I've started working on some minor improvements to CardBus and USB. I'd like to see each of them using filters. CardBus and PC Card necessarily share interrupts between the bridge and the client devices. There's only one interrupt pin. Having to schedule an ithread just to check on the status of the card (for eject events) or power (for the interrupt driven power-up sequence) imposes a large overhead on the system. In addition, it may interfere with CardBus drivers that use filter. The current ISR isn't written to be a filter (what used to be known as a fast interrupt), so some work is needed to migrate it to such an interrupt.
For USB the problem is different. There's no interrupt sharing inside of USB, per se, since all the work goes through the pci<->usb host controller. However, on many systems the designers cheaped out and forced usb and the network to share interrupts. Since usb is still Giant locked, this causes a lot of extra Giant contention. The idea hear is to introduce a filter that turns off the interrupt bits and requests that the ISR run. The ISR would be exactly like we have today, with the addition of code to turn the interrupt sources back on. The theory here is that network inspired interrupts don't have to take out Giant at all since usb can schedule its ithread only when there's real work to do. If the ithread isn't scheduled, then Giant isn't taken out.
Since the freeze is coming up soon, I need to get this done in the next two weeks.