From d395bf12d1ba61437e546eb642f0d7ea666123ff Mon Sep 17 00:00:00 2001 From: Venkatesh Pallipadi Date: Thu, 25 Aug 2005 15:59:00 -0400 Subject: [PATCH] [ACPI] Reduce acpi-cpufreq switching latency by 50% The acpi-cpufreq driver does a P-state get after a P-state set to verify whether set went through successfully. This test is kind of redundant as set goes throught most of the times, and the test is also expensive as a get of P-states can take a lot of time (same as a set operation) as it goes through SMM mode. Effectively, we are doubling the P-state latency due to this get opertion. momdule parameter "acpi_pstate_strict" restores orginal paranoia. http://bugzilla.kernel.org/show_bug.cgi?id=5129 Signed-off-by: Venkatesh Pallipadi Signed-off-by: Len Brown --- arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.c | 57 +++++++++++++-------- 1 file changed, 36 insertions(+), 21 deletions(-) diff --git a/arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.c b/arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.c index 60a9e54dd20e..822c8ce9d1f1 100644 --- a/arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.c +++ b/arch/i386/kernel/cpu/cpufreq/acpi-cpufreq.c @@ -31,6 +31,7 @@ #include #include #include +#include #include #include #include @@ -57,6 +58,8 @@ static struct cpufreq_acpi_io *acpi_io_data[NR_CPUS]; 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 @@ acpi_processor_set_performance ( } /* - * 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 @@ acpi_cpufreq_exit (void) 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); -- 2.30.2