RFC : usbatm and iso
matthieu castet
castet.matthieu at free.fr
Fri Feb 11 12:29:40 EST 2005
Hi Roman,
Roman Kagan wrote:
> Salut Matthieu,
>
> On Mon, Feb 07, 2005 at 04:36:58PM +0100, matthieu castet wrote:
>
>>I attached a patch for adding iso to the new library.
>>Seem to work well but need more testing.
>
>
> Just out of curiosity: does using of isochronous transfers improve
> throughput compared to bulk? I'd be rather surprised if it does...
Yes, with urb it don't work well for rate > 100 kbits, for iso rate it
is ok.
may be it is caused by the modem...
>
> Re. your patch, I'm missing the point of having multiple iso urbs,
> especially since you use an interval of 1. Why not use iso frames
> within a single urb where the bulk version uses separate urbs?
>
From usb.h :
* Isochronous URBs have a different data transfer model, in part because
* the quality of service is only "best effort". Callers provide specially
* allocated URBs, with number_of_packets worth of iso_frame_desc
structures
* at the end. Each such packet is an individual ISO transfer.
Isochronous
* URBs are normally queued, submitted by drivers to arrange that
* transfers are at least *double buffered*, and then explicitly
resubmitted
* in completion handlers, so
* that data (such as audio or video) streams at as constant a rate as the
* host controller scheduler can support.
Also it is ouble buffered iso urb are common :
$grep "double buffer" drivers/usb/media/*
http://64.233.183.104/search?q=cache:YcMn-AuNQsAJ:www.geocrawler.com/mail/msg.php3%3Fmsg_id%3D7830969+%22double+buffer%22+linux+iso+urb&hl=fr
> Or am I saying something totally stupid?
More information about the Usbatm
mailing list