diff --git a/fastboot/protocol.cpp b/fastboot/protocol.cpp
index 7a333ee..fda6f5d 100644
--- a/fastboot/protocol.cpp
+++ b/fastboot/protocol.cpp
@@ -278,19 +278,14 @@
     return _command_end(transport);
 }
 
-#define TRANSPORT_BUF_SIZE 1024
+static constexpr size_t TRANSPORT_BUF_SIZE = 1024;
 static char transport_buf[TRANSPORT_BUF_SIZE];
-static int transport_buf_len;
+static size_t transport_buf_len;
 
-static int fb_download_data_sparse_write(void *priv, const void *data, int len)
-{
-    int r;
-    Transport* transport = reinterpret_cast<Transport*>(priv);
-    int to_write;
-    const char* ptr = reinterpret_cast<const char*>(data);
-
+static int fb_download_data_sparse_write(void* priv, const void* data, size_t len) {
+    const char* ptr = static_cast<const char*>(data);
     if (transport_buf_len) {
-        to_write = std::min(TRANSPORT_BUF_SIZE - transport_buf_len, len);
+        size_t to_write = std::min(TRANSPORT_BUF_SIZE - transport_buf_len, len);
 
         memcpy(transport_buf + transport_buf_len, ptr, to_write);
         transport_buf_len += to_write;
@@ -298,9 +293,10 @@
         len -= to_write;
     }
 
+    Transport* transport = static_cast<Transport*>(priv);
     if (transport_buf_len == TRANSPORT_BUF_SIZE) {
-        r = _command_write_data(transport, transport_buf, TRANSPORT_BUF_SIZE);
-        if (r != TRANSPORT_BUF_SIZE) {
+        int64_t r = _command_write_data(transport, transport_buf, TRANSPORT_BUF_SIZE);
+        if (r != static_cast<int64_t>(TRANSPORT_BUF_SIZE)) {
             return -1;
         }
         transport_buf_len = 0;
@@ -311,9 +307,9 @@
             g_error = "internal error: transport_buf not empty";
             return -1;
         }
-        to_write = round_down(len, TRANSPORT_BUF_SIZE);
-        r = _command_write_data(transport, ptr, to_write);
-        if (r != to_write) {
+        size_t to_write = round_down(len, TRANSPORT_BUF_SIZE);
+        int64_t r = _command_write_data(transport, ptr, to_write);
+        if (r != static_cast<int64_t>(to_write)) {
             return -1;
         }
         ptr += to_write;