rtos/FreeRTOS: fix next pointer member offset in FreeRTOS lists

Currently, we are using offset of xListEnd.pxPrevious in List_t for
list_next_offset and offset of pxPrevious in ListItem_t for
list_elem_next_offset. This is confusing. Fix this.
As the related lists are doubly linked lists, only iteration order is
changed without breaking functionality.
Also document those offsets.

Change-Id: I8beacc235ee781ab4e3b415fccad7b72ec55b098
Signed-off-by: Chien Wong <m@xv97.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8833
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

Author: ivq

src/rtos/freertos.c

@@ -30,12 +30,12 @@ struct freertos_params {
 	const char *target_name;
 	const unsigned char thread_count_width;
 	const unsigned char pointer_width;
-	const unsigned char list_next_offset;
-	const unsigned char list_width;
-	const unsigned char list_elem_next_offset;
-	const unsigned char list_elem_content_offset;
-	const unsigned char thread_stack_offset;
-	const unsigned char thread_name_offset;
+	const unsigned char list_next_offset;			/* offsetof(List_t, xListEnd.pxNext) */
+	const unsigned char list_width;					/* sizeof(List_t) */
+	const unsigned char list_elem_next_offset;		/* offsetof(ListItem_t, pxNext) */
+	const unsigned char list_elem_content_offset;	/* offsetof(ListItem_t, pvOwner) */
+	const unsigned char thread_stack_offset;		/* offsetof(TCB_t, pxTopOfStack) */
+	const unsigned char thread_name_offset;			/* offsetof(TCB_t, pcTaskName) */
 	const struct rtos_register_stacking *stacking_info_cm3;
 	const struct rtos_register_stacking *stacking_info_cm4f;
 	const struct rtos_register_stacking *stacking_info_cm4f_fpu;
@@ -46,9 +46,9 @@ static const struct freertos_params freertos_params_list[] = {
 	"cortex_m",			/* target_name */
 	4,						/* thread_count_width; */
 	4,						/* pointer_width; */
-	16,						/* list_next_offset; */
+	12,						/* list_next_offset; */
 	20,						/* list_width; */
-	8,						/* list_elem_next_offset; */
+	4,						/* list_elem_next_offset; */
 	12,						/* list_elem_content_offset */
 	0,						/* thread_stack_offset; */
 	52,						/* thread_name_offset; */
@@ -60,9 +60,9 @@ static const struct freertos_params freertos_params_list[] = {
 	"hla_target",			/* target_name */
 	4,						/* thread_count_width; */
 	4,						/* pointer_width; */
-	16,						/* list_next_offset; */
+	12,						/* list_next_offset; */
 	20,						/* list_width; */
-	8,						/* list_elem_next_offset; */
+	4,						/* list_elem_next_offset; */
 	12,						/* list_elem_content_offset */
 	0,						/* thread_stack_offset; */
 	52,						/* thread_name_offset; */