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From:   Al Stone <ahs3@redhat.com>
To:     linux-acpi@vger.kernel.org
Cc:     Al Stone <ahs3@redhat.com>, linux-kernel@vger.kernel.org
Subject: [PATCH v3] ACPI / CPPC: do not require the _PSD method when using CPPC
Date:   Tue, 27 Aug 2019 18:21:20 -0600
Message-Id: <20190828002120.13669-1-ahs3@redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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According to the ACPI 6.3 specification, the _PSD method is optional
when using CPPC.  The underlying assumption is that each CPU can change
frequency independently from all other CPUs; _PSD is provided to tell
the OS that some processors can NOT do that.

However, the acpi_get_psd() function returns ENODEV if there is no _PSD
method present, or an ACPI error status if an error occurs when evaluating
_PSD, if present.  This makes _PSD mandatory when using CPPC, in violation
of the specification, and only on Linux.

This has forced some firmware writers to provide a dummy _PSD, even though
it is irrelevant, but only because Linux requires it; other OSPMs follow
the spec.  We really do not want to have OS specific ACPI tables, though.

So, correct acpi_get_psd() so that it does not return an error if there
is no _PSD method present, but does return a failure when the method can
not be executed properly.  This allows _PSD to be optional as it should
be.

v3:
   -- bodged the simplification in v2; simplified it properly this time
   -- verified all error statuses are handled

v2:
   -- verified simple check for AE_NOT_FOUND was sufficient
   -- simplified return status check per Rafael's suggestion

Signed-off-by: Al Stone <ahs3@redhat.com>
---
 drivers/acpi/cppc_acpi.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 15f103d7532b..3b2525908dd8 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -365,8 +365,10 @@ static int acpi_get_psd(struct cpc_desc *cpc_ptr, acpi_handle handle)
 	union acpi_object  *psd = NULL;
 	struct acpi_psd_package *pdomain;
 
-	status = acpi_evaluate_object_typed(handle, "_PSD", NULL, &buffer,
-			ACPI_TYPE_PACKAGE);
+	status = acpi_evaluate_object_typed(handle, "_PSD", NULL,
+					    &buffer, ACPI_TYPE_PACKAGE);
+	if (status == AE_NOT_FOUND)	/* _PSD is optional */
+		return 0;
 	if (ACPI_FAILURE(status))
 		return -ENODEV;
 
-- 
2.21.0