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Wysocki (Intel)" , Christian Loehle Cc: Viresh Kumar , linux-pm@vger.kernel.org, linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org, Jie Zhan , Lifeng Zheng , Pierre Gondois , Sudeep Holla , Ionela Voinescu , zhongqiu.han@oss.qualcomm.com, "linux-tegra@vger.kernel.org" References: <20260916162805.1039247-1-christian.loehle@arm.com> <20260916162805.1039247-16-christian.loehle@arm.com> <9ad67536-7b6e-4633-8cc7-52b0d68196fc@arm.com> Content-Language: en-US From: Sumit Gupta In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: PN3PEPF00000184.INDPRD01.PROD.OUTLOOK.COM (2603:1096:c04::4a) To DM4PR12MB5246.namprd12.prod.outlook.com (2603:10b6:5:399::17) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR12MB5246:EE_|MW4PR12MB6873:EE_ X-MS-Office365-Filtering-Correlation-Id: bdac6b2f-ac99-47a5-2d5d-08df14e85923 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|23010399003|376014|1800799024|7416014|366016|18002099003|4143699003|10067099003|22082099003|56012099006|5023799004|11063799006; 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Wysocki (Intel) wrote: > External email: Use caution opening links or attachments > > > On Thu, Sep 17, 2026 at 2:59 PM Christian Loehle > wrote: >> >> On 9/16/26 17:28, Christian Loehle wrote: >>> From: Sumit Gupta >>> >>> NVIDIA T41 firmware describes Performance Limited as a two-bit field at >>> offset zero in a DWord SystemMemory access unit. Generic CPPC code must >>> keep such a field read-only because preserving the remainder when clearing >>> it requires a read-modify-write which cannot be interlocked with platform >>> updates. >>> >>> The remaining bits of this access unit are unimplemented on T41: they read >>> as zero, writes have no side effects, and no other register uses them. The >>> Performance Limited register therefore owns the complete access unit, but >>> shipped firmware does not describe that property accurately. >>> >>> Add a CPPC platform-quirk table keyed by the DSDT header and carry quirk >>> behavior through explicit flags. Cache a successful table lookup, copy each >>> GAS into the driver's private descriptor, and apply fixups before layout >>> validation, mapping and overlap registration. >>> >>> Distinguish a genuine non-match from a table-header lookup failure in >>> acpi_match_platform_list(). Propagate lookup errors from CPPC probe without >>> caching them, so a transient mapping failure cannot disable the workaround >>> for every later processor. Existing matcher callers still treat all >>> negative results as no match. >>> >>> For the known T41 layout only, widen a two-bit Performance Limited field at >>> offset zero to its 32-bit access width. Clearing both status bits can then >>> be issued as one DWord write of zero without a stale read. Corrected >>> firmware which reports the full width is unchanged. The workaround >>> therefore lapses automatically when corrected firmware ships. >>> >>> Link: https://lore.kernel.org/lkml/d5f1ea9b-53b7-4db2-983a-b5be8e71a371@arm.com/ >>> Signed-off-by: Sumit Gupta >>> [ Rework quirk matching and fixup placement; propagate lookup failures >>> without caching them. ] >>> Signed-off-by: Christian Loehle >>> --- >>> drivers/acpi/cppc_acpi.c | 72 ++++++++++++++++++++++++++++++++++++++++ >>> drivers/acpi/utils.c | 13 ++++++-- >>> 2 files changed, 82 insertions(+), 3 deletions(-) >>> >>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c >>> index 6d130381245e..0e218f2be0fe 100644 >>> --- a/drivers/acpi/cppc_acpi.c >>> +++ b/drivers/acpi/cppc_acpi.c >>> @@ -330,6 +330,68 @@ static unsigned int cpc_reg_access_width(const struct cpc_reg *reg) >>> return reg->bit_width; >>> } >>> >>> +enum cpc_platform_quirk { >>> + CPC_QUIRK_PERF_LIMITED_OWNS_UNIT = BIT(0), >>> +}; >>> + >>> +static const struct acpi_platform_list cpc_platform_quirk_list[] = { >>> + { >>> + .oem_id = "NVIDIA", >>> + .oem_table_id = "T41", >> >> Sashiko: >> "Will this quirk successfully match a standard ACPI table header? >> The ACPI specification requires the OEM Table ID to be exactly 8 bytes long, >> typically padded with trailing spaces by compliant firmware (e.g., >> "T41 "). >> Looking at acpi_match_platform_list(), it compares the IDs using: >> strncmp(plat->oem_table_id, hdr.oem_table_id, ACPI_OEM_TABLE_ID_SIZE) >> Because "T41" is a null-terminated 3-character string, strncmp() will >> compare the 4th character ('\0' from the quirk definition vs ' ' from the >> ACPI table header) and immediately report a mismatch, causing the quirk to >> silently fail on compliant firmware. >> Should this be padded with spaces (e.g., "T41 ") to ensure it matches >> the firmware's table correctly?" >> >> non-padded "T41" matches exactly what Sumit proposed and was discussed in v6: >> https://lore.kernel.org/lkml/55a5c9fa-cfd3-4000-b3cc-52c343841c9f@nvidia.com/ >> So once Sumit adds Tested-by: this should be fine. > > Sure, thanks! > > Sumit, any concerns? Hi Rafael, No concerns. "T41" without space padding is correct because this firmware NUL pads the eight-byte field: # od -An -tc -j 16 -N 8 /sys/firmware/acpi/tables/DSDT T 4 1 \0 \0 \0 \0 \0 The quirk matched, producing the following message during boot: ] ACPI CPPC: firmware quirk: Performance Limited owns its access unit, using Bit Width 32 Thanks, Sumit