Re: USB performance bug since kernel 2.6.13 (CRITICAL???)

Open Source
2006-10-13T19:31:12Z

Hi Wolfgang (and all),

Thanks for the input. However, I am not understanding
exactly why kernel mode is treated any differently than
user mode for this sort of thing. I am looking at the code
in ehci-q.c and ehci-hcd.c.

It seems like the unlinking of completed URBs
happens asynchronously on a timer. This is a
surprise to me since I thought this was happening
on an IRQ from the host controller. But if what I'm
surmising is correct it would explain everything
I am seeing. I'm not able to ascertain how
user mode drivers are treated differently than
kernel mode drivers in this regard. From what I
can tell, all drivers would be broken equally!
Can anyone who has more experience
with this code confirm this for me?

Besides, we count on sub-10 ms response times all the
time in user mode. Take for example, the access of a file.
If opening a file had a fixed latency of 4 ms, people
would be up in arms. So that's not entirely a valid excuse.
A USB operation that used to take 1 ms now takes 4 ms.
That's a pretty big change.

The ability to write user-mode drivers for USB devices
is very powerful for deployment. If one writes a kernel
driver, there are severe deployment hassles. As such,
my company has chosen to write user-mode drivers
on both Windows to avoid driver deployment nightmares.
This has been extremely successful so far.. Ironically,
Windows (using libusb-win32) has had no such performance
glitches. As a matter of principle, Linux should at least be
as good as Windows, right?

Hopefully we can get this sorted out.

Cheers.

----- Original Message ----
From: WolfgangMües <wolfgang@iksw-muees.de>
To: linux-usb-devel@lists.sourceforge.net
Sent: Friday, October 13, 2006 12:11:08 PM
Subject: Re: [linux-usb-devel] USB performance bug since kernel 2.6.13 (CRITICAL???)

On Friday 13 October 2006 19:20, Open Source wrote:
> Alan -- yes, I understand the ability to increase throughput
> by transfering more bytes and I am definitely able to see
> better overall throughput when increasing the number
> of bytes per transaction. However, I needs to still have
> good transaction-level timing because I cannot always
> queue the transactions up. Recall that each transaction
> is a WRITE followed by a READ. The results of the
> READ determine the outgoing bytes for the following
> transaction's WRITE.

Relying on sub-10ms response times in userspace is broken by design.

I have written a driver with similar timing requirements, and I have
done it in the kernel. This is the right way to go. Nothing else.

regards

Wolfgang
--
Das Leben kann nur rückwärts verstanden,
muß aber vorwärts gelebt werden.

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