drtio: port 64-bit padding from mainline

src/libboard_artiq/src/drtioaux.rs

@@ -153,11 +153,10 @@ pub fn send(linkno: u8, packet: &Packet) -> Result<(), Error> {
 
         packet.write_to(&mut writer)?;
         
-        let padding = 4 - (writer.position() % 4);
-        if padding != 4 {
-            for _ in 0..padding {
-                writer.write_u8(0)?;
-            }
+        // Pad till offset 4, insert checksum there
+        let padding = (12 - (writer.position() % 8)) % 8;
+        for _ in 0..padding {
+            writer.write_u8(0)?;
         }
 
         let checksum = crc::crc32::checksum_ieee(&writer.get_ref()[0..writer.position()]);

src/libboard_artiq/src/drtioaux_async.rs

@@ -124,11 +124,10 @@ pub async fn send(linkno: u8, packet: &Packet) -> Result<(), Error> {
 
         packet.write_to(&mut writer)?;
         
-        let padding = 4 - (writer.position() % 4);
-        if padding != 4 {
-            for _ in 0..padding {
-                writer.write_u8(0)?;
-            }
+        // Pad till offset 4, insert checksum there
+        let padding = (12 - (writer.position() % 8)) % 8;
+        for _ in 0..padding {
+            writer.write_u8(0)?;
         }
 
         let checksum = crc::crc32::checksum_ieee(&writer.get_ref()[0..writer.position()]);