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

device/hid/hid_connection_linux.cc

 // Copyright (c) 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "device/hid/hid_connection_linux.h"
 
 #include <errno.h>
 #include <fcntl.h>
 #include <libudev.h>
 #include <linux/hidraw.h>
@@ skipping 86 matching lines @@
   pending_read.buffer = buffer;
   pending_read.callback = callback;
   pending_reads_.push(pending_read);
   ProcessReadQueue();
 }
 
 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);
   }
 }
 
 void HidConnectionLinux::PlatformGetFeatureReport(
     uint8_t report_id,
     scoped_refptr<net::IOBufferWithSize> buffer,
     const IOCallback& callback) {
   if (buffer->size() == 0) {
     callback.Run(false, 0);
     return;
@@ skipping 93 matching lines @@
       pending_reports_.pop();
 
       if (CompleteRead(report.buffer, read.callback)) {
         pending_reads_.pop();
       }
     }
   }
 }
 
 }  // namespace device