RE: [PATCH RFC 04/30] net: fec: fix interrupt handling races

From: Russell King - ARM Linux

On Fri, Jun 20, 2014 at 12:39:57PM +0000, David Laight wrote:

quoted

It is usually possible to reduce the interrupt count even further by:
1) processing the rings.
2) clear the IRQ.
3) check the rings for new entries, if any start again.
This means that you don't take the interrupt for anything that completes
while processing the earlier ring entries.

The slight downside is that it is easy to exit the ISR before the IRQ
line actually drops - resulting in a spurious interrupt.

(patch 24 seems to be similar).

ITYM patch 26, as patch 24 is an update to the errata workaround.

Misread my jottings.

Patch 4 is what I regard to be the bare minimum fix, patch 26 is an
improvement over it.

Remember though that the real interrupt processing happens in the NAPI
handler, not the main interrupt handler - upon seeing a transmit and/or
receive complete interrupt, the main interrupt handler masks both of
these interrupts at the chip and triggers the NAPI handler to run.

Nevertheless, your suggestion is equally valid for a NAPI handler:
- process the rings
- clear the interrupt(s)
- if more work is allowed (according to the NAPI weight), repeat

Not quite, you only want to clear the hardware IRQ and unmask the ISR
when there are no ring entries to process.
(There is no point doing the slow bus cycles until then.)

If you clear the hardware IRQ at the start of processing the ring you'll
get another interrupt when you finally enable the interrupt for a rx
packet that arrives while processing the earlier packets.
However you must check the ring AFTER clearing the IRQ - otherwise
you miss the interrupt for a packet the arrives at that time.

So the sequence is:
1) Find that there are no ring entries to process.
(a packet can arrive here)
2) Clear the IRQ request.
3) Check there are still no ring entries to process.
4) Unmask the interrupt.

This is simple enough for the RX ring, where each packet received is
equal one one NAPI unit of work.  The TX side is less obvious as
the NAPI documentation suggests that cleaning the ring partially
equates to no NAPI units of work but a clean of the entire ring
equates to the full allowance.  Maybe I've mis-understood something
there (which would mean there's another bug which needs fixing in the
tx reaping code...)

I've never tried to understand NAPI work limits.
I suspect that once the NAPI callbacks for other devices (etc) have
had a chance to run, you get called again.
So you'd leave the hardware interrupt masked until then.

	David