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

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Date:	Wed, 6 Aug 2008 16:26:29 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Simon Arlott <simon@...e.lp0.eu>
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 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.

> > "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.

> > 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"?

> 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?

> 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?

> 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?  :-)

Alan Stern

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