linux-kernel - Re: [PATCH RFC] USB: Add HCD fastboot

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Date:	Wed, 06 Aug 2008 22:49:28 +0100
From:	Simon Arlott <simon@...e.lp0.eu>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Rene Herman <rene.herman@...access.nl>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	Daniel Walker <dwalker@...sta.com>,
	USB list <linux-usb@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCH RFC] USB: Add HCD fastboot

On 06/08/08 21:26, Alan Stern wrote:
> On Wed, 6 Aug 2008, Simon Arlott wrote:
> 
>> No, by adding a 5 second delay you're intending for the device driver initcalls 
>> to complete within that 5 seconds. If they take too long then the last one 
>> blocks everything (I realise that's ridiculous, these initcalls take <1ms when 
>> there are no devices yet). The best way to do is to make the driver initcalls 
>> before the host ones, like you suggested.
> 
> Doing the HCD initcalls last certainly ought to work.

Right - so then all that's needed is to move usb/ before most other things that 
take a while to init, and it has some time to initialise usb devices in the 
background.

>> > "it'll still have to wait..."  If by "it" you mean the initcall
>> > thread, you're wrong.  If by "it" you mean the user, you still aren't
>> > necessarily correct; the user can do plenty of other things while
>> > waiting for USB devices to initialize.
>> 
>> Assuming userspace doesn't wait for all devices to settle and appear in /dev etc. 
>> before continuing.
> 
> Whatever that involves...  /dev never truly settles; it's always 
> possible to plug in a new device or remove an old one.

Yes... I'm not sure how that part would work.

>> > I suppose you could make the hub_thread delay time a module parameter 
>> > for usbcore, defaulting to 0.  Then it could be set by just the people 
>> > who want to use it -- many (most?) people keep their drivers in 
>> > modules, and it wouldn't do them any good.
>> 
>> It really needs to have hcd initcalls done very early so that device init 
> 
> Please stop using the word "it" with no antecedent!  Do you mean "we"?

Yes. I'll try to avoid doing that.

>> has the rest of the (kernel and userspace) boot process to complete in the 
>> background. This is negated by having device drivers initialised immediately 
>> afterwards. Re-ordering initcalls and doing more of the init process 
>> asynchronously is likely to expose bugs and cause inconsistent device order 
>> on some systems, so if the makefile mess could be reduced then it can be a 
>> Kconfig option.
> 
> So what exactly do you recommend?

I'm not an expert on what can be done with the Makefile process, perhaps there's 
an easier way to move things around based on a config option. If host init is 
moved before device init for everyone, wouldn't there be too many side effects?

I've not tried to use usb-storage as root but presumably that works. If that 
already uses a hack of making the kernel wait X seconds before trying to mount 
root then it'll still work.

>> How many people have *all* their USB components (hcd, drivers) as modules?
> 
> All the major distributions do, as far as I know.  (I haven't actually 
> checked them all to be certain.)
> 
>> What do they do with their USB keyboards in the period between init and module 
>> load?
> 
> Probably nothing.  What do you do on your keyboard while waiting for
> system initialization to complete?

Lack of a keyboard makes it impossible to do anything if the module fails to 
load... as I understand it when the HCD init runs, any BIOS emulation stops?
Although if HCD is a module too...

>> If even one device driver and the hcd is compiled in, they'd need to 
>> wait for every USB device to finish init before the usbhid probe could complete.
> 
> What if usbhid is the one device driver that is compiled in?  :-)

That was the situation I was thinking of - surely that would be compiled in 
if the HCDs were?

-- 
Simon Arlott
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