Re: How are halted endpoints supposed to be handled in Linux?

["Michał Pecio" <michal.pecio@gmail.com>]

On Fri, 22 Nov 2024 14:28:58 -0500, Alan Stern wrote:
> 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.

Valid point. Expecting drivers to deal with reordered completions would
be quite unintuitive, potentially laborious and bug-prone.

> > > 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 has been doing things differently for over a decade as far as I
see, and it seems to implement the usb_unlink_urb() rules absolutely
literally (restart when everything is given back), except for the BH
delay problem added later.

Maybe it was a common "idiom" before xHCI, but it seems to rely on
undocumented behavior, and other undocumented behaviors exist today
that sloppy drivers might depend on.

So I don't know, it seems risky either way.

Regards,
Michal