Re: [linux-usb-devel] *BAD* impact of usb on PCI performance

[David Brownell]

> From: "Georg Acher" <[e-mail]>
> On Sat, Jun 02, 2001 at 01:22:00PM +0200, Pavel Machek wrote:
> > 
> > With Acher's uhci, even ifconfig up of usb-to-usb networking device
> > [plusb handled by generic usb-to-usb driver; see -ac series].
> > does 50% slowdown. When fsbr is being used, systems slows down by 350%
> 
> Hm, the 50% make me curious... have to look what's happening...

For PL-2301/2302 devices, "ifconfig up" is mostly just posting a bulk read.
True with both "usbnet" and its (now obsolete) predecessor drivers "plusb"
and (for different devices) "net1080".

Laplink-style cables can often support another mode (poll via USB "interrupts",
and then issue reads only when data is available) but not every device can work
that way (like, I seem to recall, an iPaq PDA).  And that'd increase the latency
per packet by a couple milliseconds, even when it's possible.

> > (running more than 4 times slower than normally. Ouch).
> 
> Blame Intel. Either low latency or low PCI usage, you can choose...

This problem is UHCI-specific, not USB-generic, yes?
Doesn't happen with OHCI?

- Dave