This project proposal is a subtask of smp networking.

The goal of this project is to implement interrupt handling at the granularity of a networking interface. When a network device gets an interrupt, it could call <iftype>_defer(ifp) to schedule a kernel continuation (see kernel continuations) for that interface which could then invoke <iftype>_poll. Whether the interrupted source should be masked depends on if the device is a DMA device or a PIO device. This routine should then call (*ifp->if_poll)(ifp) to deal with the interrupt's servicing.

During servicing, any received packets should be passed up via (*ifp->if_input)(ifp, m) which would be responsible for ALTQ or any other optional processing as well as protocol dispatch. Protocol dispatch in <iftype>_input decodes the datalink headers, if needed, via a table lookup and call the matching protocol's pr_input to process the packet. As such, interrupt queues (e.g. ipintrq) would no longer be needed. Any transmitted packets can be processed as can MII events. Either true or false should be returned by if_poll depending on whether another invokation of <iftype>_poll for this interface should be immediately scheduled or not, respectively.

Memory allocation has to be prohibited in the interrupt routines. The device's if_poll routine should pre-allocate enough mbufs to do any required buffering. For devices doing DMA, the buffers are placed into receive descripors to be filled via DMA.

For devices doing PIO, pre-allocated mbufs are enqueued onto the softc of the device so when the interrupt routine needs one it simply dequeues one, fills in it in, and then enqueues it onto a completed queue, finally calls <iftype>_defer. If the number of pre-allocated mbufs drops below a threshold, the driver may decide to increase the number of mbufs that if_poll pre-allocates. If there are no mbufs left to receive the packet, the packets is dropped and the number of mbufs for if_poll to pre-allocate should be increased.

When interrupts are unmasked depends on a few things. If the device is interrupting "too often", it might make sense for the device's interrupts to remain masked and just schedule the device's continuation for the next clock tick. This assumes the system has a high enough value set for HZ.

Add a comment
Contact | Disclaimer | Copyright © 1994-2017 The NetBSD Foundation, Inc. ALL RIGHTS RESERVED.
NetBSD® is a registered trademark of The NetBSD Foundation, Inc.