Make sleep threshold for delays configurable

This allows the minimum time that default_delay() will choose to sleep
for instead of polling to be configured at build-time. The default
remains unchanged at 100 milliseconds for now.

The test's correctness has been checked by testing with minimum sleep
time left at its default and set to a non-default value smaller than 100
microseconds (both pass without sleeping, verified with strace) and with
the minimum sleep time set to 0 (causing the test to be skipped). The
configured value from the macro needs to be stored in a const to avoid
-Werror=type-limits errors when configured to be zero.

Change-Id: Ida96e0816ac914ed69d6fd82ad90ebe89cdef1cc
Signed-off-by: Peter Marheine <pmarheine@chromium.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/81606
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>

Makefile

@@ -542,6 +542,11 @@ CONFIG_NI845X_SPI ?= no
 # Disable wiki printing by default. It is only useful if you have wiki access.
 CONFIG_PRINT_WIKI ?= no
 
+# Minimum time in microseconds to suspend execution for (rather than polling)
+# when a delay is required. Larger values may perform better on machines with
+# low timer resolution, at the cost of increased power.
+CONFIG_DELAY_MINIMUM_SLEEP_US ?= 100000
+
 # Disable all features if CONFIG_NOTHING=yes is given unless CONFIG_EVERYTHING was also set
 ifeq ($(CONFIG_NOTHING), yes)
   ifeq ($(CONFIG_EVERYTHING), yes)
@@ -587,6 +592,7 @@ FEATURE_FLAGS += -D'CONFIG_DEFAULT_PROGRAMMER_NAME=NULL'
 endif
 
 FEATURE_FLAGS += -D'CONFIG_DEFAULT_PROGRAMMER_ARGS="$(CONFIG_DEFAULT_PROGRAMMER_ARGS)"'
+FEATURE_FLAGS += -D'CONFIG_DELAY_MINIMUM_SLEEP_US=$(CONFIG_DELAY_MINIMUM_SLEEP_US)'
 
 ################################################################################
 

meson.build

@@ -112,6 +112,9 @@ if target_machine.system() == 'dos'
   delay_src = files('udelay_dos.c')
 endif
 srcs += delay_src
+cargs += ['-DCONFIG_DELAY_MINIMUM_SLEEP_US=@0@'.format(
+  get_option('delay_minimum_sleep_us')
+)]
 
 # check for required symbols
 if cc.has_function('clock_gettime')

meson_options.txt

@@ -22,3 +22,6 @@ option('man-pages', type : 'feature', value : 'auto', description : 'build the m
 option('documentation', type : 'feature', value : 'auto', description : 'build the html documentation')
 option('ni845x_search_path', type : 'string', value : 'C:\Program Files (x86)\National Instruments\Ni-845x\MS Visual C',
        description : 'Path to search for the proprietary ni845x library and header (32-bit Windows only)')
+option('delay_minimum_sleep_us', type : 'integer', min : 0, value : 100000,
+       description : 'Minimum time in microseconds to suspend execution for (rather than polling) when a delay is required.'
+                   + ' Larger values may perform better on machines with low timer resolution, at the cost of increased power.')

tests/udelay.c

@@ -34,6 +34,8 @@ static uint64_t now_us(void) {
 #endif
 }
 
+static const int64_t min_sleep = CONFIG_DELAY_MINIMUM_SLEEP_US;
+
 /*
  * A short delay should delay for at least as long as requested,
  * and more than 10x as long would be worrisome.
@@ -43,9 +45,21 @@ static uint64_t now_us(void) {
  * test.
  */
 void udelay_test_short(void **state) {
+    /*
+     * Delay for 100 microseconds, or short enough that we won't sleep.
+     * It's not useful to test the sleep path because we assume the OS won't
+     * sleep for less time than we ask.
+     */
+    int64_t delay_us = 100;
+    if (delay_us >= min_sleep)
+      delay_us = min_sleep - 1;
+    /* No point in running this test if delay always sleeps. */
+    if (delay_us <= 0)
+      skip();
+
     uint64_t start = now_us();
-    default_delay(100);
+    default_delay(delay_us);
     uint64_t elapsed = now_us() - start;
 
-    assert_in_range(elapsed, 100, 1000);
+    assert_in_range(elapsed, delay_us, 10 * delay_us);
 }

udelay.c

@@ -92,14 +92,15 @@ void internal_sleep(unsigned int usecs)
 #endif
 }
 
+static const unsigned min_sleep = CONFIG_DELAY_MINIMUM_SLEEP_US;
+
 /* Precise delay. */
 void default_delay(unsigned int usecs)
 {
-	/* If the delay is >0.1 s, use internal_sleep because timing does not need to be so precise. */
+	if (usecs < min_sleep) {
-	if (usecs > 100000) {
-		internal_sleep(usecs);
-	} else {
 		clock_usec_delay(usecs);
+	} else {
+		internal_sleep(usecs);
 	}
 }
 

udelay_dos.c

@@ -153,19 +153,20 @@ void internal_sleep(unsigned int usecs)
 	usleep(usecs % 1000000);
 }
 
+static const unsigned min_sleep = CONFIG_DELAY_MINIMUM_SLEEP_US;
+
 /* Precise delay. */
 void default_delay(unsigned int usecs)
 {
 	static bool calibrated = false;
 
-	/* If the delay is >0.1 s, use internal_sleep because timing does not need to be so precise. */
+	if (usecs < min_sleep) {
-	if (usecs > 100000) {
-		internal_sleep(usecs);
-	} else {
 		if (!calibrated) {
 			myusec_calibrate_delay();
 			calibrated = true;
 		}
 		myusec_delay(usecs);
+	} else {
+		internal_sleep(usecs);
 	}
 }