[PATCH v7] cpufreq: Add SFI based cpufreq driver support

[PATCH v7] cpufreq: Add SFI based cpufreq driver support

[Srinidhi Kasagar]

This adds the SFI based cpu freq driver for some of the Intel's
Silvermont based Atom architectures like Z34xx and Z35xx.

Signed-off-by: Rudramuni, Vishwesh M <vishwesh.m.rudramuni@intel.com>
Signed-off-by: Srinidhi Kasagar <srinidhi.kasagar@intel.com>
---
v2:
(v2-v3: comments from Viresh)
* rephrase the Kconfig text
* merge the header file as its not being shared with anyone
* remove the unnecessary checks in sfi_cpufreq_target
* style changes in kzalloc
* fix the potential memory leak sfi_cpufreq_cpu_init
* move the cpufreq driver flag to the structure definition
* remove the redundant init of policy->cpu and policy->cur
* remove the unnecessary check while init the freq table
* merge the for loops in cpu_init
* use cpufreq_generic_attr rather than the custom one

v3:
* Add this change log
* remove the left over not used cached_freq var
* merge yet another for loop in cpu_init

v4:
* remove unnecessary pr_debug
* rephrase Kconfig text
* remove unused macro SFI_CPU_MAX
* Add Viresh's Acked-by

v5:
(comments from Len Brown)
* rename few private functions with sfi_cpufreq_ prefix
* move the PERF_CTL_MASK to msr-index.h
* make the driver.get current freq to refer PERF_CTRL
* remove sfi_processor_px, rather reuse sfi_freq_table_entry
* remove unnecessary memcpy, rather just point to the freq table
* consolidate get_performance_states with parse_freq
* remove extract_freq and make get_cur_freq to read from MSR directly
* move the state_count check while registering the perf states

v6:
* release the mutex in case of error in register_performance
(comments from Viresh)
* drop local perf pointer in ->target and ->init
* fix the order of freeing memories in the error path of ->init
* remove alloc_percpu, rather allocate it at runtime in ->init

v7:
(comments from Len Brown)
* drop the unnecessary mutex
* move sfi_table_parse to __init as it is global to the system
* drop ->get callback
* consolidate few global data structures

 arch/x86/include/uapi/asm/msr-index.h |    1 +
 drivers/cpufreq/Kconfig.x86           |   10 ++
 drivers/cpufreq/Makefile              |    1 +
 drivers/cpufreq/sfi-cpufreq.c         |  165 +++++++++++++++++++++++++++++++++
 4 files changed, 177 insertions(+)
 create mode 100644 drivers/cpufreq/sfi-cpufreq.c

diff --git a/arch/x86/include/uapi/asm/msr-index.h b/arch/x86/include/uapi/asm/msr-index.h
index e21331ce368f..4f6dae67dd10 100644
--- a/arch/x86/include/uapi/asm/msr-index.h
+++ b/arch/x86/include/uapi/asm/msr-index.h
@@ -318,6 +318,7 @@
 
 #define MSR_IA32_PERF_STATUS		0x00000198
 #define MSR_IA32_PERF_CTL		0x00000199
+#define INTEL_PERF_CTL_MASK		0xffff
 #define MSR_AMD_PSTATE_DEF_BASE		0xc0010064
 #define MSR_AMD_PERF_STATUS		0xc0010063
 #define MSR_AMD_PERF_CTL		0xc0010062
diff --git a/drivers/cpufreq/Kconfig.x86 b/drivers/cpufreq/Kconfig.x86
index 89ae88f91895..dd2b6f541f33 100644
--- a/drivers/cpufreq/Kconfig.x86
+++ b/drivers/cpufreq/Kconfig.x86
@@ -57,6 +57,16 @@ config X86_ACPI_CPUFREQ_CPB
 	  By enabling this option the acpi_cpufreq driver provides the old
 	  entry in addition to the new boost ones, for compatibility reasons.
 
