[PATCH v4 10/25] x86: mp: Support APs waiting for instructions

[Simon Glass <sjg@chromium.org>]

At present the APs (non-boot CPUs) are inited once and then parked ready
for the OS to use them. However in some cases we want to send new requests
through, such as to change MTRRs and keep them consistent across CPUs.

Change the last state of the flight plan to go into a wait loop, accepting
instructions from the main CPU.

Drop cpu_map since it is not used.

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

Changes in v4:
- Add a Kconfig to control this feature, enabled only on APL

Changes in v3:
- s/slow/slot/
- Use C code instead of assembler to read/write callback pointers
- Update commit message to mention dropping of cpu_map

Changes in v2:
- Add more comments

 arch/x86/Kconfig                |   7 ++
 arch/x86/cpu/apollolake/Kconfig |   1 +
 arch/x86/cpu/mp_init.c          | 123 +++++++++++++++++++++++++++++---
 arch/x86/include/asm/mp.h       |  11 +++
 4 files changed, 134 insertions(+), 8 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index c8eae24c07..cc4716fe12 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -603,6 +603,13 @@ config SMP
 	  only one CPU will be enabled regardless of the number of CPUs
 	  available.
 
+config SMP_AP_WORK
+	bool
+	depends on SMP
+	help
+	 Allow APs to do other work after initialisation instead of going
+	 to sleep.
+
 config MAX_CPUS
 	int "Maximum number of CPUs permitted"
 	depends on SMP
diff --git a/arch/x86/cpu/apollolake/Kconfig b/arch/x86/cpu/apollolake/Kconfig
index 942f11f566..99d4e105c2 100644
--- a/arch/x86/cpu/apollolake/Kconfig
+++ b/arch/x86/cpu/apollolake/Kconfig
@@ -15,6 +15,7 @@ config INTEL_APOLLOLAKE
 	select TPL_PCH_SUPPORT
 	select PCH_SUPPORT
 	select P2SB
+	select SMP_AP_WORK
 	imply ENABLE_MRC_CACHE
 	imply AHCI_PCI
 	imply SCSI
diff --git a/arch/x86/cpu/mp_init.c b/arch/x86/cpu/mp_init.c
index 9fdde7832a..787de92fa3 100644
--- a/arch/x86/cpu/mp_init.c
+++ b/arch/x86/cpu/mp_init.c
@@ -15,6 +15,7 @@
 #include <asm/atomic.h>
 #include <asm/cpu.h>
 #include <asm/interrupt.h>
+#include <asm/io.h>
 #include <asm/lapic.h>
 #include <asm/microcode.h>
 #include <asm/mp.h>
@@ -43,13 +44,38 @@ struct mp_flight_plan {
 	struct mp_flight_record *records;
 };
 
+/**
+ * struct mp_callback - Callback information for APs
+ *
+ * @func: Function to run
+ * @arg: Argument to pass to the function
+ * @logical_cpu_number: Either a CPU number (i.e. dev->req_seq) or a special
+ *	value like MP_SELECT_BSP. It tells the AP whether it should process this
+ *	callback
+ */
+struct mp_callback {
+	/**
+	 * func() - Function to call on the AP
+	 *
+	 * @arg: Argument to pass
+	 */
+	void (*func)(void *arg);
+	void *arg;
+	int logical_cpu_number;
+};
+
 static struct mp_flight_plan mp_info;
 
-struct cpu_map {
-	struct udevice *dev;
-	int apic_id;
-	int err_code;
-};
+/*
+ * ap_callbacks - Callback mailbox array
+ *
+ * Array of callback, one entry for each available CPU, indexed by the CPU
+ * number, which is dev->req_seq. The entry for the main CPU is never used.
+ * When this is NULL, there is no pending work for the CPU to run. When
+ * non-NULL it points to the mp_callback structure. This is shared between all
+ * CPUs, so should only be written by the main CPU.
+ */
+static struct mp_callback **ap_callbacks;
 
 static inline void barrier_wait(atomic_t *b)
 {
@@ -147,11 +173,12 @@ static void ap_init(unsigned int cpu_index)
 	debug("AP: slot %d apic_id %x, dev %s\n", cpu_index, apic_id,
 	      dev ? dev->name : "(apic_id not found)");
 
-	/* Walk the flight plan */
+	/*
+	 * Walk the flight plan, which only returns if CONFIG_SMP_AP_WORK is not
+	 * enabled
+	 */
 	ap_do_flight_plan(dev);
 
