[PATCH v4 10/15] ACPI: CPPC: Reject direct reads of write-only controls

Between _CPC revision 3 and revision 4, Desired Performance changed from
Read/Write to Write. Revision 4 also added the write-only OSPM Nominal
Performance control. ACPI 6.6 section 4.6.3 says reads of write-only bit
positions produce undefined results.

The public Desired Performance helper already rejects revision-4 readback,
but the common register accessor still permits either write-only control to
be read. Reject both centrally so new callers cannot consume undefined
values.

A partial SystemMemory field still makes cpc_write() read its complete
access unit to preserve bits outside the field. This is valid because
MASK_VAL_WRITE() replaces every bit of the write-only field, discarding its
undefined readback while preserving the surrounding bits. The descriptor
lock serializes the RMW, and the preceding SystemMemory validation rejects
unsafe cross-descriptor partial writers.

Mark an inaccessible OSPM Nominal Performance control unsupported because
it is optional. Do the same for inaccessible Desired Performance while
parsing, then let the post-parse control check accept it only for immutable
autonomous selection. This preserves the autonomous-only exception without
accepting an unusable Desired control in non-autonomous mode.

Fixes: 71e1815113f7 ("ACPI: CPPC: Add support for CPPC v4")
Reported-by: Sashiko <[email protected]>
Link: https://sashiko.dev/#/patchset/20260807111303.1062391-1-christian.loehle%40arm.com
Link: https://sashiko.dev/#/patchset/20260808082644.1251332-1-christian.loehle%40arm.com
Signed-off-by: Christian Loehle <[email protected]>
---
 drivers/acpi/cppc_acpi.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 61afb44d552c..a5920e627d00 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -333,6 +333,21 @@ static bool cpc_reg_is_writable(unsigned int reg_idx)
 	}
 }
 
+static bool cpc_reg_is_write_only(const struct cpc_desc *cpc_desc,
+				  unsigned int reg_idx)
+{
+	return cpc_desc->version >= CPPC_V4_REV &&
+	       (reg_idx == DESIRED_PERF || reg_idx == OSPM_NOMINAL_PERF);
+}
+
+static void cpc_disable_reg(struct cpc_desc *cpc_desc, unsigned int reg_idx)
+{
+	struct cpc_register_resource *reg = &cpc_desc->cpc_regs[reg_idx];
+
+	reg->type = ACPI_TYPE_INTEGER;
+	reg->cpc_entry.int_value = 0;
+}
+
 static bool cpc_sysmem_reg_needs_rmw(const struct cpc_register_resource *reg)
 {
 	const struct cpc_reg *gas = &reg->cpc_entry.reg;
@@ -1291,6 +1306,17 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
 					size_t access_width;
 
 					err = cpc_validate_sysmem_reg(cpc_ptr, gas_t, i - 2);
+					if (err && (i - 2 == DESIRED_PERF ||
+						    i - 2 == OSPM_NOMINAL_PERF)) {
+						const char *name = i - 2 == DESIRED_PERF ?
+								   "Desired Performance" :
+								   "OSPM Nominal Performance";
+
+						pr_warn("CPU%d: disabling inaccessible %s register\n",
+							pr->id, name);
+						cpc_disable_reg(cpc_ptr, i - 2);
+						continue;
+					}
 					if (err) {
 						ret = err;
 						goto out_free;
@@ -1684,6 +1710,10 @@ static int cpc_write(int cpu, struct cpc_register_resource *reg_res, u64 val)
 		}
 
 		if (reg->bit_offset || reg->bit_width != size) {
+			/*
+			 * MASK_VAL_WRITE() discards the field's old bits, so undefined
+			 * readback from a write-only field is not propagated.
+			 */
 			switch (size) {
 			case 8:
 				prev_val = readb_relaxed(vaddr);
@@ -1775,6 +1805,8 @@ static int cppc_get_reg_val(int cpu, enum cppc_regs reg_idx, u64 *val)
 		pr_debug("No CPC descriptor for CPU:%d\n", cpu);
 		return -ENODEV;
 	}
+	if (cpc_reg_is_write_only(cpc_desc, reg_idx))
+		return -EOPNOTSUPP;
 
 	reg = &cpc_desc->cpc_regs[reg_idx];
 
-- 
2.34.1