Re: forcedeth: option to disable 100Hz timer (try 2)

From: Robert Hancock
Date: Thu Sep 11 2008 - 00:25:27 EST

Andrew Morton wrote:
On Wed, 10 Sep 2008 18:18:20 -0600
Robert Hancock <hancockr@xxxxxxx> wrote:

Andrew Morton wrote:
On Tue, 9 Sep 2008 23:34:35 +0400
Mikhail Kshevetskiy <mikhail.kshevetskiy@xxxxxxxxx> wrote:

On some hardware no TX done interrupts are generated, thus special
100Hz timer interrupt is required to handle this situation properly.
Other device do not require that timer interrupt feature.

Forcedeth has a DEV_NEED_TIMERIRQ flag to mark the broken devices.
Unfortunately, nobody know the actual list of broken devices, so all
device has this flag on. Other problem, this flag is not user visible,
so the kernel recompilation is required to disable timer interrupts and
test a device.

This patch add a "disable_timerirq" option to disable interrupt timer mentioned above. This may be extremely useful for laptop users.
Why do you feel that the timer-based completions need to be disabled? Is it causing some problem?
100 unnecessary CPU wakeups per second imposes some power usage cost, especially on laptops with CPU C-states..

Is that the only reason for the change? We still don't know...

Anyway, it's certainly _sufficient_ reason, however the implementation
is pretty sad - most people won't even know that the option exists so
they'll continue to chew more power than they need to.

How do we fix this? Perhaps disable the timer by default, then wait
for the first tx timeout and then enable the timer at that stage, while
printing a message saying "add module option <foo> to prevent this
once-off timeout from happening"?

It at least provides some way forward, where those that care can add the option and find out if their chipset can have the timer disabled in the driver by default in the future..

I'm not sure how long the TX timeout is, but I suspect it would be too disruptive to only enable the interrupt after a timeout. Enabling by default and disabling after a TX done interrupt was received would likely be a better approach (or one of the more creative approaches that others have mentioned). But it's quite likely that at least some of the chipsets that this driver supports don't need this timer nonsense at all.
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