[PATCH v4] cpufreq: intel_pstate: Reporting reasons why driver prematurely exit

[Erwan Velu]

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@xxxxxxxxxx>
---
 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..eb62e5555dcc 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("Cannot detect ACPI PSS");
 	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("Cannot detect ACPI SB");
 		return true;
+	}
 
-	return !acpi_has_method(handle, "PCCH");
+	status = acpi_has_method(handle, "PCCH");
+	if (!status)
+		pr_debug("Cannot detect ACPI PCCH");
+	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("Cannot detect ACPI PPC");
 	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 reports enabled HW coordination");
 			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_warn("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_warn("Platform already taking care of power management\n");
 		return -ENODEV;
+	}
 
 	if (!hwp_active && hwp_only)
 		return -ENOTSUPP;
-- 
2.20.1