File:  [NetBSD Developer Wiki] / wikisrc / projects / project / if_poll.mdwn
Revision 1.2: download - view: text, annotated - select for diffs
Tue Jan 3 20:51:24 2017 UTC (5 years, 11 months ago) by riastradh
Branches: MAIN
CVS tags: HEAD
Remove preapproved funding designations.

Per discussion between board and core, these stale designations will
be replaced by something we hope to be more lively and maintained.

[[!template id=project

title="Implement per-interface interrupt handling"



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.

[[!tag smp_networking]]

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