x86: mp: Add iterators for CPUs

It is convenient to iterate through the CPUs performing work on each one
and processing the result. Add a few iterator functions which handle this.
These can be used by any client code. It can call mp_run_on_cpus() on
each CPU that is returned, handling them one at a time.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/cpu/mp_init.c

@@ -684,6 +684,69 @@ int mp_park_aps(void)
 	return 0;
 }
 
+int mp_first_cpu(int cpu_select)
+{
+	struct udevice *dev;
+	int num_cpus;
+	int ret;
+
+	/*
+	 * This assumes that CPUs are numbered from 0. This function tries to
+	 * avoid assuming the CPU 0 is the boot CPU
+	 */
+	if (cpu_select == MP_SELECT_ALL)
+		return 0;   /* start with the first one */
+
+	ret = get_bsp(&dev, &num_cpus);
+	if (ret < 0)
+		return log_msg_ret("bsp", ret);
+
+	/* Return boot CPU if requested */
+	if (cpu_select == MP_SELECT_BSP)
+		return ret;
+
+	/* Return something other than the boot CPU, if APs requested */
+	if (cpu_select == MP_SELECT_APS && num_cpus > 1)
+		return ret == 0 ? 1 : 0;
+
+	/* Try to check for an invalid value */
+	if (cpu_select < 0 || cpu_select >= num_cpus)
+		return -EINVAL;
+
+	return cpu_select;  /* return the only selected one */
+}
+
+int mp_next_cpu(int cpu_select, int prev_cpu)
+{
+	struct udevice *dev;
+	int num_cpus;
+	int ret;
+	int bsp;
+
+	/* If we selected the BSP or a particular single CPU, we are done */
+	if (!IS_ENABLED(CONFIG_SMP_AP_WORK) || cpu_select == MP_SELECT_BSP ||
+	    cpu_select >= 0)
+		return -EFBIG;
+
+	/* Must be doing MP_SELECT_ALL or MP_SELECT_APS; return the next CPU */
+	ret = get_bsp(&dev, &num_cpus);
+	if (ret < 0)
+		return log_msg_ret("bsp", ret);
+	bsp = ret;
+
+	/* Move to the next CPU */
+	assert(prev_cpu >= 0);
+	ret = prev_cpu + 1;
+
+	/* Skip the BSP if needed */
+	if (cpu_select == MP_SELECT_APS && ret == bsp)
+		ret++;
+	if (ret >= num_cpus)
+		return -EFBIG;
+
+	return ret;
+}
+
 int mp_init(void)
 {
 	int num_aps, num_cpus;

arch/x86/include/asm/mp.h

@@ -118,6 +118,33 @@ int mp_run_on_cpus(int cpu_select, mp_run_func func, void *arg);
  * @return 0 if OK, -ve on error
  */
 int mp_park_aps(void);
+
+/**
+ * mp_first_cpu() - Get the first CPU to process, from a selection
+ *
+ * This is used to iterate through selected CPUs. Call this function first, then
+ * call mp_next_cpu() repeatedly (with the same @cpu_select) until it returns
+ * -EFBIG.
+ *
+ * @cpu_select: Selected CPUs (either a CPU number or MP_SELECT_...)
+ * @return next CPU number to run on (e.g. 0)
+ */
+int mp_first_cpu(int cpu_select);
+
+/**
+ * mp_next_cpu() - Get the next CPU to process, from a selection
+ *
+ * This is used to iterate through selected CPUs. After first calling
+ * mp_first_cpu() once, call this function repeatedly until it returns -EFBIG.
+ *
+ * The value of @cpu_select must be the same for all calls and must match the
+ * value passed to mp_first_cpu(), otherwise the behaviour is undefined.
+ *
+ * @cpu_select: Selected CPUs (either a CPU number or MP_SELECT_...)
+ * @prev_cpu: Previous value returned by mp_first_cpu()/mp_next_cpu()
+ * @return next CPU number to run on (e.g. 0)
+ */
+int mp_next_cpu(int cpu_select, int prev_cpu);
 #else
 static inline int mp_run_on_cpus(int cpu_select, mp_run_func func, void *arg)
 {
@@ -134,6 +161,21 @@ static inline int mp_park_aps(void)
 	return 0;
 }
 
+static inline int mp_first_cpu(int cpu_select)
+{
+	/* We cannot run on any APs, nor a selected CPU */
+	return cpu_select == MP_SELECT_APS ? -EFBIG : MP_SELECT_BSP;
+}
+
+static inline int mp_next_cpu(int cpu_select, int prev_cpu)
+{
+	/*
+	 * When MP is not enabled, there is only one CPU and we did it in
+	 * mp_first_cpu()
+	 */
+	return -EFBIG;
+}
+
 #endif
 
 #endif /* _X86_MP_H_ */