+config X86_SFI_CPUFREQ
+	tristate "SFI P-States driver"
+	depends on X86_INTEL_MID && SFI
+	help
+	  This adds a CPUFreq driver for some Silvermont based Intel Atom
+	  architectures like Z34xx and Z35xx which enumerate processor
+	  performance states through SFI.
+
+	  If in doubt, say N.
+
 config ELAN_CPUFREQ
 	tristate "AMD Elan SC400 and SC410"
 	depends on MELAN
diff --git a/drivers/cpufreq/Makefile b/drivers/cpufreq/Makefile
index db6d9a2fea4d..c3b51efd4a85 100644
--- a/drivers/cpufreq/Makefile
+++ b/drivers/cpufreq/Makefile
@@ -41,6 +41,7 @@ obj-$(CONFIG_X86_P4_CLOCKMOD)		+= p4-clockmod.o
 obj-$(CONFIG_X86_CPUFREQ_NFORCE2)	+= cpufreq-nforce2.o
 obj-$(CONFIG_X86_INTEL_PSTATE)		+= intel_pstate.o
 obj-$(CONFIG_X86_AMD_FREQ_SENSITIVITY)	+= amd_freq_sensitivity.o
+obj-$(CONFIG_X86_SFI_CPUFREQ)		+= sfi-cpufreq.o
 
