hid: Linux expects the report ID in the output report buffer.

The Linux hidraw interface expects the buffer written to the device to
always have the report ID as the first byte. Remove the conditional that
only added this data when the report ID was non-zero.

BUG=
R=rockot@chromium.org

Review URL: https://codereview.chromium.org/411463005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285041 0039d316-1c4b-4281-b951-d872f2087c98

device/hid/hid_connection_linux.cc

@@ -104,17 +104,20 @@ void HidConnectionLinux::PlatformWrite(
     uint8_t report_id,
     scoped_refptr<net::IOBufferWithSize> buffer,
     const IOCallback& callback) {
-  // If report ID is non-zero, insert it into a new copy of the buffer.
-  if (report_id != 0)
-    buffer = CopyBufferWithReportId(buffer, report_id);
-  int bytes_written = HANDLE_EINTR(
+  // Linux always expects the first byte of the buffer to be the report ID.
+  buffer = CopyBufferWithReportId(buffer, report_id);
+  const int bytes_written = HANDLE_EINTR(
       write(device_file_.GetPlatformFile(), buffer->data(), buffer->size()));
   if (bytes_written < 0) {
     VPLOG(1) << "Write failed";
     Disconnect();
     callback.Run(false, 0);
   } else {
-    callback.Run(true, bytes_written);
+    if (bytes_written != buffer->size()) {
+      LOG(WARNING) << "Incomplete HID write: "
+                   << bytes_written << " != " << buffer->size();
+    }
+    callback.Run(true, bytes_written == 0 ? 0 : bytes_written - 1);
   }
 }