powerpc/secvar: Handle format string in the consumer

The code that handles the format string in secvar-sysfs.c is entirely
OPAL specific, so create a new "format" op in secvar_operations to make
the secvar code more generic.  No functional change.
Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-8-ajd@linux.ibm.com

arch/powerpc/include/asm/secvar.h

@@ -17,6 +17,7 @@ struct secvar_operations {
 	int (*get)(const char *key, u64 key_len, u8 *data, u64 *data_size);
 	int (*get_next)(const char *key, u64 *key_len, u64 keybufsize);
 	int (*set)(const char *key, u64 key_len, u8 *data, u64 data_size);
+	ssize_t (*format)(char *buf, size_t bufsize);
 };
 
 #ifdef CONFIG_PPC_SECURE_BOOT

arch/powerpc/kernel/secvar-sysfs.c

@@ -21,26 +21,17 @@ static struct kset *secvar_kset;
 static ssize_t format_show(struct kobject *kobj, struct kobj_attribute *attr,
 			   char *buf)
 {
-	ssize_t rc = 0;
-	struct device_node *node;
-	const char *format;
-
-	node = of_find_compatible_node(NULL, NULL, "ibm,secvar-backend");
-	if (!of_device_is_available(node)) {
-		rc = -ENODEV;
-		goto out;
-	}
+	char tmp[32];
+	ssize_t len = secvar_ops->format(tmp, sizeof(tmp));
 
-	rc = of_property_read_string(node, "format", &format);
-	if (rc)
-		goto out;
+	if (len > 0)
+		return sysfs_emit(buf, "%s\n", tmp);
+	else if (len < 0)
+		pr_err("Error %zd reading format string\n", len);
+	else
+		pr_err("Got empty format string from backend\n");
 
-	rc = sysfs_emit(buf, "%s\n", format);
-
-out:
-	of_node_put(node);
-
-	return rc;
+	return -EIO;
 }
 
 

arch/powerpc/platforms/powernv/opal-secvar.c

@@ -98,10 +98,35 @@ static int opal_set_variable(const char *key, u64 ksize, u8 *data, u64 dsize)
 	return opal_status_to_err(rc);
 }
 
+static ssize_t opal_secvar_format(char *buf, size_t bufsize)
+{
+	ssize_t rc = 0;
+	struct device_node *node;
+	const char *format;
+
+	node = of_find_compatible_node(NULL, NULL, "ibm,secvar-backend");
+	if (!of_device_is_available(node)) {
+		rc = -ENODEV;
+		goto out;
+	}
+
+	rc = of_property_read_string(node, "format", &format);
+	if (rc)
+		goto out;
+
+	rc = snprintf(buf, bufsize, "%s", format);
+
+out:
+	of_node_put(node);
+
+	return rc;
+}
+
 static const struct secvar_operations opal_secvar_ops = {
 	.get = opal_get_variable,
 	.get_next = opal_get_next_variable,
 	.set = opal_set_variable,
+	.format = opal_secvar_format,
 };
 
 static int opal_secvar_probe(struct platform_device *pdev)