On 05/25/2009 05:27 PM, Alan Stern wrote:
On Mon, 25 May 2009, Hans de Goede wrote:
Hi All,
Please keep me in the CC, I'm not subscribed to the list.
Short intro: I'm a FOSS / Linux developer for 10 years now,
the last year or so I've been working on improving webcam
support under Linux, see:
http://fedoraproject.org/wiki/Features/BetterWebcamSupport
While working on this I noticed that one of the older
Logitech cams I have for testing with, did not work due to
bandwidth problems. It turns out this one only has 1
(non-zero isoc speed) altsetting, claiming max usb 1.1 bandwidth
for the isoc endpoint. Since I was also playing audio through
a usb-1.1 audio device at the time, the not working was not
a surprise. Stopping the audio fixes this.
As a workaround to be able to play both audio and test the
cam I plugged the cam into the usb-2.0 hub built into my
dell monitor, but even with the audio not playing it will
not work when connected this way.
Doing isochronous transfers to a USB-1.1 device through a USB-2.0 hub
is a weak point in the ehci-hcd driver. (In fact, it's a weak point of
the USB-2.0 specification!)
Yes, it seems this whole bandwidth allocation / sharing thing is a weakpoint.
Anyways even though not recommended, isn't this supposed to work ?
I got the idea to do this after reading:
http://www.linux-usb.org/usb2.html
Which sort of makes it sound, that the way to get multiple full-speed
1.1 devices to work without bandwidth problem is actually using a hub.
I'm not claiming the hub in my monitor will be able to handle multiple
full speed devices (no idea) but shouldn't it be able to handle atleast one?
Note: The being a PITA is not deliberate, I can live with what the workaounds,
but I'm wondering how this will reflect upon us when encountered by the
mythical mother in law using the computer.
Regards,
Hans
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