display_task: Pre-ack render commands

The drop-oldest path in display_task_consume_mailbox can silently
drop an update message to make room for a newer one (rendering a
stale frame is wasted when a fresher one is queued). The dropped
caller was left without a reply and timed out at gen_server:call
— an ugly failure mode for a frame-loss-tolerant design.

Send {Ref, ok} on arrival, before the enqueue-or-drop decision.
Drivers skip the reply on the update path so the caller never
sees a duplicate. draw_buffer gets the same treatment: it is
gen_call-shaped but fire-and-forget at the driver, today timing
out the caller.

register_font and load_image still reply synchronously: they
need to convey the operation outcome (registered handle /
decoded binary) and do not share the drop-for-newer rationale.

Author: bettio

dcs_lcd_display_driver.c

@@ -223,6 +223,7 @@ static void process_message(Message *message, Context *ctx)
                                       "update")) {
         term display_list = term_get_tuple_element(req, 1);
         do_update(ctx, display_list);
+        return;
 
     } else if (cmd == context_make_atom(ctx, "\xB"
                                              "draw_buffer")) {
@@ -237,7 +238,8 @@ static void process_message(Message *message, Context *ctx)
 
         dcs_lcd_draw_buffer(&driver->bus, &driver->screen, driver->desc->pixel_bytes, x, y, width, height, data);
 
-        // draw_buffer is a kind of cast, no need to reply
+        // Reply already sent at enqueue time by
+        // try_pre_ack_render_cmd in display_task.c.
         return;
 
     } else if (cmd == globalcontext_make_atom(ctx->global, "\xA" "load_image")) {

display_task.c

@@ -32,13 +32,47 @@
 
 UFontManager *ufont_manager;
 
+static bool try_pre_ack_render_cmd(Message *message, Context *ctx)
+{
+    GenMessage gen_message;
+    if (UNLIKELY(port_parse_gen_message(message->message,
+                &gen_message) != GenCallMessage)) {
+        return false;
+    }
+
+    term req = gen_message.req;
+    if (UNLIKELY(!term_is_tuple(req) || term_get_tuple_arity(req) < 1)) {
+        return false;
+    }
+    term cmd = term_get_tuple_element(req, 0);
+
+    if (cmd != globalcontext_make_atom(ctx->global, "\x6" "update")
+            && cmd != globalcontext_make_atom(ctx->global,
+                    "\xB" "draw_buffer")) {
+        return false;
+    }
+
+    BEGIN_WITH_STACK_HEAP(TUPLE_SIZE(2) + REF_SIZE, heap);
+    term return_tuple = term_alloc_tuple(2, &heap);
+    term_put_tuple_element(return_tuple, 0, gen_message.ref);
+    term_put_tuple_element(return_tuple, 1, OK_ATOM);
+    display_message_send(gen_message.pid, return_tuple, ctx->global);
+    END_WITH_STACK_HEAP(heap, ctx->global);
+
+    return true;
+}
+
 NativeHandlerResult display_task_consume_mailbox(Context *ctx)
 {
     struct DisplayTaskArgs *args = ctx->platform_data;
 
     MailboxMessage *mbox_msg = mailbox_take_message(&ctx->mailbox);
     Message *msg = CONTAINER_OF(mbox_msg, Message, base);
 
+    // Pre-ack update / draw_buffer before the enqueue-or-drop
+    // dance so the caller doesn't time out if dropped.
+    try_pre_ack_render_cmd(msg, ctx);
+
     // Non-blocking enqueue; drop oldest on overflow.
     if (xQueueSend(args->messages_queue, &msg, 0) != pdTRUE) {
 

epaper_display_driver.c

@@ -260,6 +260,7 @@ static void process_message(Message *message, Context *ctx)
 
         term display_list = term_get_tuple_element(req, 1);
         do_update(ctx, display_list);
+        return;
 
     } else if (cmd == globalcontext_make_atom(ctx->global, "\xA" "load_image")) {
         handle_load_image(req, gen_message.ref, gen_message.pid, ctx);

memory_display_driver.c

@@ -179,6 +179,7 @@ static void process_message(Message *message, Context *ctx)
                                       "update")) {
         term display_list = term_get_tuple_element(req, 1);
         do_update(ctx, display_list);
+        return;
 
     } else if (cmd == globalcontext_make_atom(ctx->global, "\xA" "load_image")) {
         handle_load_image(req, gen_message.ref, gen_message.pid, ctx);

oled_display_driver.c

@@ -193,6 +193,7 @@ static void process_message(Message *message, Context *ctx)
                                       "update")) {
         term display_list = term_get_tuple_element(req, 1);
         do_update(ctx, display_list);
+        return;
 
     } else if (cmd == globalcontext_make_atom(ctx->global, "\xA" "load_image")) {
         handle_load_image(req, gen_message.ref, gen_message.pid, ctx);