[settings] Formalise notion of setting applicability

Expose a function setting_applies() to allow a caller to determine
whether or not a particular setting is applicable to a particular
settings block.

Restrict DHCP-backed settings blocks to accepting only DHCP-based
settings.

Restrict network device settings blocks to accepting only DHCP-based
settings and network device-specific settings such as "mac".

Inspired-by: Glenn Brown <glenn@myri.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/80211/net80211.c

@@ -206,6 +206,7 @@ struct setting net80211_ssid_setting __setting = {
 	.name = "ssid",
 	.description = "802.11 SSID (network name)",
 	.type = &setting_type_string,
+	.tag = NET80211_SETTING_TAG_SSID,
 };
 
 /** Whether to use active scanning
@@ -218,6 +219,7 @@ struct setting net80211_active_setting __setting = {
 	.name = "active-scan",
 	.description = "Use an active scan during 802.11 association",
 	.type = &setting_type_int8,
+	.tag = NET80211_SETTING_TAG_ACTIVE_SCAN,
 };
 
 /** The cryptographic key to use
@@ -230,6 +232,7 @@ struct setting net80211_key_setting __setting = {
 	.name = "key",
 	.description = "Encryption key for protected 802.11 networks",
 	.type = &setting_type_string,
+	.tag = NET80211_SETTING_TAG_KEY,
 };
 
 /** @} */

src/net/dhcpopts.c

@@ -347,6 +347,18 @@ static int set_dhcp_option ( struct dhcp_options *options, unsigned int tag,
 	return offset;
 }
 
+/**
+ * Check applicability of DHCP option setting
+ *
+ * @v tag		Setting tag number
+ * @ret applies		Setting applies to this option block
+ */
+int dhcpopt_applies ( unsigned int tag ) {
+
+	return ( tag && ( tag <= DHCP_ENCAP_OPT ( DHCP_MAX_OPTION,
+						  DHCP_MAX_OPTION ) ) );
+}
+
 /**
  * Store value of DHCP option setting
  *

src/net/dhcppkt.c

@@ -134,6 +134,19 @@ find_dhcp_packet_field ( unsigned int tag ) {
 	return NULL;
 }
 
+/**
+ * Check applicability of DHCP setting
+ *
+ * @v dhcppkt		DHCP packet
+ * @v tag		Setting tag number
+ * @ret applies		Setting applies within this settings block
+ */
+static int dhcppkt_applies ( struct dhcp_packet *dhcppkt __unused,
+			     unsigned int tag ) {
+
+	return dhcpopt_applies ( tag );
+}
+
 /**
  * Store value of DHCP packet setting
  *
@@ -204,6 +217,21 @@ int dhcppkt_fetch ( struct dhcp_packet *dhcppkt, unsigned int tag,
  *
  */
 
+/**
+ * Check applicability of DHCP setting
+ *
+ * @v settings		Settings block
+ * @v setting		Setting
+ * @ret applies		Setting applies within this settings block
+ */
+static int dhcppkt_settings_applies ( struct settings *settings,
+				      struct setting *setting ) {
+	struct dhcp_packet *dhcppkt =
+		container_of ( settings, struct dhcp_packet, settings );
+
+	return dhcppkt_applies ( dhcppkt, setting->tag );
+}
+
 /**
  * Store value of DHCP setting
  *
@@ -242,6 +270,7 @@ static int dhcppkt_settings_fetch ( struct settings *settings,
 
 /** DHCP settings operations */
 static struct settings_operations dhcppkt_settings_operations = {
+	.applies = dhcppkt_settings_applies,
 	.store = dhcppkt_settings_store,
 	.fetch = dhcppkt_settings_fetch,
 };

src/net/netdev_settings.c

@@ -22,6 +22,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #include <errno.h>
 #include <byteswap.h>
 #include <ipxe/dhcp.h>
+#include <ipxe/dhcpopts.h>
 #include <ipxe/settings.h>
 #include <ipxe/device.h>
 #include <ipxe/netdevice.h>
@@ -37,13 +38,29 @@ struct setting mac_setting __setting = {
 	.name = "mac",
 	.description = "MAC address",
 	.type = &setting_type_hex,
+	.tag = NETDEV_SETTING_TAG_MAC,
 };
 struct setting busid_setting __setting = {
 	.name = "busid",
 	.description = "Bus ID",
 	.type = &setting_type_hex,
+	.tag = NETDEV_SETTING_TAG_BUS_ID,
 };
 
+/**
+ * Check applicability of network device setting
+ *
+ * @v settings		Settings block
+ * @v setting		Setting
+ * @ret applies		Setting applies within this settings block
+ */
+static int netdev_applies ( struct settings *settings __unused,
+			    struct setting *setting ) {
+
+	return ( IS_NETDEV_SETTING_TAG ( setting->tag ) ||
+		 dhcpopt_applies ( setting->tag ) );
+}
+
 /**
  * Store value of network device setting
  *
@@ -114,6 +131,7 @@ static void netdev_clear ( struct settings *settings ) {
 
 /** Network device configuration settings operations */
 struct settings_operations netdev_settings_operations = {
+	.applies = netdev_applies,
 	.store = netdev_store,
 	.fetch = netdev_fetch,
 	.clear = netdev_clear,