libpayload/x86/delay: Add an x86 arch_ndelay

This method has a pause instruction to help the CPU relax a little bit.
Measuring grunt it saves about 80mW.

BUG=b:109749762
TEST=Made sure that grunt boots.

Change-Id: I045a941ed42fcc4f2dbdd65b5cbb42d84813f50c
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://review.coreboot.org/28244
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/arch/x86/Makefile.inc

@@ -35,6 +35,7 @@ libc-y += timer.c coreboot.c util.S
 libc-y += exec.S virtual.c
 libc-y += selfboot.c
 libc-y += exception_asm.S exception.c
+libc-y += delay.c
 
 # Will fall back to default_memXXX() in libc/memory.c if GPL not allowed.
 libc-$(CONFIG_LP_GPL) += string.c

payloads/libpayload/arch/x86/delay.c

@@ -0,0 +1,52 @@
+/*
+ * This file is part of the libpayload project.
+ *
+ * Copyright (C) 2018 Google LLC.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. The name of the author may not be used to endorse or promote products
+ *    derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <libpayload.h>
+
+/* The pause instruction can delay 10-140 CPU cycles, so avoid calling it when
+ * getting close to finishing. Depending on the timer source, the timer can be
+ * running at CPU frequency, bus frequency, or some arbitrary value. We assume
+ * that the timer is running at the CPU frequency. */
+#define PAUSE_THRESHOLD_TICKS		150
+
+void arch_ndelay(uint64_t ns)
+{
+	uint64_t delta = ns * timer_hz() / NSECS_PER_SEC;
+	uint64_t pause_delta = 0;
+	uint64_t start = timer_raw_value();
+
+	if (delta > PAUSE_THRESHOLD_TICKS)
+		pause_delta = delta - PAUSE_THRESHOLD_TICKS;
+
+	while (timer_raw_value() - start < pause_delta)
+		asm volatile("pause\n\t");
+
+	while (timer_raw_value() - start < delta)
+		continue;
+}