DOESN'T WORK and would break OnDiskBitmap if it did...

Expand displayio transactions

In principle, we would like to use background SPI DMA so that pixels
can be sent while new pixels are being calculated. This could potentially
reduce frame time from ~252ms to ~185ms in my test (3.9fps to 5.4fps).

However, a number of things stand in the way of this. In order for a
background write to be useful, the bus has to remain owned by the displayio
core. To end a transaction, the last background write has to finish.
Thus, initial efforts didn't bear any fruit since immediately after
every data chunk was sent, the transaction was ended.

On the other hand, there's at least one difficult-to-change reason that
it is this way: OnDiskBitmap. OnDiskBitmap is already a problem for
bus locking and performance, but also the OnDiskBitmap _could_ be on
the same SPI bus as the display, so you can't enter displayio_display_core_fill_area
with the bus lock held.

So .. our hands are tied by the system of displayio as it currently
exists. There appears to be no way to incrementally evolve it to overlap
pixel calculation & display bus writing.

And anyway this patch as it is does not work, it never even initializes
and clears the LCD.

Author: jepler

shared-module/_stage/__init__.c

@@ -46,14 +46,14 @@ void render_stage(
     area.y1 = y0 * scale;
     area.x2 = x1 * scale;
     area.y2 = y1 * scale;
+    while (!displayio_display_core_begin_transaction(&display->core)) {
+        RUN_BACKGROUND_TASKS;
+    }
     displayio_display_core_set_region_to_update(
         &display->core, display->set_column_command, display->set_row_command,
         NO_COMMAND, NO_COMMAND, display->data_as_commands, false, &area,
         display->SH1107_addressing);
 
-    while (!displayio_display_core_begin_transaction(&display->core)) {
-        RUN_BACKGROUND_TASKS;
-    }
     display->core.send(display->core.bus, DISPLAY_COMMAND,
         CHIP_SELECT_TOGGLE_EVERY_BYTE,
         &display->write_ram_command, 1);

shared-module/displayio/Display.c

@@ -274,6 +274,13 @@ STATIC bool _refresh_area(displayio_display_obj_t *self, const displayio_area_t
     uint32_t mask[mask_length];
     uint16_t remaining_rows = displayio_area_height(&clipped);
 
+    // Can't acquire display bus; skip the refresh
+    if (!displayio_display_core_begin_transaction(&self->core)) {
+        return false;
+    }
+    displayio_display_core_set_region_to_update(&self->core, self->set_column_command,
+        self->set_row_command, NO_COMMAND, NO_COMMAND, self->data_as_commands, false,
+        &clipped, self->SH1107_addressing);
     for (uint16_t j = 0; j < subrectangles; j++) {
         displayio_area_t subrectangle = {
             .x1 = clipped.x1,
@@ -286,9 +293,6 @@ STATIC bool _refresh_area(displayio_display_obj_t *self, const displayio_area_t
         }
         remaining_rows -= rows_per_buffer;
 
-        displayio_display_core_set_region_to_update(&self->core, self->set_column_command,
-            self->set_row_command, NO_COMMAND, NO_COMMAND, self->data_as_commands, false,
-            &subrectangle, self->SH1107_addressing);
 
         uint16_t subrectangle_size_bytes;
         if (self->core.colorspace.depth >= 8) {
@@ -302,21 +306,16 @@ STATIC bool _refresh_area(displayio_display_obj_t *self, const displayio_area_t
 
         displayio_display_core_fill_area(&self->core, &subrectangle, mask, buffer);
 
-        // Can't acquire display bus; skip the rest of the data.
-        if (!displayio_display_core_bus_free(&self->core)) {
-            return false;
-        }
 
-        displayio_display_core_begin_transaction(&self->core);
         _send_pixels(self, (uint8_t *)buffer, subrectangle_size_bytes);
-        displayio_display_core_end_transaction(&self->core);
 
