[Libusb-devel] permission denied for opening HID device

[amruth <amr...@ya...>]

Hi
I am using libusb-0.9.2 and libcompat-0.1-beta with libhid-0.1.
I am facing this problem whenever I am trying to open my HID device.
libusb is always trying to open from
libusb could'nt open USB device /dev/bus/usb/001/001 permission denied.
I have unplugged the HID device several times but libusb is always trying open from /dev/bus/usb/001/001 location. Please let me know if anybody has any idea.


Thanks
Amruth p.v


--- On Mon, 8/25/08, Charles Lepple <cl...@gh...> wrote:

> From: Charles Lepple <cl...@gh...>
> Subject: Re: [libhid-discuss] Logic of HID_RET_FAIL_INT_READ vs HID_RET_TIMEOUT
> To: "Marian Aldenhoevel" <mar...@ma...>
> Cc: "libhid-discuss List" <lib...@li...>
> Date: Monday, August 25, 2008, 6:30 AM
> On Aug 25, 2008, at 6:02 AM, Marian Aldenhoevel wrote:
> 
> > 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.
> 
> See http://libhid.alioth.debian.org/doc/hid__exchange_8c- 
> source.html#l00332
> 
> > 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);
> 
> Yeah, that won't work as expected. Because USB is
> packet-oriented,  
> read requests are not buffered. If you read one byte,
> libhid sends a  
> request to libusb to read one byte, which asks the device
> for one  
> byte. I believe the device can answer with however many
> bytes it  
> wants (not sure, because I have only tested the case where
> you  
> request more bytes than the device would be expected to
> return).
> 
> >
> > 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)?
> 
> After you read that one byte, the next read will retrieve
> the next  
> report. Consider the other 74 bytes gone.
> 
> > 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?
> 
> I think you're hitting a limitation in the libhid API.
> Check the  
> logic at line 365:
> 
> 00365   if (len != (signed)size) {
> 00366     WARNING("failed to get all of interrupt
> report from device % 
> s; "
> 00367       "requested: %d bytes, sent: %d
> bytes.", hidif->id,
> 00368       size, len);
> 00369     return HID_RET_FAIL_INT_READ;
> 00370   }
> 
> I forget who contributed the interrupt read functions, but
> it looks  
> like they only needed to read fixed-length reports.
> 
> > 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?
> 
> Again, check the code - it's pretty simple. The USB
> stack won't keep  
> re-reading until the buffer is filled, so you should pick
> the timeout  
> based on how many bytes you want to read (after all, USB
> transaction  
> times are measured in milliseconds, and you also need to
> account for  
> the processing time on the microcontroller at the other
> end).
> 
> > Sorry for these very basic questions, I am only just
> learning the  
> > stuff.
> >
> > Ciao, MM
> >
> >
> > _______________________________________________
> > libhid-discuss mailing list
> > lib...@li...
> >
> http://lists.alioth.debian.org/mailman/listinfo/libhid-discuss
> 
> 
> -- 
> Charles Lepple
> 
> 
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