AW: [ECOS] Splitting USB-Packets

Fuchs, Torsten
Thu Apr 14 08:24:00 GMT 2005


I'm using a philips ISP1583 with a self-written driver in a digital camera to implement the PTP (Picture transfer protocol). So there are only bulk and some interrupt transfers. The hardware does not send the zero packet automatically. Modifying the device driver is not the problem but getting a flag through the overlying slave package, that I want to use. It seems I have to modify it and be no more compatible to the original package. I just wondered if no-one had the problem of splitting larger packets into pieces until now. 
The PTP-Standard says that the first short or zero packet ends the transfer, so I think there is no other way than modifying the driver.

The host side is a PC or MAC. I did not know that WinXP breaks up the packets in smaller units when they exceed a certain size. But relying on this "feature" is not a safe way for the device.

Torsten Fuchs

-----Ursprüngliche Nachricht-----
Von: Frank Pagliughi []
Gesendet: Mittwoch, 13. April 2005 23:41
An: Fuchs, Torsten;
Betreff: RE: [ECOS] Splitting USB-Packets

Which USBS device/driver are you using? Does the hardware automatically send
the zero packet? I assume not, so you could modify the driver for your
specific application - if that's what you really want to do. I've been
working with Philips chips, where you have to send the zero packet
explicitly. And you do *need* to do this by default, especially if you're
using the higher level File I/O functions.

BTW, are you talking about Bulk transfers?

And what are you using on the host side? I don't know that low-level USB
functions guarantee undivided packets back to user mode code (or even upper
level drivers). In a Windows XP device driver I just wrote I would see Bulk
IN packets broken up by the underlying bus driver when they were around the
4kB page size. That would require repeated read() calls by the application
to reconstruct the full packet, sort of like reading a TCP stream.

Frank Pagliughi

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