 ##################################################################################
 # ARM SoC drivers
diff --git a/drivers/cpufreq/sfi-cpufreq.c b/drivers/cpufreq/sfi-cpufreq.c
new file mode 100644
index 000000000000..118aa5eba628
--- /dev/null
+++ b/drivers/cpufreq/sfi-cpufreq.c
@@ -0,0 +1,165 @@
+/*
+ *  SFI Processor P-States Driver
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License.
+ *
+ *  This program is distributed in the hope that it will be useful, but
+ *  WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ *  General Public License for more details.
+ *
+ *  Author: Vishwesh M Rudramuni <vishwesh.m.rudramuni@intel.com>
+ *  Author: Srinidhi Kasagar <srinidhi.kasagar@intel.com>
+ */
+
+#include <linux/cpufreq.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/sfi.h>
+#include <linux/slab.h>
+#include <linux/smp.h>
+
+#include <asm/msr.h>
+
+#define SFI_FREQ_MAX		32
+#define SFI_FREQ_MASK		0xff00
+
+static DEFINE_PER_CPU(struct cpufreq_frequency_table *, drv_data);
+static struct sfi_freq_table_entry sfi_cpufreq_array[SFI_FREQ_MAX];
+static int num_freq_table_entries;
+
+static int sfi_parse_freq(struct sfi_table_header *table)
+{
+	struct sfi_table_simple *sb;
+	struct sfi_freq_table_entry *pentry;
+	int totallen;
+
+	sb = (struct sfi_table_simple *)table;
+	num_freq_table_entries = SFI_GET_NUM_ENTRIES(sb,
+			struct sfi_freq_table_entry);
+	if (num_freq_table_entries <= 1) {
+		pr_err("No p-states discovered\n");
+		return -ENODEV;
+	}
+
+	pentry = (struct sfi_freq_table_entry *)sb->pentry;
+	totallen = num_freq_table_entries * sizeof(*pentry);
+	memcpy(sfi_cpufreq_array, pentry, totallen);
+
+	return 0;
+}
+
+static int sfi_cpufreq_target(struct cpufreq_policy *policy,
+				unsigned int index)
+{
+	struct cpufreq_frequency_table *freq_table =
+			per_cpu(drv_data, policy->cpu);
+	unsigned int next_perf_state = 0; /* Index into perf table */
+	u32 lo, hi;
+
+	next_perf_state = freq_table[index].driver_data;
+
+	rdmsr_on_cpu(policy->cpu, MSR_IA32_PERF_CTL, &lo, &hi);
+	lo = (lo & ~INTEL_PERF_CTL_MASK) |
+		((u32) sfi_cpufreq_array[next_perf_state].ctrl_val &
+		INTEL_PERF_CTL_MASK);
+	wrmsr_on_cpu(policy->cpu, MSR_IA32_PERF_CTL, lo, hi);
+
+	return 0;
+}
+
+static int sfi_cpufreq_cpu_init(struct cpufreq_policy *policy)
+{
+	struct cpufreq_frequency_table *freq_table;
+	unsigned int i, result, valid_states = 0;
+	unsigned int cpu = policy->cpu;
+
+	freq_table = kzalloc(sizeof(*freq_table) *
+			(num_freq_table_entries + 1), GFP_KERNEL);
+	if (!freq_table)
+		return -ENOMEM;
+
+	per_cpu(drv_data, cpu) = freq_table;
+
+	policy->shared_type = CPUFREQ_SHARED_TYPE_HW;
+
+	policy->cpuinfo.transition_latency = 0;
+	for (i = 0; i < num_freq_table_entries; i++) {
+		/* detect transition latency */
+		if ((sfi_cpufreq_array[i].latency * 1000) >
+		    policy->cpuinfo.transition_latency)
+			policy->cpuinfo.transition_latency =
+				sfi_cpufreq_array[i].latency * 1000;
+
+		/* initialize the freq table */
+		freq_table[valid_states].driver_data = i;
+		freq_table[valid_states].frequency =
+				sfi_cpufreq_array[i].freq_mhz * 1000;
+		valid_states++;
+
+		pr_debug("     P%d: %d MHz, %d uS\n",
+			i,
+			(u32) sfi_cpufreq_array[i].freq_mhz,
+			(u32) sfi_cpufreq_array[i].latency);
+	}
+	freq_table[valid_states].frequency = CPUFREQ_TABLE_END;
+
+	result = cpufreq_table_validate_and_show(policy, freq_table);
+	if (result)
+		goto err_free;
+
+	pr_debug("CPU%u - SFI performance management activated.\n", cpu);
+
+	return result;
+
+err_free:
+	per_cpu(drv_data, cpu) = NULL;
+	kfree(freq_table);
+
+	return result;
+}
+
+static int sfi_cpufreq_cpu_exit(struct cpufreq_policy *policy)
+{
+	struct cpufreq_frequency_table *freq_table =
+			per_cpu(drv_data, policy->cpu);
+
+	kfree(freq_table);
+	return 0;
+}
+
+static struct cpufreq_driver sfi_cpufreq_driver = {
+	.flags		= CPUFREQ_CONST_LOOPS,
+	.verify		= cpufreq_generic_frequency_table_verify,
+	.target_index	= sfi_cpufreq_target,
+	.init		= sfi_cpufreq_cpu_init,
+	.exit		= sfi_cpufreq_cpu_exit,
+	.name		= "sfi-cpufreq",
+	.attr		= cpufreq_generic_attr,
+};
+
+static int __init sfi_cpufreq_init(void)
+{
+	int ret;
+
+	/* parse the freq table from SFI */
+	ret = sfi_table_parse(SFI_SIG_FREQ, NULL, NULL, sfi_parse_freq);
+	if (ret)
+		return ret;
+
+	return cpufreq_register_driver(&sfi_cpufreq_driver);
+}
+late_initcall(sfi_cpufreq_init);
+
+static void __exit sfi_cpufreq_exit(void)
+{
+	cpufreq_unregister_driver(&sfi_cpufreq_driver);
+}
+module_exit(sfi_cpufreq_exit);
+
+MODULE_AUTHOR("Vishwesh M Rudramuni <vishwesh.m.rudramuni@intel.com>");
+MODULE_DESCRIPTION("SFI P-States Driver");
+MODULE_LICENSE("GPL");
-- 
1.7.9.5


-- 

Re: [PATCH v7] cpufreq: Add SFI based cpufreq driver support

[Viresh Kumar]

On 11 December 2014 at 13:44, Srinidhi Kasagar
<srinidhi.kasagar@intel.com> wrote:
> diff --git a/drivers/cpufreq/sfi-cpufreq.c b/drivers/cpufreq/sfi-cpufreq.c

> +static DEFINE_PER_CPU(struct cpufreq_frequency_table *, drv_data);

You don't need to store this information anywhere, its already present
in policy->freq_table; Just use that and get rid of the per-cpu stuff..

