drivers: construct driver list at runtime

Relying on linker politeness could fail when compiled with -flto :
https://sigrok.org/bugzilla/show_bug.cgi?id=1433

Instead of using a special section that holds a pointer to each of the
driver descriptors, use "__attribute__((constructor))" to run a certain
function at runtime just before main() entry.
That is also before glib init and possibly (unclear) before heap is
usable, so it may not be possible to directly build a GArray or GSList.
A simple linked-list provides the intermediarey step, and
sr_drivers_init() still builds the final driver list via a GArray.

This compiler attribute is well-supported by gcc and clang.

The SR_REGISTER_DEV_DRIVER_LIST() implementation is also not very
elegant, but is a drop-in replacement.

Author: fenugrec

Makefile.am

@@ -216,22 +216,16 @@ libsigrok_la_SOURCES += \
 	src/scale/kern.c
 
 # Hardware drivers
-noinst_LTLIBRARIES = src/libdrivers.la \
-	src/libdrivers_head.la src/libdrivers_tail.la
+noinst_LTLIBRARIES = src/libdrivers.la
+
+src/libdrivers.o: src/libdrivers.la
+	$(AM_V_CCLD)$(LINK) src/libdrivers.la
 
-src/libdrivers.o: src/libdrivers.la \
-		src/libdrivers_head.la src/libdrivers_tail.la
-	$(AM_V_CCLD)$(LINK) src/libdrivers_head.la src/libdrivers.la \
-		src/libdrivers_tail.la
 src/libdrivers.lo: src/libdrivers.o
 	$(AM_V_GEN)echo "# Generated by libtool" > $@
 	$(AM_V_at)echo "pic_object='libdrivers.o'" >> $@
 	$(AM_V_at)echo "non_pic_object='libdrivers.o'" >> $@
 
-src_libdrivers_head_la_SOURCES = src/driver_list_start.c
-
-src_libdrivers_tail_la_SOURCES = src/driver_list_stop.c
-
 src_libdrivers_la_SOURCES = src/drivers.c
 
 if HW_AGILENT_DMM

src/driver_list_start.c

@@ -4,6 +4,7 @@
  * Copyright (C) 2016 Lars-Peter Clausen <[email protected]>
  * Copyright (C) 2016 Aurelien Jacobs <[email protected]>
  * Copyright (C) 2017 Marcus Comstedt <[email protected]>
+ * Copyright (C) 2023 fenugrec <[email protected]>
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -24,16 +25,28 @@
 #include <libsigrok/libsigrok.h>
 #include "libsigrok-internal.h"
 
-/*
- * The special __sr_driver_list section contains pointers to all hardware
- * drivers which were built into the library according to its configuration
- * (will depend on the availability of dependencies, as well as user provided
- * specs). The __start and __stop symbols point to the start and end of the
- * section. They are used to iterate over the list of all drivers which were
- * included in the library.
+static struct device_node *devlist_head = NULL;
+
+/* constructors will call this, before main() and GLib init.
+ * We only assume static variables were initialized (i.e. contents of .bss
+ * and .data sections).
  */
-SR_PRIV extern const struct sr_dev_driver *sr_driver_list__start[];
-SR_PRIV extern const struct sr_dev_driver *sr_driver_list__stop[];
+void sr_register_dev_node(struct device_node *devnode) {
+	devnode->next = devlist_head;
+	devlist_head = devnode;
+}
+
+void sr_register_dev_array(struct sr_dev_driver *driver_array[], struct device_node *node_array, unsigned num) {
+	unsigned i;
+	struct device_node *dnode;
+
+	for (i = 0; i < num; i++) {
+		dnode = &node_array[i];
+		dnode->dev = driver_array[i];
+		sr_register_dev_node(dnode);
+	}
+}
+
 
