On Fri, Nov 22, 2024 at 01:57:33PM +0100, Michał Pecio wrote: > On Thu, 21 Nov 2024 10:06:50 -0500, Alan Stern wrote:= > > > One way I see with existing APIs is to wait until the driver > > > submits a new URB, but are drivers prepared for this? Is EHCI doing > > > the same? > > > > Yes; see above. > > That's comforting to hear. > But still seems to have races, see Mathias and my reply to him. > > I suppose alternative solutions include: bypassing the BH on unlink and > error paths, or making it call the HCD back when it's done. The latter > may not be so trivial, as it would ideally be per-endpoint. Bypassing the BH might not be a good idea, because driver's completion handlers expect to be called in order of URB completion. It probably wouldn't make any difference, but it's hard to be sure. > > What about automatic unlinking? > > Maybe it could make things go faster and smoother. > > Networking can tolerate dropped packets, but considering that their MTU > is larger than USB MTU, I suppose they have to split payloads across > multiple USB packets and may get out of sync if only part of a payload > is dropped. But I don't know, they could use packet headers to resync. > > > Note that some class drivers treat -EPROTO as a fatal error. That > > is, they don't retry and their completion-resubmission loop breaks > > down. > > Well, that's on EHCI. No, it's the behavior of the class driver and is independent of the type of host controller. > xHCI gives them a chance to continue with the > remaining URBs. Hopefuly nobody relies on that... > > > However, this seems impractical if the class driver wants to retain > > the existing URBs already on the endpoint's queue. (I don't know of > > any drivers that do this, but still...) Perhaps we should adopt the > > policy that -EPROTO, -EILSEQ, and -ETIME cause all outstanding URBs > > to fail and enforce this policy in usbcore by automatic unlinking so > > that HC drivers don't have to do it. > > I wouldn't exclude the possibility of sloppy drivers leaving URBs > simply because they don't care. Hard to tell what's right for them. If they don't care then they won't care if the URBs get unlinked. Alan Stern
