[PATCH 6.18.y] ACPI: CPPC: Reject desired_perf reads on _CPC revision 4+

[Sasha Levin <sashal@kernel.org>]

From: Christian Loehle <christian.loehle@arm.com>

[ Upstream commit df5a1d4a8cdfda20eb2581a85e81c7d436866534 ]

When CPPC feedback counters cannot provide a usable sample, cppc-cpufreq
calls cppc_get_desired_perf() because some platforms repurpose Desired
Performance to report actual delivered performance.

ACPI 6.5 defines _CPC revision 3 and lists Read/Write as the Optional
Attribute of Desired Performance. ACPI 6.6 advances _CPC to revision 4 and
lists only Write, so invoking that workaround for revision 4 or later would
require a register read that the interface no longer specifies.

Make cppc_get_desired_perf() return -EOPNOTSUPP for _CPC revision 4 or
later. Use the revision retained in the per-CPU CPC descriptor rather than
the platform-wide FADT revision.

The _CPC revision may still not accurately describe the implemented
register semantics. If a nominally revision 3 platform implements a
non-readable Desired Performance register, a read may return zero and make
cppc_cpufreq_get_rate() report 0 kHz. Treat a zero read as unusable and
fall back to the cached OSPM request, just as for a failed read.

Fixes: c47195631960 ("cppc_cpufreq: Use desired perf if feedback ctrs are 0 or unchanged")
Cc: stable@vger.kernel.org
Suggested-by: Sumit Gupta <sumitg@nvidia.com>
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/20260803203531.1268651-2-christian.loehle@arm.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
[ preserved the raw firmware revision while limiting parsing to the v3-sized register prefix ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/acpi/cppc_acpi.c       | 13 +++++++++++--
 drivers/cpufreq/cppc_cpufreq.c |  2 +-
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 60d2548f0eea4..531777557b47c 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -768,7 +768,6 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
 	}
 	if (cpc_rev > CPPC_V3_REV) {
 		num_ent = CPPC_V3_NUM_ENT;
-		cpc_rev = CPPC_V3_REV;
 	}
 
 	cpc_ptr->num_entries = num_ent;
@@ -1289,10 +1288,20 @@ static int cppc_set_reg_val(int cpu, enum cppc_regs reg_idx, u64 val)
  * @cpunum: CPU from which to get desired performance.
  * @desired_perf: Return address.
  *
- * Return: 0 for success, -EIO otherwise.
+ * Return: 0 for success, -EOPNOTSUPP for _CPC revision 4 or later, and a
+ * negative errno otherwise.
  */
 int cppc_get_desired_perf(int cpunum, u64 *desired_perf)
 {
+	struct cpc_desc *cpc_desc = per_cpu(cpc_desc_ptr, cpunum);
+
+	if (!cpc_desc)
+		return -ENODEV;
+
+	/* _CPC revision 4 no longer specifies Desired Performance as readable. */
+	if (cpc_desc->version > CPPC_V3_REV)
+		return -EOPNOTSUPP;
+
 	return cppc_get_reg_val(cpunum, DESIRED_PERF, desired_perf);
 }
 EXPORT_SYMBOL_GPL(cppc_get_desired_perf);
diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index e23d9abea1359..0330bc25fbe1a 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -766,7 +766,7 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
 	 * value first as some platforms may update the actual delivered perf
 	 * there; if failed, resort to the cached desired perf.
 	 */
-	if (cppc_get_desired_perf(cpu, &delivered_perf))
+	if (cppc_get_desired_perf(cpu, &delivered_perf) || !delivered_perf)
 		delivered_perf = cpu_data->perf_ctrls.desired_perf;
 
 	return cppc_perf_to_khz(&cpu_data->perf_caps, delivered_perf);
-- 
2.53.0