[PATCH v6] cpufreq: intel_pstate: Reporting reasons why driver prematurely exit

[Erwan Velu <erwanaliasr1@gmail.com>]

The init code path has several exceptions where the module can decide not to load.
As CONFIG_X86_INTEL_PSTATE is generally set to Y, the return code is not reachable.
The initialization code is neither verbose of the reason why it did choose to prematurely exit.

This situation leads to a situation where its difficult for a user to determine,
on a given platform, why the driver didn't load properly.

This patch is about reporting to the user the reason/context of why the driver failed to load.
That is a precious hint when debugging a platform.

Signed-off-by: Erwan Velu <e.velu@criteo.com>
---
 drivers/cpufreq/intel_pstate.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index dd66decf2087..e1ae309923bf 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -2475,6 +2475,7 @@ static bool __init intel_pstate_no_acpi_pss(void)
 		kfree(pss);
 	}
 
+	pr_debug("ACPI _PSS not found\n");
 	return true;
 }
 
@@ -2484,10 +2485,15 @@ static bool __init intel_pstate_no_acpi_pcch(void)
 	acpi_handle handle;
 
 	status = acpi_get_handle(NULL, "\\_SB", &handle);
-	if (ACPI_FAILURE(status))
+	if (ACPI_FAILURE(status)) {
+		pr_debug("ACPI PCCH not found\n");
 		return true;
+	}
 
-	return !acpi_has_method(handle, "PCCH");
+	status = acpi_has_method(handle, "PCCH");
+	if (!status)
+		pr_debug("ACPI PCCH not found\n");
+	return !status;
 }
 
 static bool __init intel_pstate_has_acpi_ppc(void)
@@ -2502,6 +2508,7 @@ static bool __init intel_pstate_has_acpi_ppc(void)
 		if (acpi_has_method(pr->handle, "_PPC"))
 			return true;
 	}
+	pr_debug("ACPI _PPC not found\n");
 	return false;
 }
 
@@ -2539,8 +2546,10 @@ static bool __init intel_pstate_platform_pwr_mgmt_exists(void)
 	id = x86_match_cpu(intel_pstate_cpu_oob_ids);
 	if (id) {
 		rdmsrl(MSR_MISC_PWR_MGMT, misc_pwr);
-		if ( misc_pwr & (1 << 8))
+		if (misc_pwr & (1 << 8)) {
+			pr_debug("MSR_MISC_PWR_MGMT enabled\n");
 			return true;
+		}
 	}
 
 	idx = acpi_match_platform_list(plat_info);
@@ -2606,22 +2615,28 @@ static int __init intel_pstate_init(void)
 		}
 	} else {
 		id = x86_match_cpu(intel_pstate_cpu_ids);
-		if (!id)
+		if (!id) {
+			pr_info("CPU ID is not in the list of supported devices\n");
 			return -ENODEV;
+		}
 
 		copy_cpu_funcs((struct pstate_funcs *)id->driver_data);
 	}
 
-	if (intel_pstate_msrs_not_valid())
+	if (intel_pstate_msrs_not_valid()) {
+		pr_warn("Cannot enable driver as per invalid MSRs\n");
 		return -ENODEV;
+	}
 
 hwp_cpu_matched:
 	/*
 	 * The Intel pstate driver will be ignored if the platform
 	 * firmware has its own power management modes.
 	 */
-	if (intel_pstate_platform_pwr_mgmt_exists())
+	if (intel_pstate_platform_pwr_mgmt_exists()) {
+		pr_info("Platform already taking care of power management\n");
 		return -ENODEV;
+	}
 
 	if (!hwp_active && hwp_only)
 		return -ENOTSUPP;
-- 
2.20.1