Re: future developments of usbnet

From: Oliver Neukum <redacted>
Date: Wed, 11 May 2011 19:37:47 +0200

How is the frequency NAPI uses to poll determined? We could abuse
this and resubmit the rx URBs only at poll time, but this feels dirty,
because we would still leave interrupts enabled.

It's not a frequency determined internally by the networking.

It is purely event based (meaning triggered by the device's interrupt).

Control flow is:

IRQ --> irq_handler()
	my_netdevice_disable_device_irq();
	napi_schedule();
		--> schedule POLL soft irq
SOFTIRQ --> net_rx_action()
	budget = netdev_budget;
	for_each_net_device_needing_polling() {
		...
		weight = napi_state->weight;
		...
		work = n->poll(n, weight);
		...
		budget -= work;
		...
	}

That's the general idea.

Basically once you take you interrupt, and disable device interrupts,
the generic net device layer calls your ->poll() routing with a "weight"
You should not process more RX packets than this value.

If you have less than "weight" work to do, you should do a napi_complete(),
which takes you out of the polling group, and re-enable device interrupts.

So the idea is that you keep getting ->poll()'d until there is no more
RX work to do.

The "weight" argument implements fairness amongst competing, actively
polling, devices on the same CPU.
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Am Mittwoch, 11. Mai 2011, 19:47:27 schrieb David Miller:

Basically once you take you interrupt, and disable device interrupts,
the generic net device layer calls your ->poll() routing with a "weight"
You should not process more RX packets than this value.

If you have less than "weight" work to do, you should do a napi_complete(),
which takes you out of the polling group, and re-enable device interrupts.

So the idea is that you keep getting ->poll()'d until there is no more
RX work to do.

The "weight" argument implements fairness amongst competing, actively
polling, devices on the same CPU.

Thank you, this is very informative. Our problem here is that USB doesn't work
sanely without interrupts. We can stop IO regarding rx, but we cannot stop
interrupts if we want to do rx.

	Regards
		Oliver