 /**
  * Initialize the driver list in a fresh libsigrok context.
@@ -48,9 +61,9 @@ SR_API void sr_drivers_init(struct sr_context *ctx)
 
 	array = g_array_new(TRUE, FALSE, sizeof(struct sr_dev_driver *));
 #ifdef HAVE_DRIVERS
-	for (const struct sr_dev_driver **drivers = sr_driver_list__start + 1;
-	     drivers < sr_driver_list__stop; drivers++)
-		g_array_append_val(array, *drivers);
+	for (struct device_node *cur = devlist_head; cur; cur = cur->next) {
+		g_array_append_val(array, cur->dev);
+	}
 #endif
 	ctx->driver_list = (struct sr_dev_driver **)g_array_free(array, FALSE);
 }

src/driver_list_stop.c

@@ -1127,20 +1127,28 @@ static inline void write_dblle_inc(uint8_t **p, double x)
 #define ALL_ZERO { 0 }
 #endif
 
-#ifdef __APPLE__
-#define SR_DRIVER_LIST_SECTION "__DATA,__sr_driver_list"
-#else
-#define SR_DRIVER_LIST_SECTION "__sr_driver_list"
-#endif
 
-#if !defined SR_DRIVER_LIST_NOREORDER && defined __has_attribute
-#if __has_attribute(no_reorder)
-#define SR_DRIVER_LIST_NOREORDER __attribute__((no_reorder))
-#endif
-#endif
-#if !defined SR_DRIVER_LIST_NOREORDER
-#define SR_DRIVER_LIST_NOREORDER /* EMPTY */
-#endif
+/* constructors will assemble a LL with every driver.
+ * Every driver defines one of these struct by using one of the
+ * SR_REGISTER_DEV_DRIVER_* macros below.
+ * This could be greatly simplified if struct sr_dev_driver had a '->next' member. Maybe someday.
+*/
+struct device_node {
+	struct sr_dev_driver *dev;
+	struct device_node *next;
+};
+
+/** should only be called via SR_REGISTER_DEV_DRIVER_LIST() macro */
+void sr_register_dev_array(struct sr_dev_driver *driver_array[], struct device_node *node_array, unsigned num);
+
+/** should only be called via SR_REGISTER_DEV_DRIVER() macro */
+void sr_register_dev_node(struct device_node *devnode);
+
+
+/* glib uses this internally but doesn't seem to make it available.
+ * This is a much simplified def that will only work on gcc/clang.
+ */
+#define G_DEFINE_CONSTRUCTOR(_func) static void __attribute__((constructor)) _func (void);
 
 /**
  * Register a list of hardware drivers.
@@ -1170,13 +1178,14 @@ static inline void write_dblle_inc(uint8_t **p, double x)
  * @param ... Comma separated list of pointers to sr_dev_driver structs.
  */
 #define SR_REGISTER_DEV_DRIVER_LIST(name, ...) \
-	static const struct sr_dev_driver *name[] \
-		SR_DRIVER_LIST_NOREORDER \
-		__attribute__((section (SR_DRIVER_LIST_SECTION), used, \
-			aligned(sizeof(struct sr_dev_driver *)))) \
-		= { \
-			__VA_ARGS__ \
-		};
+	static struct sr_dev_driver *name[] = { \
+		__VA_ARGS__ \
+	}; \
+	static struct device_node name##_nodes[ARRAY_SIZE(name)]; \
+	G_DEFINE_CONSTRUCTOR(register_##name) \
+	static void register_##name(void) { \
+		sr_register_dev_array(name, name##_nodes, ARRAY_SIZE(name)); \
+	}
 
 /**
  * Register a hardware driver.
@@ -1200,7 +1209,11 @@ static inline void write_dblle_inc(uint8_t **p, double x)
  * @param name Identifier name of sr_dev_driver struct to register.
  */
 #define SR_REGISTER_DEV_DRIVER(name) \
-	SR_REGISTER_DEV_DRIVER_LIST(name##_list, &name);
+	static struct device_node name##_node = { .dev = &name }; \
+	G_DEFINE_CONSTRUCTOR(register_##name) \
+	static void register_##name(void) { \
+		sr_register_dev_node(&name##_node); \
+	}
 
 SR_API void sr_drivers_init(struct sr_context *context);