[PATCH 10/15] ACPI: CPPC: Reject reads and RMW 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 exported 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 bypass the
revision rule.

A partial SystemMemory field would also make cpc_write() read its complete
access unit to preserve bits outside the field.  Reject revision-4
descriptions of either write-only control when their geometry requires RMW.
A full-width description remains supported and is written without a
preceding read.

Fixes: 71e1815113f7 ("ACPI: CPPC: Add support for CPPC v4")
Signed-off-by: Christian Loehle <[email protected]>
---
 drivers/acpi/cppc_acpi.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 2bb7d74fc8e7..14f672392433 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -333,6 +333,13 @@ 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 bool cpc_sysmem_reg_needs_rmw(const struct cpc_register_resource *reg)
 {
 	const struct cpc_reg *gas = &reg->cpc_entry.reg;
@@ -363,6 +370,17 @@ static int cpc_validate_sysmem_reg(const struct cpc_desc *cpc_desc,
 	if (gas->address & (access_size - 1))
 		goto invalid;
 
+	if (cpc_reg_is_write_only(cpc_desc, reg_idx) &&
+	    (gas->bit_offset || gas->bit_width != access_width)) {
+		const char *name = reg_idx == DESIRED_PERF ?
+				   "Desired Performance" :
+				   "OSPM Nominal Performance";
+
+		pr_err("CPU%d: _CPC v%d %s register requires unsupported read-modify-write\n",
+		       cpc_desc->cpu_id, cpc_desc->version, name);
+		return -EINVAL;
+	}
+
 	return 0;
 
 invalid:
@@ -1737,6 +1755,8 @@ 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