Re: Rationale for delay_use in USB storage?

Matthew Dharm <mdharm-usb@xxxxxxxxxxxxxxxxxx> · Mon, 26 Nov 2007 09:40:01 -0800

On Mon, Nov 26, 2007 at 10:40:16AM -0500, Alan Stern wrote:
> On Mon, 26 Nov 2007, Mike Nuss wrote:
> 
> > I'm using a USB Mass Storage flash device to boot on an embedded
> > system and I'm trying to reduce the boot time as much as possible. One
> > delay I'm encountering is the "Waiting for device to settle" delay
> > after the kernel detects the mass storage device.  I see that this is
> > controlled by the delay_use parameter in usb-storage but I couldn't
> > find any documentation as to why a device would need to settle.  Could
> > someone explain?
> 
> Not really.  All we know is that some devices (some Apple iPods, for
> example) really do seem to need it.  That is, with the delay they work
> and without it they don't work.
> 
> 5 seconds is overkill in all likelihood, but we wanted the default to 
> work as widely as possible.  You can, of course, always set the module 
> parameter to 0.

That said, we've also seen devices which need more than this.  One I recall
recently needed at least 7, so we told that user to set it to 10.

This parameter originated in our quest to act like the "popular" OSes; we
were crashing device firmware by sending the first commands to the device
too quickly otherwise.

Matt

-- 
Matthew Dharm                              Home: mdharm-usb@xxxxxxxxxxxxxxxxxx 
Maintainer, Linux USB Mass Storage Driver

A:  The most ironic oxymoron wins ...
DP: "Microsoft Works"
A:  Uh, okay, you win.
					-- A.J. & Dust Puppy
User Friendly, 1/18/1998