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Subject[PATCH] acpi-cpufreq: Remove P-state read after a P-state write in normal path


Remove P-state read status after a P-state write in normal case. As
that operation is expensive. There are no known failures of a set and we can
assume success in the common case. acpi_pstate_strict parameter can be used
to force it back on any systems that is known to have errors.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>

Index: linux-2.6.12/arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.c
===================================================================
--- linux-2.6.12.orig/arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.c 2005-08-16 09:41:52.977456128 -0700
+++ linux-2.6.12/arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.c 2005-08-17 13:32:33.034651136 -0700
@@ -31,6 +31,7 @@
#include <linux/cpufreq.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
+#include <linux/compiler.h>
#include <asm/io.h>
#include <asm/delay.h>
#include <asm/uaccess.h>
@@ -57,6 +58,8 @@

static struct cpufreq_driver acpi_cpufreq_driver;

+static unsigned int acpi_pstate_strict;
+
static int
acpi_processor_write_port(
u16 port,
@@ -163,34 +166,44 @@
}

/*
- * Then we read the 'status_register' and compare the value with the
- * target state's 'status' to make sure the transition was successful.
- * Note that we'll poll for up to 1ms (100 cycles of 10us) before
- * giving up.
+ * Assume the write went through when acpi_pstate_strict is not used.
+ * As read status_register is an expensive operation and there
+ * are no specific error cases where an IO port write will fail.
*/
-
- port = data->acpi_data.status_register.address;
- bit_width = data->acpi_data.status_register.bit_width;
-
- dprintk("Looking for 0x%08x from port 0x%04x\n",
- (u32) data->acpi_data.states[state].status, port);
-
- for (i=0; i<100; i++) {
- ret = acpi_processor_read_port(port, bit_width, &value);
- if (ret) {
- dprintk("Invalid port width 0x%04x\n", bit_width);
- retval = ret;
- goto migrate_end;
+ if (acpi_pstate_strict) {
+ /* Then we read the 'status_register' and compare the value
+ * with the target state's 'status' to make sure the
+ * transition was successful.
+ * Note that we'll poll for up to 1ms (100 cycles of 10us)
+ * before giving up.
+ */
+
+ port = data->acpi_data.status_register.address;
+ bit_width = data->acpi_data.status_register.bit_width;
+
+ dprintk("Looking for 0x%08x from port 0x%04x\n",
+ (u32) data->acpi_data.states[state].status, port);
+
+ for (i=0; i<100; i++) {
+ ret = acpi_processor_read_port(port, bit_width, &value);
+ if (ret) {
+ dprintk("Invalid port width 0x%04x\n", bit_width);
+ retval = ret;
+ goto migrate_end;
+ }
+ if (value == (u32) data->acpi_data.states[state].status)
+ break;
+ udelay(10);
}
- if (value == (u32) data->acpi_data.states[state].status)
- break;
- udelay(10);
+ } else {
+ i = 0;
+ value = (u32) data->acpi_data.states[state].status;
}

/* notify cpufreq */
cpufreq_notify_transition(&cpufreq_freqs, CPUFREQ_POSTCHANGE);

- if (value != (u32) data->acpi_data.states[state].status) {
+ if (unlikely(value != (u32) data->acpi_data.states[state].status)) {
unsigned int tmp = cpufreq_freqs.new;
cpufreq_freqs.new = cpufreq_freqs.old;
cpufreq_freqs.old = tmp;
@@ -537,6 +550,8 @@
return;
}

+module_param(acpi_pstate_strict, uint, 0644);
+MODULE_PARM_DESC(acpi_pstate_strict, "value 0 or non-zero. non-zero -> strict ACPI checks are performed during frequency changes.");

late_initcall(acpi_cpufreq_init);
module_exit(acpi_cpufreq_exit);
-
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