Other than that, Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

Re: [PATCH v7] cpufreq: Add SFI based cpufreq driver support

[Kasagar, Srinidhi]

On Thu, Dec 11, 2014 at 01:52:33PM +0530, Viresh Kumar wrote:
> On 11 December 2014 at 13:44, Srinidhi Kasagar
> <srinidhi.kasagar@intel.com> wrote:
> > diff --git a/drivers/cpufreq/sfi-cpufreq.c b/drivers/cpufreq/sfi-cpufreq.c
> 
> > +static DEFINE_PER_CPU(struct cpufreq_frequency_table *, drv_data);
> 
> You don't need to store this information anywhere, its already present
> in policy->freq_table; Just use that and get rid of the per-cpu stuff..

That's wonderful and keeping freq table per policy makes more sense..
Will fix that; finally you made me to get rid of that
per-cpu stuff :). Thank you Viresh.

Srinidhi

> 
> Other than that, Acked-by: Viresh Kumar <viresh.kumar@linaro.org>

-- 

Re: [PATCH v7] cpufreq: Add SFI based cpufreq driver support

[Len Brown]

> diff --git a/arch/x86/include/uapi/asm/msr-index.h b/arch/x86/include/uapi/asm/msr-index.h
> index e21331ce368f..4f6dae67dd10 100644
> --- a/arch/x86/include/uapi/asm/msr-index.h
> +++ b/arch/x86/include/uapi/asm/msr-index.h
> @@ -318,6 +318,7 @@
>
>  #define MSR_IA32_PERF_STATUS           0x00000198
>  #define MSR_IA32_PERF_CTL              0x00000199
> +#define INTEL_PERF_CTL_MASK            0xffff
>  #define MSR_AMD_PSTATE_DEF_BASE                0xc0010064
>  #define MSR_AMD_PERF_STATUS            0xc0010063
>  #define MSR_AMD_PERF_CTL               0xc0010062
> diff --git a/drivers/cpufreq/Kconfig.x86 b/drivers/cpufreq/Kconfig.x86
> index 89ae88f91895..dd2b6f541f33 100644
> --- a/drivers/cpufreq/Kconfig.x86
> +++ b/drivers/cpufreq/Kconfig.x86
> @@ -57,6 +57,16 @@ config X86_ACPI_CPUFREQ_CPB
>           By enabling this option the acpi_cpufreq driver provides the old
>           entry in addition to the new boost ones, for compatibility reasons.
>
> +config X86_SFI_CPUFREQ
> +       tristate "SFI P-States driver"

I guess since you are editing it anyway, spell out "Performance-State"
instead of P-state.  bytes are free:-)

> +       depends on X86_INTEL_MID && SFI
> +       help
> +         This adds a CPUFreq driver for some Silvermont based Intel Atom
> +         architectures like Z34xx and Z35xx which enumerate processor
> +         performance states through SFI.
> +
> +         If in doubt, say N.
> +
>  config ELAN_CPUFREQ
>         tristate "AMD Elan SC400 and SC410"
>         depends on MELAN
...
> diff --git a/drivers/cpufreq/sfi-cpufreq.c b/drivers/cpufreq/sfi-cpufreq.c
> new file mode 100644
> index 000000000000..118aa5eba628
> --- /dev/null
> +++ b/drivers/cpufreq/sfi-cpufreq.c
> @@ -0,0 +1,165 @@
> +/*
> + *  SFI Processor P-States Driver
> + *
> + *  This program is free software; you can redistribute it and/or modify
> + *  it under the terms of the GNU General Public License as published by
> + *  the Free Software Foundation; either version 2 of the License.
> + *
> + *  This program is distributed in the hope that it will be useful, but
> + *  WITHOUT ANY WARRANTY; without even the implied warranty of
> + *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + *  General Public License for more details.
> + *
> + *  Author: Vishwesh M Rudramuni <vishwesh.m.rudramuni@intel.com>
> + *  Author: Srinidhi Kasagar <srinidhi.kasagar@intel.com>
> + */
> +
> +#include <linux/cpufreq.h>
> +#include <linux/init.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/sfi.h>
> +#include <linux/slab.h>
> +#include <linux/smp.h>
> +
> +#include <asm/msr.h>
> +
> +#define SFI_FREQ_MAX           32

please delete definition of SFI_FREQ_MAX, per comment below  in sfi_parse_freq()

