[PATCH] ACPI: CPPC: Gate desired performance getter on ACPI <6.6

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 6.6 changed the register's Optional Attribute from Read/Write to
Write. The existing cppc-cpufreq error path now falls back to its
cached OSPM request on ACPI >=6.6.
The original (and existing) workaround was for a specific HiSilicon
platform which isn't expected to report >=6.6.

Fixes: c47195631960 ("cppc_cpufreq: Use desired perf if feedback ctrs are 0 or unchanged")
---
 drivers/acpi/cppc_acpi.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 53d09ca98f06..218fdea09c1c 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -1321,10 +1321,17 @@ 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 ACPI 6.6 or later, and a negative
+ * errno otherwise.
  */
 int cppc_get_desired_perf(int cpunum, u64 *desired_perf)
 {
+	/* ACPI 6.6 no longer specifies Desired Performance as readable. */
+	if (acpi_gbl_FADT.header.revision > 6 ||
+	    (acpi_gbl_FADT.header.revision == 6 &&
+	     acpi_gbl_FADT.minor_revision >= 6))
+		return -EOPNOTSUPP;
+
 	return cppc_get_reg_val(cpunum, DESIRED_PERF, desired_perf);
 }
 EXPORT_SYMBOL_GPL(cppc_get_desired_perf);
-- 
2.34.1