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List:       linux-usb-users
Subject:    [Linux-usb-users] Re: [linux-usb-devel] usb-uhci problem
From:       Georg Acher <acher () in ! tum ! de>
Date:       2001-11-26 19:19:00
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On Mon, Nov 26, 2001 at 11:02:08PM +0530, Masoodur Rahman wrote:
> hi ,
>  I was not able to figure out how the packets coming to the usb device are 
> handed over to the USB device drivers through the usb-uhci interface .
>    AFAICS, while sending the packets, it goes through 
> -->uhci_submit_urb()---->uhci_submit_xxx_urb()  (xxx->ctrl/bulk/iso/int)
> 
> In each of these routines Tds are allocates and filled and the URB gets 
> queued.
> After this the host controller takes charge of putting them on the bus.
> Is my understanding correct ???
> 
> What happens when a packet(bulk/isoc/control/interrupt) comes to the usb 
> device from the other side ??
> What are the functions that gets called upto the point where it delivers 
> to the specific device driver ?? Does it start from uhci_interrupt ???

No, you missed the point that USB is a master/slave protocol, with the PC as
the ONE AND ONLY master. Thus the PC actively sends a "read-request" to the
device, which answers (hopefully). So the if the user wants to receive data,
a TD with the IN-flag is assembled. This TD triggers a IN request on USB, if
the device sends data, the TD gets filled and completed. If no data arrives
(either an active NAK or a timeout), the transfer is retried.

So everything is started with the submit_urb, the interrupts only signal the
completion or the failing of the transfer.
-- 
         Georg Acher, acher@in.tum.de         
         http://www.in.tum.de/~acher/
          "Oh no, not again !" The bowl of petunias          

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