Re: External USB2 HDD affects speed hda

[Rene Herman <rene.herman@keyaccess.nl>]

David Brownell wrote:

>>Added EHCI maintainer to this one as well. If possible, this looks like 
>>a good candidate for a /proc or /sys knob?
> 
> 
> No, it's based on a mis-understanding of the hardware.
> 
> The controller should only be doing DMA when some driver has submitted
> an URB and that URB hasn't yet completed.  Pretty much like any other
> hardware, like a disk or network controller.

Okay, thanks, and okay, crap. Sounded like a nice, plausible explanation...

> For periodic transfers -- interrupt, isochronous, neither used for
> disk I/O -- the driver issuing the transfer always has control over
> the polling period.  But that's mostly related to the USB activity;
> if a periodic transfer is active, then the current segment of the
> periodic schedule has to be scanned (by DMA) every microframe (8x/msec).
> If that segment is empty, that's just one word (32 bits).  If there
> are transfers, it's got to read them and maybe perform them.

I see. Well, sort of at least. "Even if the HDD were using periodic 
transfers, which it isn't, it would be DMAing 32-bits 8x per msec while 
idle, which certainly isn't going to cost 8MB/s bus bandwidth". Right?

Rene.