Re: [libusb] Issue with raspberry pi and libusb_bulk_transfer if buffer size is too big

[Tim R. <ti...@pr...>]

David Levy wrote:
> The things that bugs me is that a buffer size of 0x4400 (so an even multiple of 512) will not work on the raspberry. And the device is sending data slowly (less than 1kb/sec) so we have time to read the buffer.
>
> lsusb -v return this if it can give any useful information:
>
>        Endpoint Descriptor:
>          bLength                 7
>          bDescriptorType         5
>          bEndpointAddress     0x81  EP 1 IN
>          bmAttributes            2
>            Transfer Type            Bulk
>            Synch Type               None
>            Usage Type               Data
>          wMaxPacketSize     0x0040  1x 64 bytes
>          bInterval               0
>        Endpoint Descriptor:
>          bLength                 7
>          bDescriptorType         5
>          bEndpointAddress     0x01  EP 1 OUT
>          bmAttributes            2
>            Transfer Type            Bulk
>            Synch Type               None
>            Usage Type               Data
>          wMaxPacketSize     0x0040  1x 64 bytes
>          bInterval               0

The fact that your max packet size is 64 means that you must have a 
full-speed device, not a high-speed device.  That's fine, it just means 
that the "multiple of 512" thing I mentioned actually only needs to be 
"multiple of 64".

Your description seemed to say that all of the returned packets were 
less than the full packet size.  Is that true?  If so, there's really no 
reason to have your buffer be so large. A transfer is completed as soon 
as there is a packet shorter than the max packet size.  If they're all 
less than 64 bytes, then you can make the buffer 64 bytes.


> And on the endpoint 1, I dont have any issue with another buffer of the same size(but the endpoint 1 doesnt stream any data, it just answers to requests).

Well, endpoint 1 receives your requests, but the results have to appear 
somewhere else (endpoint 0x81?).  An OUT endpoint can't return any 
data.  All endpoints (except the control endpoint) are unidirectional.

But if you send a buffer of 0x4000 bytes to an OUT endpoint, it's always 
going to send 0x4000 bytes, which will be 256 packets and is going to 
span many frames.  I assume your writes are actually specifying the real 
length of the data you're sending.  Right?

-- 
Tim Roberts, ti...@pr...
Providenza & Boekelheide, Inc.