> +#define SFI_FREQ_MASK          0xff00

please delete definition of SFI_FREQ_MASK
it is unused since get_cur_freq_on_cpu() is gone -- yay:-)

> +
> +static DEFINE_PER_CPU(struct cpufreq_frequency_table *, drv_data);
> +static struct sfi_freq_table_entry sfi_cpufreq_array[SFI_FREQ_MAX];

above will become a pointer to a kzalloc array.

> +static int num_freq_table_entries;
> +
> +static int sfi_parse_freq(struct sfi_table_header *table)
> +{
> +       struct sfi_table_simple *sb;
> +       struct sfi_freq_table_entry *pentry;
> +       int totallen;
> +
> +       sb = (struct sfi_table_simple *)table;
> +       num_freq_table_entries = SFI_GET_NUM_ENTRIES(sb,
> +                       struct sfi_freq_table_entry);
> +       if (num_freq_table_entries <= 1) {
> +               pr_err("No p-states discovered\n");
> +               return -ENODEV;
> +       }
> +
> +       pentry = (struct sfi_freq_table_entry *)sb->pentry;
> +       totallen = num_freq_table_entries * sizeof(*pentry);

if num_freq_table_entries exceeds SFI_FREQ_MAX,
then the following line would over-write random kernel data.

Better to simply kzalloc this array,
and delete the arbitrary #define SFI_FREQ_MAX

> +       memcpy(sfi_cpufreq_array, pentry, totallen);
> +
> +       return 0;
> +}
> +
> +static int sfi_cpufreq_target(struct cpufreq_policy *policy,
> +                               unsigned int index)

fits into 80 columns on 1 line, yes?

> +{
> +       struct cpufreq_frequency_table *freq_table =
> +                       per_cpu(drv_data, policy->cpu);
> +       unsigned int next_perf_state = 0; /* Index into perf table */
> +       u32 lo, hi;
> +
> +       next_perf_state = freq_table[index].driver_data;
> +
> +       rdmsr_on_cpu(policy->cpu, MSR_IA32_PERF_CTL, &lo, &hi);
> +       lo = (lo & ~INTEL_PERF_CTL_MASK) |
> +               ((u32) sfi_cpufreq_array[next_perf_state].ctrl_val &
> +               INTEL_PERF_CTL_MASK);
> +       wrmsr_on_cpu(policy->cpu, MSR_IA32_PERF_CTL, lo, hi);
> +
> +       return 0;
> +}
> +
> +static int sfi_cpufreq_cpu_init(struct cpufreq_policy *policy)
> +{
> +       struct cpufreq_frequency_table *freq_table;
> +       unsigned int i, result, valid_states = 0;
> +       unsigned int cpu = policy->cpu;
> +
> +       freq_table = kzalloc(sizeof(*freq_table) *
> +                       (num_freq_table_entries + 1), GFP_KERNEL);
> +       if (!freq_table)
> +               return -ENOMEM;
> +
> +       per_cpu(drv_data, cpu) = freq_table;
> +
> +       policy->shared_type = CPUFREQ_SHARED_TYPE_HW;
> +
> +       policy->cpuinfo.transition_latency = 0;

replace line above with...

policy->cpuinfo.transition_latency = CPUFREQ_ETERNAL;

ondemand will compare this to TRANSITION_LATENCY_LIMIT (10 * 1000 * 1000)
and since -1 is less than 10,000,000; it will be happy.

