DV Display: Fix unaligned read across page boundary

drivers/aps6404/aps6404.cpp

@@ -192,9 +192,9 @@ namespace pimoroni {
     void APS6404::read(uint32_t addr, uint32_t* read_buf, uint32_t len_in_words) {
         start_read(read_buf, len_in_words);
 
-        uint32_t first_page_len = (PAGE_SIZE - (addr & (PAGE_SIZE - 1))) >> 2;
+        uint32_t first_page_len = (PAGE_SIZE - (addr & (PAGE_SIZE - 1)));
 
-        if (first_page_len >= len_in_words) {
+        if (first_page_len >= len_in_words << 2) {
             pio_sm_put_blocking(pio, pio_sm, (len_in_words * 8) - 4);
             pio_sm_put_blocking(pio, pio_sm, 0xeb000000u | addr);
             pio_sm_put_blocking(pio, pio_sm, pio_offset + sram_offset_do_read);
@@ -261,19 +261,33 @@ namespace pimoroni {
     }
 
     uint32_t* APS6404::add_read_to_cmd_buffer(uint32_t* cmd_buf, uint32_t addr, uint32_t len_in_words) {
-        uint32_t len = (PAGE_SIZE - (addr & (PAGE_SIZE - 1))) >> 2;
+        int32_t len_remaining = len_in_words << 2;
+        uint32_t len = std::min((PAGE_SIZE - (addr & (PAGE_SIZE - 1))), (uint32_t)len_remaining);
+        bool clear_isr = false;
 
         while (true) {
-            *cmd_buf++ = (len * 8) - 4;
+            if (len < 2) {
+                // This is guaranteed to leave at least one byte in the ISR, 
+                // which we then clear with an additional command
+                len = 2;
+                clear_isr = true;
+            }
+            *cmd_buf++ = (len * 2) - 4;
             *cmd_buf++ = 0xeb000000u | addr;
             *cmd_buf++ = pio_offset + sram_offset_do_read;
-            len_in_words -= len;
-            addr += len << 2;
+            len_remaining -= len;
+            addr += len;
 
-            if (len_in_words == 0) break;
+            if (len_remaining <= 0) break;
 
-            len = len_in_words;
-            if (len > (PAGE_SIZE >> 2)) len = PAGE_SIZE >> 2;
+            len = len_remaining;
+            if (len > PAGE_SIZE) len = PAGE_SIZE;
+        }
+
+        if (clear_isr) {
+            *cmd_buf++ = 0;
+            *cmd_buf++ = 0xeb000000u | addr;
+            *cmd_buf++ = pio_offset + sram_offset_do_clear;
         }
 
         return cmd_buf;

drivers/aps6404/aps6404.pio

@@ -34,6 +34,8 @@ PUBLIC do_write:
   jmp y--, do_write side 0b10
   set pindirs, 0    side 0b00
   out null, 32      side 0b01
+PUBLIC do_clear:
+  mov isr, null     side 0b01
   jmp top           side 0b01
 
 ; Read command
@@ -76,6 +78,8 @@ PUBLIC do_write:
   jmp y--, do_write side 0b10
   set pindirs, 0    side 0b00
   out null, 32      side 0b01
+PUBLIC do_clear:
+  mov isr, null     side 0b01
   jmp top           side 0b01
 
 ; Read command
@@ -121,6 +125,8 @@ PUBLIC do_write:
   jmp y--, do_write [1] side 0b10
   set pindirs, 0        side 0b00
   out null, 32          side 0b01
+PUBLIC do_clear:
+  mov isr, null         side 0b01
   jmp top               side 0b01
 
 ; Read command

drivers/dv_display/dv_display.cpp

@@ -378,6 +378,7 @@ namespace pimoroni {
       ram.write(point_to_address_palette(p), &val, 1);
       --l;
       offset++;
+      ram.wait_for_finish_blocking();
     }
     if (l > 0) {
       ram.write(point_to_address_palette(p) + offset, (uint32_t*)data, l);