[PATCH 1/3] ACPI: CPPC: Reject desired_perf reads on _CPC revision 4+

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: [email protected]
Suggested-by: Sumit Gupta <[email protected]>
Signed-off-by: Christian Loehle <[email protected]>
---
 drivers/acpi/cppc_acpi.c       | 17 ++++++++++++++++-
 drivers/cpufreq/cppc_cpufreq.c |  2 +-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 53d09ca98f06..42aeb749ebe0 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -1316,15 +1316,30 @@ static int cppc_set_reg_val(int cpu, enum cppc_regs reg_idx, u64 val)
 	return cpc_write(cpu, reg, val);
 }
 
+static bool cppc_desired_perf_readable(const struct cpc_desc *cpc_desc)
+{
+	return cpc_desc->version < CPPC_V4_REV;
+}
+
 /**
  * cppc_get_desired_perf - Get the desired performance register value.
  * @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 (!cppc_desired_perf_readable(cpc_desc))
+		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 6fe0e972952a..80893844353c 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -836,7 +836,7 @@ 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);
-- 
2.34.1