-	/* Park the AP */
-	debug("parking\n");
 done:
 	stop_this_cpu();
 }
@@ -456,6 +483,81 @@ static int get_bsp(struct udevice **devp, int *cpu_countp)
 	return dev->req_seq >= 0 ? dev->req_seq : 0;
 }
 
+/**
+ * read_callback() - Read the pointer in a callback slot
+ *
+ * This is called by APs to read their callback slot to see if there is a
+ * pointer to new instructions
+ *
+ * @slot: Pointer to the AP's callback slot
+ * @return value of that pointer
+ */
+static struct mp_callback *read_callback(struct mp_callback **slot)
+{
+	dmb();
+
+	return *slot;
+}
+
+/**
+ * store_callback() - Store a pointer to the callback slot
+ *
+ * This is called by APs to write NULL into the callback slot when they have
+ * finished the work requested by the BSP.
+ *
+ * @slot: Pointer to the AP's callback slot
+ * @val: Value to write (e.g. NULL)
+ */
+static void store_callback(struct mp_callback **slot, struct mp_callback *val)
+{
+	*slot = val;
+	dmb();
+}
+
+/**
+ * ap_wait_for_instruction() - Wait for and process requests from the main CPU
+ *
+ * This is called by APs (here, everything other than the main boot CPU) to
+ * await instructions. They arrive in the form of a function call and argument,
+ * which is then called. This uses a simple mailbox with atomic read/set
+ *
+ * @cpu: CPU that is waiting
+ * @unused: Optional argument provided by struct mp_flight_record, not used here
+ * @return Does not return
+ */
+static int ap_wait_for_instruction(struct udevice *cpu, void *unused)
+{
+	struct mp_callback lcb;
+	struct mp_callback **per_cpu_slot;
+
+	if (!IS_ENABLED(CONFIG_SMP_AP_WORK))
+		return 0;
+
+	per_cpu_slot = &ap_callbacks[cpu->req_seq];
+
+	while (1) {
+		struct mp_callback *cb = read_callback(per_cpu_slot);
+
+		if (!cb) {
+			asm ("pause");
+			continue;
+		}
+
+		/* Copy to local variable before using the value */
+		memcpy(&lcb, cb, sizeof(lcb));
+		mfence();
+		if (lcb.logical_cpu_number == MP_SELECT_ALL ||
+		    lcb.logical_cpu_number == MP_SELECT_APS ||
+		    cpu->req_seq == lcb.logical_cpu_number)
+			lcb.func(lcb.arg);
+
+		/* Indicate we are finished */
+		store_callback(per_cpu_slot, NULL);
+	}
+
+	return 0;
+}
+
 static int mp_init_cpu(struct udevice *cpu, void *unused)
 {
 	struct cpu_platdata *plat = dev_get_parent_platdata(cpu);
@@ -468,6 +570,7 @@ static int mp_init_cpu(struct udevice *cpu, void *unused)
 
 static struct mp_flight_record mp_steps[] = {
 	MP_FR_BLOCK_APS(mp_init_cpu, NULL, mp_init_cpu, NULL),
+	MP_FR_BLOCK_APS(ap_wait_for_instruction, NULL, NULL, NULL),
 };
 
 int mp_init(void)
@@ -505,6 +608,10 @@ int mp_init(void)
 	if (ret)
 		log_warning("Warning: Device tree does not describe all CPUs. Extra ones will not be started correctly\n");
 
+	ap_callbacks = calloc(num_cpus, sizeof(struct mp_callback *));
+	if (!ap_callbacks)
+		return -ENOMEM;
+
 	/* Copy needed parameters so that APs have a reference to the plan */
 	mp_info.num_records = ARRAY_SIZE(mp_steps);
 	mp_info.records = mp_steps;
diff --git a/arch/x86/include/asm/mp.h b/arch/x86/include/asm/mp.h
index 94af819ad9..41b1575f4b 100644
--- a/arch/x86/include/asm/mp.h
+++ b/arch/x86/include/asm/mp.h
@@ -11,6 +11,17 @@
 #include <asm/atomic.h>
 #include <asm/cache.h>
 
+enum {
+	/* Indicates that the function should run on all CPUs */
+	MP_SELECT_ALL	= -1,
+
+	/* Run on boot CPUs */
+	MP_SELECT_BSP	= -2,
+
+	/* Run on non-boot CPUs */
+	MP_SELECT_APS	= -3,
+};
+
 typedef int (*mp_callback_t)(struct udevice *cpu, void *arg);
 
 /*
-- 
2.27.0.383.g050319c2ae-goog