ACPI: CPPC: Optimize cppc_get_perf()

Optimize cppc_get_perf() with three changes:

 1. Change the error kind to "no such device" when pcc_ss_id < 0, as
    other register value getting functions.

 2. Add a check to ensure the pointer 'perf' is no null.

 3. Add a check to verify if the register is supported to be read before
    using it. The logic is:

    (1) If the register is of the integer type, check whether the
        register is optional and its value is 0. If yes, the register
        is not supported.

    (2) If the register is of other types, a null one is not supported.

 4. Return the result of cpc_read() instead of 0.

Reviewed-by: Pierre Gondois <[email protected]>
Signed-off-by: Lifeng Zheng <[email protected]>
Reviewed-by: Mario Limonciello <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Lifeng Zheng

drivers/acpi/cppc_acpi.c

@@ -1184,27 +1184,39 @@ static int cppc_get_perf(int cpunum, enum cppc_regs reg_idx, u64 *perf)
 	struct cpc_desc *cpc_desc = per_cpu(cpc_desc_ptr, cpunum);
 	struct cpc_register_resource *reg;
 
+	if (perf == NULL)
+		return -EINVAL;
+
 	if (!cpc_desc) {
 		pr_debug("No CPC descriptor for CPU:%d\n", cpunum);
 		return -ENODEV;
 	}
 
 	reg = &cpc_desc->cpc_regs[reg_idx];
 
+	if ((reg->type == ACPI_TYPE_INTEGER && IS_OPTIONAL_CPC_REG(reg_idx) &&
+	     !reg->cpc_entry.int_value) || (reg->type != ACPI_TYPE_INTEGER &&
+	     IS_NULL_REG(&reg->cpc_entry.reg))) {
+		pr_debug("CPC register is not supported\n");
+		return -EOPNOTSUPP;
+	}
+
 	if (CPC_IN_PCC(reg)) {
 		int pcc_ss_id = per_cpu(cpu_pcc_subspace_idx, cpunum);
 		struct cppc_pcc_data *pcc_ss_data = NULL;
-		int ret = 0;
+		int ret;
 
-		if (pcc_ss_id < 0)
-			return -EIO;
+		if (pcc_ss_id < 0) {
+			pr_debug("Invalid pcc_ss_id\n");
+			return -ENODEV;
+		}
 
 		pcc_ss_data = pcc_data[pcc_ss_id];
 
 		down_write(&pcc_ss_data->pcc_lock);
 
 		if (send_pcc_cmd(pcc_ss_id, CMD_READ) >= 0)
-			cpc_read(cpunum, reg, perf);
+			ret = cpc_read(cpunum, reg, perf);
 		else
 			ret = -EIO;
 
@@ -1213,9 +1225,7 @@ static int cppc_get_perf(int cpunum, enum cppc_regs reg_idx, u64 *perf)
 		return ret;
 	}
 
-	cpc_read(cpunum, reg, perf);
-
-	return 0;
+	return cpc_read(cpunum, reg, perf);
 }
 
 /**