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[67.165.232.89]) by smtp.gmail.com with ESMTPSA id e22sm16141388iog.2.2019.08.13.15.26.39 (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Tue, 13 Aug 2019 15:26:39 -0700 (PDT) Reply-To: ahs3@redhat.com Subject: Re: [PATCH] ACPI / CPPC: do not require the _PSD method when using CPPC To: "Rafael J. Wysocki" Cc: linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org, Len Brown References: <20190805170338.29493-1-ahs3@redhat.com> <521915646.RcUJINxfhL@kreacher> From: Al Stone Organization: Red Hat, Inc. Message-ID: Date: Tue, 13 Aug 2019 16:26:38 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <521915646.RcUJINxfhL@kreacher> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-acpi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org On 8/13/19 3:59 PM, Rafael J. Wysocki wrote: > On Monday, August 5, 2019 7:03:38 PM CEST Al Stone wrote: >> According to the ACPI 6.3 specification, the _PSD method is optional >> when using CPPC. The underlying assumption appears to be 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 essentially 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. >> >> Signed-off-by: Al Stone >> Cc: Rafael J. Wysocki >> Cc: Len Brown >> --- >> drivers/acpi/cppc_acpi.c | 11 +++++++---- >> 1 file changed, 7 insertions(+), 4 deletions(-) >> >> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c >> index 15f103d7532b..e9ecfa13e997 100644 >> --- a/drivers/acpi/cppc_acpi.c >> +++ b/drivers/acpi/cppc_acpi.c >> @@ -365,10 +365,13 @@ 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); >> - if (ACPI_FAILURE(status)) >> - return -ENODEV; >> + if (acpi_has_method(handle, "_PSD")) { > > It would be better to compare the status below to AE_NOT_FOUND > and return 0 if that's the case. > > A couple of code lines could be saved this way at least. D'oh. Good point. Let me dig back through the ACPICA code again; I had some reason for not relying on AE_NOT_FOUND alone that I apparently did not write down in my notes. I'll send out a v2 when I figure out what it was, and if it was of any consequence. >> + status = acpi_evaluate_object_typed(handle, "_PSD", NULL, >> + &buffer, ACPI_TYPE_PACKAGE); >> + if (ACPI_FAILURE(status)) >> + return -ENODEV; >> + } else >> + return 0; /* _PSD is optional */ >> >> psd = buffer.pointer; >> if (!psd || psd->package.count != 1) { >> Thanks. -- ciao, al ----------------------------------- Al Stone Software Engineer Red Hat, Inc. ahs3@redhat.com -----------------------------------