[libhid-discuss] Trying to get reports from hid device - unsuccessful

[Vladimir Bashkirtsev]

Hello,

I have some questions and cannot find answers. Hope that you will be 
able to help.

I have a USB scale which appears as HID device. On plugin it claimed by 
hidraw driver and appears as /dev/hidraw4 . Scales repeatedly send 6 
bytes blocks containing current status. I would like to read these 6 
bytes, decode them and then feed relevant representation to /dev/uinput 
so it would act similar to barcode scanners which act as keyboards. So I 
am going to make a daemon which will do it. However something tells me 
that most appropriate way to access HID will be libhid instead of 
hidraw. I tried to use it and run into problems and need some guidance.

1. As I understand libhid does not support asynchronous transfer? ie I 
cannot register a callback function which will be triggered each time as 
data appears on interface?
2. I have compiled test_libhid.c and I can detach the device from hidraw 
and can dump its tree:

TRACE: hid_dump_tree(): iterating the parse tree for USB device 
003/005[0]...
parse tree of HIDInterface 003/005[0]:
   path: 0x008d0020.0x008d0030.0x008d0027.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x008d0050.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0032.0x008d0070.0x00000000; type: 0x80
   path: 0x008d0020.0x008d0032.0x008d0050.0x00000000; type: 0x80
   path: 0x008d0020.0x008d0032.0x008d0041; type: 0x80
   path: 0x008d0020.0x008d0032.0x008d0040; type: 0x80
   path: 0x008d0020.0x008d0031.0x008d0080; type: 0x90
   path: 0x008d0020.0x008d0031.0x008d0081; type: 0x90
   path: 0x008d0020.0x008d0031.0x00000000; type: 0x90
   path: 0x008d0020.0x008d003a.0x008d00ff; type: 0x90
   path: 0x008d0020.0x008d003a.0x00000000; type: 0x90
   path: 0x008d0020.0x008d003a.0x00000000; type: 0x90
   path: 0x008d0020.0x008d003a.0x00000000; type: 0x90
   path: 0x008d0020.0x008d003a.0x00000000; type: 0x90
   path: 0x008d0020.0x008d003a.0x00000000; type: 0x90
   path: 0x008d0020.0x008d003a.0x00000000; type: 0x90
   path: 0x008d0020.0x008d0030.0x008d0040; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0
   path: 0x008d0020.0x008d0030.0x00000000; type: 0xb0

As I understand I need to read reports from path of type 0x80. So I do:

   unsigned char const PATHLEN = 4;
   int const PATH_OUT[4] = { 0x008d0020, 0x008d0032, 0x008d0070, 
0x00000000 };

   unsigned char const RECV_PACKET_LEN = 6;
   char packet[RECV_PACKET_LEN];
   ret = hid_get_input_report(hid, PATH_OUT, PATHLEN, packet, 
RECV_PACKET_LEN);
   if (ret != HID_RET_SUCCESS) {
     fprintf(stderr, "hid_get_input_report failed with return code 
%d\n", ret);
   }

and it comes back with

   TRACE: hid_get_input_report(): looking up report ID...
   TRACE: hid_prepare_parse_path(): preparing search path of depth 4 for 
parse tree of USB device 003/005[0]...
   TRACE: hid_prepare_parse_path(): search path prepared for parse tree 
of USB device 003/005[0].
  NOTICE: hid_find_object(): found requested item.
   TRACE: hid_get_input_report(): retrieving report ID 0x03 (length: 6) 
from USB device 003/005[0]...
WARNING: hid_get_input_report(): failed to retrieve report from USB 
device 003/005[0]:error sending control message: Broken pipe.
hid_get_input_report failed with return code 20

And it is the same for all paths of type 0x80. What I do wrong? Perhaps 
answer is simple but I have tried everything and cannot get scales to 
report as expected.

Regards,
Vladimir