Re: [libusb] Issue with Bulk Transfer in Raspberry PI

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

Alan Stern wrote:
>
> But if the byte count is a multiple of 512 then sending a short packet 
> will involve an extra, unnecessary transfer.  For example, suppose the 
> byte count is 614400.  You send that many bytes (broken up into 1200 
> 512-byte packets) and then send a short packet containing 0 bytes.
>
> The host receives the 1200 packets of data and realizes the transfer 
> has ended, because the byte count has been fulfilled.  So then the host 
> starts the next transfer.  But this one ends immediately, because it is 
> terminated when the 0-byte packet arrives.  This causes extra trouble 
> for the host.

That's an excellent point.  If the device follows the short packet
religion consistently, then the host driver/application needs to send
requests that are larger than expected.  So, if I expected 614,400
bytes, I'd send a 700k byte buffer.

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