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SubjectRe: ehci-hcd affects hda speed
On 15-04-08 21:56, Lev A. Melnikovsky wrote:

[ sorry, hit send prematurely by accident ]

> Sorry, I had virtually no time to answer earlier. If (hopefully) someone
> is still interested, here's my feedback

Interested yes, although being no longer in posession of the hardware it's a
somewhat academic interest...

> I have repeated experiments with P3B-F and VT6212L combination (to
> improve visibility the AsyncSchedSleepTime is set to 1us):
>
> #0. Nothing is connected to USB, no ehci-hcd loaded
> hda throughput 28+-1MB/s
>
> #1. ehci-hcd loaded, still no USB peripherals
> hda throughput 28+-1 MB/s
>
> #2. Something (USB hub and FLASH drive tested) is attached
> hda throughput 15+-1 MB/s
>
> #3. All USB peripherals are removed
> hda throughput 15+-1 MB/s
>
> #4. ehci-hcd is rmmod'ed
> hda throughput 28+-1MB/s
>
> The oddest peculiarity for me is the hysteretic difference between #1 and
> #3 states. I mean experimental data (hda throughput) depends not on the
> state (hardware/loaded modules), but on the path we followed.

On the chip having inited itself at least. Yes, your results match what I
experienced precisely.

> Interestingly enough, sampling registers (via /sys) often shows Async bit
> set of the status register in the state #3. It is always cleared in #1.
> The async file is empty in both states. I wonder, how many degrees of
> freedom does an empty schedule have? Does "empty" mean "has no incomplete
> requests" or "has no requests at all"? Just guessing...

Should leave this up to David, but as far as I'm aware "no at all".

> RH> The sleep time wasn't the core problem, so I wonder of later VIA chips do
> RH> still have the active async schedule problem...
> I don't think this is purely VIA problem. I did not _try_ to install that
> VT6212L card into newer motherboard, but my _feeling_ is that we see an
> "incompatibility" between older PCI mobo chipsets and VIA USB controller.

I very much doubt that. Can't really imagine an off-silicon reason the chip
would keep scanning the async schedule. I'm also now using a NEC controller
card in that same machine and it shows no problems.

> Actually, taking into account superior PCI bandwidth of modern PCs (if
> compared with my old P3B-F motherboard) I am not sure we can perform a
> clean reliable test without PCI bus analyzer.

In my original thread on the issue:

http://lkml.org/lkml/2005/5/30/259

there's was some indication that a rev 0x86 VIA (more modern) does something
similar due to Grant Coady in there also being able to see the Async bit on
at all while looking at it so while throughput may not be hindered much, the
issue could probably still be debugged/seen on newer (VIA) hardware by
examing that state file.

You are probably by now one of the best positioned users to debug the issue
though with a VT2612 in an older machine so perhaps you can work with David
Brownell to perhaps for once and all confirm that it's just unfixable VIA
silicon or something which can be helped. As indicated, I just gave up on
the bloody controller :-)

Rene.


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