         // TODO(tannewt): Make refresh displays faster so we don't starve other
         // background tasks.
         #if CIRCUITPY_USB
         usb_background();
         #endif
     }
+    displayio_display_core_end_transaction(&self->core);
     return true;
 }
 

shared-module/displayio/EPaperDisplay.c

@@ -281,6 +281,11 @@ STATIC bool displayio_epaperdisplay_refresh_area(displayio_epaperdisplay_obj_t *
     for (uint8_t pass = 0; pass < passes; pass++) {
         uint16_t remaining_rows = displayio_area_height(&clipped);
 
+        if (!displayio_display_core_begin_transaction(&self->core)) {
+            // Can't acquire display bus; bail
+            return false;
+        }
+        displayio_display_core_begin_transaction(&self->core);
         if (self->set_row_window_command != NO_COMMAND) {
             displayio_display_core_set_region_to_update(&self->core, self->set_column_window_command,
                 self->set_row_window_command, self->set_current_column_command, self->set_current_row_command,
@@ -291,9 +296,7 @@ STATIC bool displayio_epaperdisplay_refresh_area(displayio_epaperdisplay_obj_t *
         if (pass == 1) {
             write_command = self->write_color_ram_command;
         }
-        displayio_display_core_begin_transaction(&self->core);
         self->core.send(self->core.bus, DISPLAY_COMMAND | self->chip_select, &write_command, 1);
-        displayio_display_core_end_transaction(&self->core);
 
         for (uint16_t j = 0; j < subrectangles; j++) {
             displayio_area_t subrectangle = {
@@ -335,19 +338,15 @@ STATIC bool displayio_epaperdisplay_refresh_area(displayio_epaperdisplay_obj_t *
                 }
             }
 
-            if (!displayio_display_core_begin_transaction(&self->core)) {
-                // Can't acquire display bus; skip the rest of the data. Try next display.
-                return false;
-            }
             self->core.send(self->core.bus, DISPLAY_DATA | self->chip_select, (uint8_t *)buffer, subrectangle_size_bytes);
-            displayio_display_core_end_transaction(&self->core);
 
             // TODO(tannewt): Make refresh displays faster so we don't starve other
             // background tasks.
             #if CIRCUITPY_USB
             usb_background();
             #endif
         }
+        displayio_display_core_end_transaction(&self->core);
     }
 
     return true;

shared-module/displayio/display_core.c

@@ -251,7 +251,6 @@ void displayio_display_core_set_region_to_update(displayio_display_core_t *self,
     }
 
     // Set column.
-    displayio_display_core_begin_transaction(self);
     uint8_t data[5];
     data[0] = column_command;
     uint8_t data_length = 1;
@@ -282,19 +281,15 @@ void displayio_display_core_set_region_to_update(displayio_display_core_t *self,
     }
 
     self->send(self->bus, data_type | chip_select, data, data_length);
-    displayio_display_core_end_transaction(self);
 
     if (set_current_column_command != NO_COMMAND) {
         uint8_t command = set_current_column_command;
-        displayio_display_core_begin_transaction(self);
         self->send(self->bus, DISPLAY_COMMAND | chip_select, &command, 1);
         self->send(self->bus, DISPLAY_DATA | chip_select, data, data_length / 2);
-        displayio_display_core_end_transaction(self);
     }
 
 
     // Set row.
-    displayio_display_core_begin_transaction(self);
     data[0] = row_command;
     data_length = 1;
     if (!data_as_commands) {
@@ -321,14 +316,11 @@ void displayio_display_core_set_region_to_update(displayio_display_core_t *self,
     }
 
     self->send(self->bus, data_type | chip_select, data, data_length);
-    displayio_display_core_end_transaction(self);
 
     if (set_current_row_command != NO_COMMAND) {
         uint8_t command = set_current_row_command;
-        displayio_display_core_begin_transaction(self);
         self->send(self->bus, DISPLAY_COMMAND | chip_select, &command, 1);
         self->send(self->bus, DISPLAY_DATA | chip_select, data, data_length / 2);
-        displayio_display_core_end_transaction(self);
     }
 }