Re: [RFC PATCH v1] usb: dwc2: reduce dwc2 driver probe time

From: Julius Werner
Date: Tue Feb 10 2015 - 13:23:47 EST

> @@ -2703,7 +2703,7 @@ int dwc2_get_hwparams(struct dwc2_hsotg *hsotg)
> gusbcfg = readl(hsotg->regs + GUSBCFG);
> writel(gusbcfg, hsotg->regs + GUSBCFG);
> - usleep_range(100000, 150000);
> + usleep_range(25000, 50000);

The point of usleep_range() is to coalesce multiple timer interrupts
in idle systems for power efficiency. It's pretty pointless/harmful
during probe anyway and there's almost never a reason to make the span
larger than a few milliseconds. You should reduce this to something
reasonable (e.g. usleep_range(25000, 26000) or even
usleep_range(25000, 25000)) to save another chunk of time. Same
applies to other delays above.

> do you know what's the upper boundary for AHB clock ? How fast can it
> be? It's not wise to change timers because "it works on my RK3288
> board", you need to guarantee that this won't break anybody else.

But this code is already a loop that spins on the AHBIdle bit, right?
It should work correctly regardless of the delay. The only question is
whether the code could be more efficient with a longer sleep... but
since the general recommendation is to delay for ranges less than
10us, and the AHB clock would need to be lower than 100KHz (the ones I
see are usually in the range of tens or hundreds of MHz) to take
longer than that, this seems reasonable to me.
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