> +       for (i = 0; i < num_freq_table_entries; i++) {
> +               /* detect transition latency */
> +               if ((sfi_cpufreq_array[i].latency * 1000) >
> +                   policy->cpuinfo.transition_latency)
> +                       policy->cpuinfo.transition_latency =
> +                               sfi_cpufreq_array[i].latency * 1000;

delete above 5 lines.  every SFI system will have latency
plenty lower than that required by ondemand governor.

> +
> +               /* initialize the freq table */
> +               freq_table[valid_states].driver_data = i;
> +               freq_table[valid_states].frequency =
> +                               sfi_cpufreq_array[i].freq_mhz * 1000;
> +               valid_states++;
> +
> +               pr_debug("     P%d: %d MHz, %d uS\n",
> +                       i,
> +                       (u32) sfi_cpufreq_array[i].freq_mhz,
> +                       (u32) sfi_cpufreq_array[i].latency);

I don't think anybody cares what the latency field is,
so you don't have to print it.

cpufreq_table_validate_and_show already does a pr_debug()
showing the mhz for each p-state -- so you can delete this pr_debug entirely.

> +       }
> +       freq_table[valid_states].frequency = CPUFREQ_TABLE_END;
> +
> +       result = cpufreq_table_validate_and_show(policy, freq_table);
> +       if (result)
> +               goto err_free;
> +
> +       pr_debug("CPU%u - SFI performance management activated.\n", cpu);
> +
> +       return result;
> +
> +err_free:
> +       per_cpu(drv_data, cpu) = NULL;
> +       kfree(freq_table);
> +
> +       return result;
> +}
> +
> +static int sfi_cpufreq_cpu_exit(struct cpufreq_policy *policy)
> +{
> +       struct cpufreq_frequency_table *freq_table =
> +                       per_cpu(drv_data, policy->cpu);
> +
> +       kfree(freq_table);
> +       return 0;
> +}
> +
> +static struct cpufreq_driver sfi_cpufreq_driver = {
> +       .flags          = CPUFREQ_CONST_LOOPS,
> +       .verify         = cpufreq_generic_frequency_table_verify,
> +       .target_index   = sfi_cpufreq_target,
> +       .init           = sfi_cpufreq_cpu_init,
> +       .exit           = sfi_cpufreq_cpu_exit,
> +       .name           = "sfi-cpufreq",
> +       .attr           = cpufreq_generic_attr,
> +};
> +
> +static int __init sfi_cpufreq_init(void)
> +{
> +       int ret;
> +
> +       /* parse the freq table from SFI */
> +       ret = sfi_table_parse(SFI_SIG_FREQ, NULL, NULL, sfi_parse_freq);
> +       if (ret)
> +               return ret;
> +
> +       return cpufreq_register_driver(&sfi_cpufreq_driver);
> +}
> +late_initcall(sfi_cpufreq_init);
> +
> +static void __exit sfi_cpufreq_exit(void)
> +{
> +       cpufreq_unregister_driver(&sfi_cpufreq_driver);
> +}
> +module_exit(sfi_cpufreq_exit);
> +
> +MODULE_AUTHOR("Vishwesh M Rudramuni <vishwesh.m.rudramuni@intel.com>");
> +MODULE_DESCRIPTION("SFI P-States Driver");
> +MODULE_LICENSE("GPL");
> --
> 1.7.9.5


-- 
Len Brown, Intel Open Source Technology Center

Re: [PATCH v7] cpufreq: Add SFI based cpufreq driver support

[Kasagar, Srinidhi]

On Tue, Dec 16, 2014 at 03:15:33AM -0500, Len Brown wrote:

[...]

> replace line above with...
> 
> policy->cpuinfo.transition_latency = CPUFREQ_ETERNAL;

But we know the transition latency of this system. Why should we
claim it as "unknown"? The sfi_freq_table_entry has latency
field - does it not make irrelevant then?

