[libhid-discuss] Logic of HID_RET_FAIL_INT_READ vs HID_RET_TIMEOUT

[Marian Aldenhoevel]

Hi,

What is the exact difference between the results HID_RET_FAIL_INT_READ and
HID_RET_TIMEOUT?

I send a report to my device and it is supposed to answer on the interrupt
endpoint. And it does, but I am having trouble fetching the data.

The first byte read is the report-ID, right? And amongst other things that
implies a certain amount of data following that byte? In the case of my
device report-IDs seem to ONLY specify the size, anything else, any content
is inside the report, encoded with it's own length-field and command-code
and so on.

So my plan is to first read the report-ID:

   char reportID;
   hid_interrupt_read(hid,0x81,&reportID,1);

libhid traces:

   TRACE: hid_interrupt_read(): retrieving interrupt report from device 
001/019[0] ...
  NOTICE: hid_interrupt_read(): successfully got interrupt report from device 
001/019[0]
2

Fine. I got my byte. And it looks like a valid report-ID.

I then look up the length for that report-ID (as given by my docs and the
lsusb output, which agree), allocate a buffer and try to read the rest. In
this case the ID is 0x45, report length 75 bytes.

Q: Does that mean I can now expect 75 bytes? Or is it 74 (75 minus the
    report-ID that I have already read)?

Anyway the next hid_interrupt_read() fails, wether I try 75 or 74:

   TRACE: hid_interrupt_read(): retrieving interrupt report from device 
001/019[0] ...
WARNING: hid_interrupt_read(): failed to get all of interrupt report from 
device 001/019[0]; requested: 74 bytes, sent: 0 bytes.

The result is 21 = HID_RET_FAIL_INT_READ

Am I doing something fundamentally wrong? Does it make sense to try the
read again? Do I need to reset something between reads?

What is the logic in case of HID_RET_TIMEOUT? I am occasionally getting that
result, too. I have tried much much longer timeouts and it helps, but I'd
rather keep them short and try again instead of picking some maximum.
Especially as the device as replies ranging from 3 bytes to 2KB. Can I just
call hid_interrupt_read() again?

Sorry for these very basic questions, I am only just learning the stuff.

Ciao, MM