From: Alan Stern <[EMAIL PROTECTED]>
To: Steve Calfee <[EMAIL PROTECTED]>
CC: [EMAIL PROTECTED], <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]>
Subject: Re: [linux-usb-devel] USBDEVFS_CONTROL behaviour difference between 2.4.27 and 2.6.7
Date: Sun, 5 Sep 2004 11:13:55 -0400 (EDT)
On Sat, 4 Sep 2004, Steve Calfee wrote:
> One thing to keep in mind while looking at these traces is that windows
> cannot respond to any I/O event in a couple of milliseconds. Therefore in
> the initial get device descriptors you see the setup packet, a few frames
> pause where the windows interrupt routine makes sure something is out there
> and then the IN request for 64 (0x40) bytes followed immediately by the zero
> length OUT packet completing the transaction.
It's probably not worth harping on this, but...
That few frames' delay after the Setup packet isn't because a Windows interrupt routine is doing something. It's because the device takes a little while to respond to the Get-Device-Descriptor request. You can see some (but not all) of the IN packets with NAK responses in the mouse trace; apparently they've been edited out of the modem trace.
Alan Stern
Hi Alan,
Yes you are right. I got confused by my own editing. So this implies the entire setup, IN and zero length OUT are all scheduled to the H/W at once. The control endpoint size determines how many bytes are sent. After the control endpoint size is known (after the initial get descriptor, device transaction), the proper number of IN requests are prebuilt and again queued as one transaction by the host.
It is kind of hard to understand how windows does bandwidth allocation for control packets for low speed devices, it looks like during enum a low speed device gets priority over all ongoing FS bulk transfers.
Regards, Steve
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