> 
> ondemand will compare this to TRANSITION_LATENCY_LIMIT (10 * 1000 * 1000)
> and since -1 is less than 10,000,000; it will be happy.
> 
> > +       for (i = 0; i < num_freq_table_entries; i++) {
> > +               /* detect transition latency */
> > +               if ((sfi_cpufreq_array[i].latency * 1000) >
> > +                   policy->cpuinfo.transition_latency)
> > +                       policy->cpuinfo.transition_latency =
> > +                               sfi_cpufreq_array[i].latency * 1000;
> 
> delete above 5 lines.  every SFI system will have latency
> plenty lower than that required by ondemand governor.

Srinidhi

Re: [PATCH v7] cpufreq: Add SFI based cpufreq driver support

[Len Brown]

>> policy->cpuinfo.transition_latency = CPUFREQ_ETERNAL;
>
> But we know the transition latency of this system. Why should we
> claim it as "unknown"? The sfi_freq_table_entry has latency
> field - does it not make irrelevant then?

I wrote the SFI spec.
If I did it again, I would not include the latency field in the FREQ table,
because Linux does not need it.

>> ondemand will compare this to TRANSITION_LATENCY_LIMIT (10 * 1000 * 1000)
>> and since -1 is less than 10,000,000; it will be happy.
>>
>> > +       for (i = 0; i < num_freq_table_entries; i++) {
>> > +               /* detect transition latency */
>> > +               if ((sfi_cpufreq_array[i].latency * 1000) >
>> > +                   policy->cpuinfo.transition_latency)
>> > +                       policy->cpuinfo.transition_latency =
>> > +                               sfi_cpufreq_array[i].latency * 1000;
>>
>> delete above 5 lines.  every SFI system will have latency
>> plenty lower than that required by ondemand governor.

There is effectively no customer for the information being provided,
as there are only two possible outcomes:

1. The largest value in the table is < 10ms, and ondemand loads
2. The largest value in the table is > 10ms and ondemand fails to load.
#1 is how it should always work on all known SFI platforms.
#2 is a firmware bug.

Well, we can do #1 with 1 line, and at the same time eliminate
exposure to firmware bugs in #2.
Less is more, the choice is simple.

thanks,
Len Brown, Intel Open Source Technology Center

Re: [PATCH v7] cpufreq: Add SFI based cpufreq driver support

[Kasagar, Srinidhi]

On Wed, Dec 17, 2014 at 05:12:43PM -0500, Len Brown wrote:
> >> policy->cpuinfo.transition_latency = CPUFREQ_ETERNAL;
> >
> > But we know the transition latency of this system. Why should we
> > claim it as "unknown"? The sfi_freq_table_entry has latency
> > field - does it not make irrelevant then?
> 
> I wrote the SFI spec.
> If I did it again, I would not include the latency field in the FREQ table,
> because Linux does not need it.
> 
> >> ondemand will compare this to TRANSITION_LATENCY_LIMIT (10 * 1000 * 1000)
> >> and since -1 is less than 10,000,000; it will be happy.
> >>
> >> > +       for (i = 0; i < num_freq_table_entries; i++) {
> >> > +               /* detect transition latency */
> >> > +               if ((sfi_cpufreq_array[i].latency * 1000) >
> >> > +                   policy->cpuinfo.transition_latency)
> >> > +                       policy->cpuinfo.transition_latency =
> >> > +                               sfi_cpufreq_array[i].latency * 1000;
> >>
> >> delete above 5 lines.  every SFI system will have latency
> >> plenty lower than that required by ondemand governor.
> 
> There is effectively no customer for the information being provided,
> as there are only two possible outcomes:
> 
> 1. The largest value in the table is < 10ms, and ondemand loads
> 2. The largest value in the table is > 10ms and ondemand fails to load.
> #1 is how it should always work on all known SFI platforms.
> #2 is a firmware bug.
> 
> Well, we can do #1 with 1 line, and at the same time eliminate
> exposure to firmware bugs in #2.

Ok, as we know the latency of this system (as 100 us), I will hardcode this
with 1 line and remove the rest of detection stuff..

Srinidhi