Re: External USB2 HDD affects speed hda

From: Rene Herman (rene.herman_at_keyaccess.nl)
Date: 06/02/05

  • Next message: Steve French: "[GIT] cifs fixes" 
    Date:	Thu, 02 Jun 2005 00:24:29 +0200
To: David Brownell <david-b@pacbell.net>

    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.
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