Re: [linux-usb-devel] [LinuxBIOS] [RFC][PATCH 0/2] x86_64 Early usb debug port support.

On Thu, 7 Dec 2006, Peter Stuge wrote:

On Wed, Dec 06, 2006 at 01:08:14PM -0800, Lu, Yinghai wrote:
-----Original Message-----
From: Andi Kleen [mailto:ak@xxxxxxx]
Sent: Wednesday, December 06, 2006 12:59 PM

I haven't looked how the other usb_debug works -- if it's polled
too then it wouldn't have much advantage.

Need to verify if the two sides of debug cable are identical.

In fact they are not. They are almost identical but not exactly the same.

I got my device yesterday and after a small plugfest I can confirm
that only one end of the device enumerates when connected to an ICH7
EHCI driven by 2.6.19.

This is incorrect. Both sides will enumerate. In fact, both sides will
enumerate even at full speed (when plugged in to a UHCI controller instead
of EHCI).

The difference is that the device will accept power only from one side --
let's call it the A side (the side to the right when you have the PLX logo
facing you).

The device is bus-powered, so nothing will happen if the A side isn't
plugged in. But if it is then both the A and B sides will enumerate.
Their descriptors are almost the same; only the serial numbers are
different. The A side's serial number string is "1", and the B side's
serial number string is "0". Oddly enough, both sides use the same string
index number for the descriptor (3).

Bus 001 Device 027: ID 0525:127a Netchip Technology, Inc.
Device Descriptor:
bLength 18
bDescriptorType 1
bcdUSB 2.00
bDeviceClass 255 Vendor Specific Class
bDeviceSubClass 0
bDeviceProtocol 0
bMaxPacketSize0 64
idVendor 0x0525 Netchip Technology, Inc.
idProduct 0x127a
bcdDevice 1.01
iManufacturer 1 NetChip
iProduct 2 NetChip TurboCONNECT 2.0
iSerial 3 1
...

The device is in fact not self-powered.

My theory is that the same set of descriptors are used for both ends,
but one end has been locked to address 127 in order to work with
simpler debug port drivers that assume it will be there.

No, you are wrong. The device will accept any address on either side.

I guess that the self-powered error is also to simplify life for
debug port drivers. IIRC most if not all USB power management
concerns are noops for debug ports.

I don't see how it would make life any simpler, but the device certainly
does lie about its power source.

Alan Stern

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