Re: [PATCH 2/2] PM / Domains: Avoid infinite loops in attach/detach code

From: Geert Uytterhoeven
Date: Tue Jun 23 2015 - 09:45:52 EST

Hi Rafael,

On Tue, Jun 23, 2015 at 3:38 PM, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>>>> @@ -2218,10 +2227,13 @@ int genpd_dev_pm_attach(struct device *dev)
>>>> dev_dbg(dev, "adding to PM domain %s\n", pd->name);
>>>> - while (1) {
>>>> + for (i = 0; i < GENPD_RETRIES; i++) {
>>>> ret = pm_genpd_add_device(pd, dev);
>>>> if (ret != -EAGAIN)
>>>> break;
>>>> +
>>>> + if (i > GENPD_RETRIES / 2)
>>>> + udelay(GENPD_DELAY_US);
>>> In this execution path, we retry when getting -EAGAIN while believing
>>> the reason to the error are only *temporary* as we are soon waiting
>>> for all devices in the genpd to be system PM resumed. At least that's
>>> my understanding to why we want to deal with -EAGAIN here, but I might
>>> be wrong.
>>> In this regards, I wonder whether it could be better to re-try only a
>>> few times but with a far longer interval time than a couple us. What
>>> do you think?
>> That's indeed viable. I have no idea for how long this temporary state can
>> extend.
> A usual approach to this kind of thing is to use exponential fallback
> where you increase the delay twice with respect to the previous one
> every time.

Right, but when do you give up?

Note that udelay() is a busy loop. Should it fall back to msleep() after
a while?



Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
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