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

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Date:	Wed, 11 Feb 2015 19:33:17 -0800
From:	John Youn <John.Youn@...opsys.com>
To:	Roy <yunzhi.li@...mail.com>,
	"John.Youn@...opsys.com" <John.Youn@...opsys.com>,
	Felipe Balbi <balbi@...com>
CC:	Yunzhi Li <lyz@...k-chips.com>,
	"jwerner@...omium.org" <jwerner@...omium.org>,
	"gregory.herrero@...el.com" <gregory.herrero@...el.com>,
	"yousaf.kaukab@...el.com" <yousaf.kaukab@...el.com>,
	"r.baldyga@...sung.com" <r.baldyga@...sung.com>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>,
	Eddie Cai <cf@...k-chips.com>, Lin Huang <hl@...k-chips.com>,
	wulf <wulf@...k-chips.com>,
	杨凯 <yk@...k-chips.com>,
	Tao Huang <huangtao@...k-chips.com>,
	"walkrain@....com" <walkrain@....com>,
	"Douglas Anderson" <dianders@...omium.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1] usb: dwc2: reduce dwc2 driver probe time

On 2/11/2015 3:42 AM, Roy wrote:
> Hi John Youn:
> 
>      Could you please give some suggestions from your point of view, 
> about this probe time issue ?
> 
>      Thanks a lot.
> 
> at 2015/2/11 2:23, Julius Werner wrote:
>>> @@ -2703,7 +2703,7 @@ int dwc2_get_hwparams(struct dwc2_hsotg *hsotg)
>>>          gusbcfg = readl(hsotg->regs + GUSBCFG);
>>>          gusbcfg &= ~GUSBCFG_FORCEHOSTMODE;
>>>          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.

Databook does say 25ms. From what I could gather this has to
do with the debounce filter time on the IDDIG pin after the
ForceHstMode/ForceDevMode is programmed. There is no way to
poll this. I think the change is acceptable, even to lower
the range as Julius suggested.

>>
>>> 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.

Agree with this. It shouldn't take nearly that long and you are
polling anyways.


As for the other change:

> It seems that usleep_range() at boot time will pick the longest
> value in the range. In dwc2_core_reset() there is a very long
> delay takes 200ms, and this function run twice when probe, could
> any one tell me is this delay time resonable ?

I'm not sure about this value or the reasoning/history behind
it. It is not in our internal code. It looks like it is taking
into account the delay for the ForceHstMode/ForceDevMode
programming. However, I think your change is conservative and
should be ok. Maybe Samsung engineers know about this?

John




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