Re: New USB stack on FreeBSD w/Linux emulation layer coming
David Brownell <[email protected]>
| Newsgroups | gmane.linux.usb.devel |
|---|---|
| Message-ID | <[email protected]> |
On Thursday 17 May 2007, Pete Zaitcev wrote: > On Thu, 17 May 2007 12:08:15 -0700, David Brownell <[email protected]> wrote: > > > > > As it happens, USB callbacks cannot be interrupted. That's a somewhat > > > > artificial restriction; in theory there's no reason we couldn't allow > > > > interrupts. > > > > > > Do you remember why we're doing this? I did not touch that part since > > > the attempt to keep usb->lock across the callback (read: years back). > > > > HCDs have, for historical reasons, issued URB completions in their IRQ > > handlers. > > > > In order to do that correctly -- since completion handlers need to be > > able to submit URBs -- that must be done while (a) dropping the HCD's > > internal spinlock, but also (b) not re-enabling the HCD's IRQ, and a bit > > more subtly (c) ensuring certain HCD-internal state doesn't change until > > after the completion returns. > > I understand the points a-c above. The question is, why do we do > local_irq_save across the URB callback? It seems to be completely > non-functional. Where is that done? The HCDs just seem to call usb_hcd_giveback_urb() after dropping their internal spinlock (except the u132 one, which has always been kind of dubious and acts unmaintained). And that routine doesn't do a local_irq_save. So the primary paths certainly don't do such a thing. I could believe other paths have that in case they're not called from an IRQ handler context. Like for example with root hub URBs... - Dave > Linux already guarantees that interrupt handlers are > not re-entered. If the interrupt controller on the platform allows it, > this is done by masking a specific IRQ. If that is impossible, interrupts > are masked in the CPU (maybe only partially, e.g. on SPARC). However > it is done though, the local_irq_save that we do adds nothing that > I can see. > > -- Pete > ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ [email protected] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel