check display-bus transaction status and act accordingly

2 files changed, 18 insertions, 4 deletions

diff --git a/shared-module/displayio/Display.c b/shared-module/displayio/Display.c
index 95c5cdfec..2f2ba0665 100644
--- a/shared-module/displayio/Display.c
+++ b/shared-module/displayio/Display.c
@@ -69,7 +69,11 @@ void common_hal_displayio_display_construct(displayio_display_obj_t* self,
     self->bus = bus;
 
     uint32_t i = 0;
-    self->begin_transaction(self->bus);
+    while (!self->begin_transaction(self->bus)) {
+#ifdef MICROPY_VM_HOOK_LOOP
+        MICROPY_VM_HOOK_LOOP ;
+#endif
+    }
     while (i < init_sequence_len) {
         uint8_t *cmd = init_sequence + i;
         uint8_t data_size = *(cmd + 1);
@@ -198,9 +202,14 @@ bool common_hal_displayio_display_set_brightness(displayio_display_obj_t* self,
     return ok;
 }
 
-void displayio_display_start_region_update(displayio_display_obj_t* self, uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1) {
+// This routine is meant to be called as a background task. If it cannot acquire the display bus,
+// it will return false immediately to indicate it didn't do anything.
+bool displayio_display_start_region_update(displayio_display_obj_t* self, uint16_t x0, uint16_t y0, uint16_t x1, uint16_t y1) {
     // TODO(tannewt): Handle displays with single byte bounds.
-    self->begin_transaction(self->bus);
+    if (!self->begin_transaction(self->bus)) {
+        // Could not acquire the bus; give up.
+        return false;
+    }
     uint16_t data[2];
     self->send(self->bus, true, &self->set_column_command, 1);
     data[0] = __builtin_bswap16(x0 + self->colstart);
@@ -211,6 +220,7 @@ void displayio_display_start_region_update(displayio_display_obj_t* self, uint16
     data[1] = __builtin_bswap16(y1 - 1 + self->rowstart);
     self->send(self->bus, false, (uint8_t*) data, 4);
     self->send(self->bus, true, &self->write_ram_command, 1);
+    return true;
 }
 
 void displayio_display_finish_region_update(displayio_display_obj_t* self) {
diff --git a/shared-module/displayio/__init__.c b/shared-module/displayio/__init__.c
index 79014fb5a..58a6ce471 100644
--- a/shared-module/displayio/__init__.c
+++ b/shared-module/displayio/__init__.c
@@ -60,7 +60,11 @@ void displayio_refresh_displays(void) {
             if (display->transpose_xy) {
                 swap(&c1, &r1);
             }
-            displayio_display_start_region_update(display, c0, r0, c1, r1);
+            // Someone else is holding the bus used to talk to the display,
+            // so skip update for now.
+            if (!displayio_display_start_region_update(display, c0, r0, c1, r1)) {
+                return;
+            }
 
             uint16_t x0 = 0;
             uint16_t x1 = display->width - 1;