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
