firmware/dmi_scan: Uninline dmi_get_bios_year() helper

Uninline dmi_get_bios_year() which, in particular, allows us
to optimize it in the future.

While doing this, convert the function to return an error code
when BIOS date is not present or not parsable, or CONFIG_DMI=n.

Additionally, during the move, add a bit of documentation.

Suggested-by: Bjorn Helgaas <[e-mail]>
Suggested-by: Rafael J. Wysocki <[e-mail]>
Signed-off-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Jean Delvare <[e-mail]>
Reviewed-by: Rafael J. Wysocki <[e-mail]>
Acked-by: Thomas Gleixner <[e-mail]>
Cc: Bjorn Helgaas <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Lukas Wunner <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Rafael J . Wysocki <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Fixes: 492a1abd61e4 ("dmi: Introduce the dmi_get_bios_year() helper function")
Signed-off-by: Ingo Molnar <[e-mail]>

drivers/firmware/dmi_scan.c

···
 EXPORT_SYMBOL(dmi_get_date);
 
 /**
+ *	dmi_get_bios_year - get a year out of DMI_BIOS_DATE field
+ *
+ *	Returns year on success, -ENXIO if DMI is not selected,
+ *	or a different negative error code if DMI field is not present
+ *	or not parseable.
+ */
+int dmi_get_bios_year(void)
+{
+	bool exists;
+	int year;
+
+	exists = dmi_get_date(DMI_BIOS_DATE, &year, NULL, NULL);
+	if (!exists)
+		return -ENODATA;
+
+	return year ? year : -ERANGE;
+}
+EXPORT_SYMBOL(dmi_get_bios_year);
+
+/**
  *	dmi_walk - Walk the DMI table and get called back for every record
  *	@decode: Callback function
  *	@private_data: Private data to be passed to the callback function

include/linux/dmi.h

···
 extern void dmi_memdev_walk(void);
 extern void dmi_set_dump_stack_arch_desc(void);
 extern bool dmi_get_date(int field, int *yearp, int *monthp, int *dayp);
+extern int dmi_get_bios_year(void);
 extern int dmi_name_in_vendors(const char *str);
 extern int dmi_name_in_serial(const char *str);
 extern int dmi_available;
···
 		*dayp = 0;
 	return false;
 }
+static inline int dmi_get_bios_year(void) { return -ENXIO; }
 static inline int dmi_name_in_vendors(const char *s) { return 0; }
 static inline int dmi_name_in_serial(const char *s) { return 0; }
 #define dmi_available 0
···
 	dmi_first_match(const struct dmi_system_id *list) { return NULL; }
 
 #endif
-
-static inline int dmi_get_bios_year(void)
-{
-	int year;
-
-	dmi_get_date(DMI_BIOS_DATE, &year, NULL, NULL);
-
-	return year;
-}
 
 #endif	/* __DMI_H__ */