Longhaul - Add ignore_latency option

Some laptops with VIA C3 processor, CLE266 chipset and AMI BIOS have incorrect latency values in FADT table. These laptops seems to be C3 capable, but latency values are to
big: 101 for C2 and 1017 for C3. This option will allow user to skip C3 latency test but not C3 address test. AMI BIOS is setting C3 address to correct value in DSDT table.

Signed-off-by Rafa³ Bilski <[email protected]>

---

arch/i386/kernel/cpu/cpufreq/longhaul.c +6 -3

@@ -65,7 +65,7 @@ static int port22_en = 0;
 
 /* Module parameters */
 static int dont_scale_voltage;
-
+static int ignore_latency = 0;
 
 #define dprintk(msg...) cpufreq_debug_printk(CPUFREQ_DEBUG_DRIVER, "longhaul", msg)
 

@@ -665,8 +665,10 @@ static int __init longhaul_cpu_init(stru
 	if (longhaul_version == TYPE_POWERSAVER) {
 		/* Check ACPI support for C3 state */
 		cx = &pr->power.states[ACPI_STATE_C3];
-		if (cx->address == 0 || cx->latency > 1000)
+		if (cx->address == 0 || 
+		   (cx->latency > 1000 && ignore_latency == 0) )
 			goto err_acpi;
+
 	} else {
 		/* Check ACPI support for bus master arbiter disable */
 		if (!pr->flags.bm_control) {

@@ -773,6 +775,8 @@ static void __exit longhaul_exit(void)
 
 module_param (dont_scale_voltage, int, 0644);
 MODULE_PARM_DESC(dont_scale_voltage, "Don't scale voltage of processor");
+module_param(ignore_latency, int, 0644);
+MODULE_PARM_DESC(ignore_latency, "Skip ACPI C3 latency test");
 
 MODULE_AUTHOR ("Dave Jones <[email protected]>");
 MODULE_DESCRIPTION ("Longhaul driver for VIA Cyrix processors.");

@@ -780,4 +784,3 @@ MODULE_LICENSE ("GPL");
 
 late_initcall(longhaul_init);
 module_exit(longhaul_exit);
-

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Quoted 6 lines

Some laptops with VIA C3 processor, CLE266 chipset and AMI BIOS have incorrect latency values in FADT table. These laptops seems to be C3 capable, but latency values are to
big: 101 for C2 and 1017 for C3. This option will allow user to skip C3 latency test but not C3 address test. AMI BIOS is setting C3 address to correct value in DSDT table.

This looks very fishy.

C2 latency above 100usec and C3 latency above 1000 usec are the official way for the BIOS to tell the OS to NOT USE those C-states. Under no conditions should Linux second guess those explicit instructions -- the BIOS may have put those values there because you get silent data corruption on that particular stepping of the processor or chipset if they are enabled.

But the real question is why the longhaul driver is checking for C2 and C3 support in the first place -- as they are not directly related to the availability of P-states.

-Len

Quoted 8 lines

Quoted 6 lines

Some laptops with VIA C3 processor, CLE266 chipset and AMI BIOS have incorrect latency values in FADT table. These laptops seems to be C3 capable, but latency values are to
big: 101 for C2 and 1017 for C3. This option will allow user to skip C3 latency test but not C3 address test. AMI BIOS is setting C3 address to correct value in DSDT table.

This looks very fishy.

So why P_BLK address is valid and have proper lenght?

Quoted 8 lines

C2 latency above 100usec and C3 latency above 1000 usec are the official way for the BIOS to tell the OS to NOT USE those C-states. Under no conditions should Linux second guess those explicit instructions -- the BIOS may have put those values there because you get silent data corruption on that particular stepping of the processor or chipset if they are enabled.

It is disabled by default.
It is only needed for some laptops.
As far I know there is no data corruption. Chipset and processor are exacly the same as on Epia M10000 so I don't expect any.

Why force user to recompilation of distro kernel?

Quoted 3 lines

But the real question is why the longhaul driver is checking for C2 and C3 support in the first place -- as they are not directly related to the availability of P-states.

Because Longhaul isn't using P-states.

Quoted 1 line

So why P_BLK address is valid and have proper lenght?

One possible scenario is that the same BIOS source runs on multiple hardware steppings. If a broken stepping is found, the BIOS writer knows that setting the latency above the legal threshold is sufficient to disable the C-state in all known ACPI-enabled operating systems, per below.

Quoted 2 lines

Why force user to recompilation of distro kernel?

Why should a distro support this if the BIOS vendor disabled it? Is the fact that you have not noticed data corruption a sufficient test such that they can suport this?

Quoted 4 lines

Quoted 3 lines

But the real question is why the longhaul driver is checking for C2 and C3 support in the first place -- as they are not directly related to the availability of P-states.

Because Longhaul isn't using P-states.

I guess I don't understand what longhaul.c is doing. Why is it using ACPI's C2 and C3 register addresses in order to do frequency/voltage scaling?

In the case there those addresses have been deemed invalid for the purpose of ACPI C-states, why are they still valid for longhaul?

-Len