[PATCH v2 1/2] hw/core: Support device reset handler priority

[Stephanos Ioannidis <root@stephanos.io>]

The device reset handler invocation order is currently dependent on
the order of handler registration, and this is less than ideal because
there may exist dependencies among the handlers that require them to
be invoked in a specific order.

This commit adds the `priority` field to the reset entry struct and
introduces the `qemu_register_reset_with_priority` function that
implements descending-order insertion into the reset handler list based
on the priority value, in order to allow the reset handler invocation
order to be specified by the caller.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
---
 hw/core/reset.c        | 32 ++++++++++++++++++++++++++++++--
 include/sysemu/reset.h | 24 ++++++++++++++++++++++++
 2 files changed, 54 insertions(+), 2 deletions(-)

diff --git a/hw/core/reset.c b/hw/core/reset.c
index 9c477f2bf5..74f3677d96 100644
--- a/hw/core/reset.c
+++ b/hw/core/reset.c
@@ -31,6 +31,7 @@
 
 typedef struct QEMUResetEntry {
     QTAILQ_ENTRY(QEMUResetEntry) entry;
+    uint16_t priority;
     QEMUResetHandler *func;
     void *opaque;
 } QEMUResetEntry;
@@ -38,13 +39,40 @@ typedef struct QEMUResetEntry {
 static QTAILQ_HEAD(, QEMUResetEntry) reset_handlers =
     QTAILQ_HEAD_INITIALIZER(reset_handlers);
 
-void qemu_register_reset(QEMUResetHandler *func, void *opaque)
+void qemu_register_reset_with_priority(uint16_t priority,
+    QEMUResetHandler *func, void *opaque)
 {
+    /* Initialise reset entry */
     QEMUResetEntry *re = g_malloc0(sizeof(QEMUResetEntry));
 
+    re->priority = priority;
     re->func = func;
     re->opaque = opaque;
-    QTAILQ_INSERT_TAIL(&reset_handlers, re, entry);
+
+    /* Insert sorted by priority into the reset entry list */
+    if (QTAILQ_EMPTY(&reset_handlers) ||
+        QTAILQ_LAST(&reset_handlers)->priority >= priority)
+    {
+        QTAILQ_INSERT_TAIL(&reset_handlers, re, entry);
+    }
+    else
+    {
+        QEMUResetEntry *cur = QTAILQ_LAST(&reset_handlers);
+
+        while (QTAILQ_PREV(cur, entry) != NULL &&
+               QTAILQ_PREV(cur, entry)->priority < priority)
+        {
+            cur = QTAILQ_PREV(cur, entry);
+        }
+
+        QTAILQ_INSERT_BEFORE(cur, re, entry);
+    }
+}
+
+void qemu_register_reset(QEMUResetHandler *func, void *opaque)
+{
+    qemu_register_reset_with_priority(
+        QEMU_RESET_PRIORITY_DEFAULT, func, opaque);
 }
 
 void qemu_unregister_reset(QEMUResetHandler *func, void *opaque)
diff --git a/include/sysemu/reset.h b/include/sysemu/reset.h
index 0b0d6d7598..39a0fe55f0 100644
--- a/include/sysemu/reset.h
+++ b/include/sysemu/reset.h
@@ -1,8 +1,32 @@
 #ifndef QEMU_SYSEMU_RESET_H
 #define QEMU_SYSEMU_RESET_H
 
+/*
+ * The device reset handlers are executed in descending order of their priority
+ * values (i.e. the reset handler with the greatest numerical priority value
+ * will be executed first).
+ */
+#define QEMU_RESET_PRIORITY_DEFAULT     (0x8000)
+
+/*
+ * The priority level can range from -8 to +7, where the reset handler with the
+ * highest priority level is executed first.
+ */
+#define QEMU_RESET_PRIORITY_LEVEL(l) \
+    (QEMU_RESET_PRIORITY_DEFAULT + (l * 0x1000))
+
+/*
+ * Each priority level may be further divided into a maximum of 4096 sub-levels
+ * (0 to 4095).
+ */
+#define QEMU_RESET_PRIORITY_SUBLEVEL(l, s) \
+    (QEMU_RESET_PRIORITY_DEFAULT + (l * 0x1000) + s)
+
 typedef void QEMUResetHandler(void *opaque);
 
+void qemu_register_reset_with_priority(uint16_t priority,
+    QEMUResetHandler *func, void *opaque);
+
 void qemu_register_reset(QEMUResetHandler *func, void *opaque);
 void qemu_unregister_reset(QEMUResetHandler *func, void *opaque);
 void qemu_devices_reset(void);
-- 
2.17.1