From: Alan Stern <stern@rowland.harvard.edu>
To: Oliver Neukum <oneukum@suse.com>
Cc: linux-usb@vger.kernel.org
Subject: Re: [RFC]extension of the anchor API
Date: Thu, 15 Apr 2021 11:18:10 -0400 [thread overview]
Message-ID: <20210415151810.GA6837@rowland.harvard.edu> (raw)
In-Reply-To: <72d092726448607af2fd453c48be5b0ba69e617a.camel@suse.com>
On Thu, Apr 15, 2021 at 01:23:21PM +0200, Oliver Neukum wrote:
> Am Mittwoch, den 14.04.2021, 10:56 -0400 schrieb Alan Stern:
> > On Wed, Apr 14, 2021 at 10:12:01AM +0200, Oliver Neukum wrote:
> > > Am Montag, den 12.04.2021, 11:06 -0400 schrieb Alan Stern:
> > > > On Mon, Apr 12, 2021 at 11:58:16AM +0200, Oliver Neukum wrote:
> > > > > That presumes that the URBs will finish in order. I don't think such
> > > > > an assumption can be made.
> > > >
> > > > I don't understand -- I can't detect any such presumption.
> > >
> > > OK, this shows that I am bad at explaining.
> > > > As far as I can tell, the only reason for maintaining the URBs in any
> > > > particular order on the anchor list is so that usb_kill_anchored_urbs
> > > > and usb_poison_anchored_urbs can kill them in reverse order of
> > > > submission. THat's why the current code moves completed URBs to the end
> > > > of the list.
> > >
> > > No longer strictly true, as the API has a call to submit everything
> > > on an anchor, but I think it boils down to the same thing.
> > >
> > > > If you keep a pointer to the most recently submitted URB, killing them
> > > > easy enough to do. Start with that URB, then go backward through the
> > > > list (wrapping to the end when you reach the beginning of the list).
> > >
> > > Yes, but that supposes that the next on the list has not been
> > > resubmitted _before_ the one after it.
> > >
> > > If you do not keep the list ordered, but in the initial order,
> > > we can have the situation that A (happens most recently submitted)
> > > is followed by B and C, but C was submitted before B.
> >
> > I think the only reasonable alternative is to move an URB to the end of
> > the list when it is submitted, rather than when it completes. Have you
> > considered doing it that way?
>
> No, that did not occur to me. Back to the drawing board.
> Still I have to put it somewhere when I anchor an URB. Head or tail?
Tail, so that the list's order will be the same as the order in which
URBs were added.
> > The real problem with usb_submit_anchored_urbs is that the core can't
> > know in what order the caller wants the URBs to be submitted. If the
>
> I think the reasonable assumption is that they need to be submitted
> in the order they were anchored.
>
> > In the kerneldoc you can explain that if the anchor has not been used
> > since its URBs were added then the URBs will be submitted in the order
> > they were added to the anchor, but otherwise they will be submitted in
> > an unspecified order, which may not be suitable.
>
> Yes.
>
> > > > The order in which the URBs complete doesn't matter, because trying to
> > > > unlink a completed URB won't cause any harm.
> > >
> > > As long as it stays completed.
> >
> > Rather, as long as they complete in order of submission.
> >
> > > > The only assumption here
> > > > is that URBs get submitted in the list's order (possibly circularly) --
> > > > this should always be true.
> > >
> > > I am afraid we cannot guarantee that. It might intuitively seem so,
> > > but nothing guarantees that all URBs are going to the same endpoint.
> >
> > I hadn't thought of that. Do anchors get used that way anywhere?
>
> I haven't found an example, but I thought it could not be ruled out.
> So you think that that case should be discouraged in documentation
> and henceforth ignored?
It should work okay. Really I was just concerned about
usb_submit_anchored_urbs using the wrong order. People need to be aware
that the the order may be wrong. Put a big warning about it in the
kerneldoc.
As for reordering the URBs in the list... I suppose it's unavoidable if
the URBs can be for different endpoints. I think it makes more sense to
do it when the URBs are submitted; that way you know that the list order
matches the submission order at all times. But it means you have to be
careful when submitting all the URBs at once -- especially if a
completion handler resubmits -- because you want to avoid submitting an
URB twice.
> So we do agree that we need the following:
>
> a - submit in the order you
> anchored
> b - kill or poison in the reverse order
> c - unpoison does not really matter but better do it in the submit
> order?
Doesn't matter. Whatever is easiest.
> Does that mean that the list needs to be kept ordered by sequence
> of submission? I think so.
Yes.
Alan Stern
prev parent reply other threads:[~2021-04-15 15:20 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-25 11:03 [RFC]extension of the anchor API Oliver Neukum
2021-03-25 15:06 ` Alan Stern
2021-03-25 16:04 ` Oliver Neukum
2021-03-25 18:38 ` Alan Stern
2021-04-08 9:23 ` Oliver Neukum
2021-04-08 15:07 ` Alan Stern
2021-04-12 9:58 ` Oliver Neukum
2021-04-12 15:06 ` Alan Stern
2021-04-14 8:12 ` Oliver Neukum
2021-04-14 14:56 ` Alan Stern
2021-04-15 11:23 ` Oliver Neukum
2021-04-15 15:18 ` Alan Stern [this message]
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