1, xzr, [sp], #16
+	/* retrieve pointer to the current thread */
+	ldr	x1, [x0, #___cpu_t_current_OFFSET]
 
 	/*
-	 * x0: 1st thread in the ready queue
-	 * x1: _current thread
+	 * Get next thread to schedule with z_get_next_switch_handle().
+	 * We pass it a NULL as we didn't save the whole thread context yet.
+	 * If no scheduling is necessary then NULL will be returned.
 	 */
+	str	x1, [sp, #-16]!
+	mov	x0, xzr
+	bl	z_get_next_switch_handle
+	ldr	x1, [sp], #16
+	cbz	x0, exit
 
-#ifdef CONFIG_SMP
 	/*
-	 * 2 possibilities here:
-	 *  - x0 != NULL (implies x0 != x1): we need to context switch and set
-	 *    the switch_handle in the context switch code
-	 *  - x0 == NULL: no context switch
+	 * Switch thread
+	 * x0: new thread
+	 * x1: old thread
 	 */
-	cmp	x0, #0x0
-	bne	switch
-
-	/*
-	 * No context switch. Restore x0 from x1 (they are the same thread).
-	 * See also comments to z_arch_get_next_switch_handle()
-	 */
-	mov	x0, x1
-	b	exit
-switch:
-#else
-	cmp	x0, x1
-	beq	exit
-#endif
-	/* Switch thread */
 	bl	z_arm64_context_switch
 
 exit:

arch/arm64/core/switch.S

@@ -68,12 +68,10 @@ SECTION_FUNC(TEXT, z_arm64_context_switch)
 	ldp	x0, x1, [sp], #16
 #endif
 
-#ifdef CONFIG_SMP
 	/* save old thread into switch handle which is required by
 	 * wait_for_switch
 	 */
 	 str	x1, [x1, #___thread_t_switch_handle_OFFSET]
-#endif
 
 #ifdef CONFIG_THREAD_LOCAL_STORAGE
 	/* Grab the TLS pointer */

arch/arm64/core/thread.c

@@ -120,32 +120,6 @@ void arch_new_thread(struct k_thread *thread, k_thread_stack_t *stack,
 	thread->switch_handle = thread;
 }
 
-void *z_arch_get_next_switch_handle(struct k_thread **old_thread)
-{
-	/*
-	 * When returning from this function we will have the current thread
-	 * onto the stack to be popped in x1 and the next thread in x0 returned
-	 * from z_get_next_switch_handle() (see isr_wrapper.S)
-	 */
-	*old_thread =  _current;
-
-#ifdef CONFIG_SMP
-	/*
-	 * XXX: see thread in #41840 and #40795
-	 *
-	 * The scheduler API requires a complete switch handle here, but arm64
-	 * optimizes things such that the callee-save registers are still
-	 * unsaved here (they get written out in z_arm64_context_switch()
-	 * below).  So pass a NULL instead, which the scheduler will store into
-	 * the thread switch_handle field.  The resulting thread won't be
-	 * switched into until we write that ourselves.
-	 */
-	return z_get_next_switch_handle(NULL);
-#else
-	return z_get_next_switch_handle(*old_thread);
-#endif
-}
-
 #ifdef CONFIG_USERSPACE
 FUNC_NORETURN void arch_user_mode_enter(k_thread_entry_t user_entry,
 					void *p1, void *p2, void *p3)