Re: [PATCH 1/3] driver core: Add API to wait for deferred probe tocomplete during init

[Jean-Christophe PLAGNIOL-VILLARD]

On 16:07 Thu 09 May     , Russell King - ARM Linux wrote:
> On Thu, May 09, 2013 at 03:37:02PM +0100, Mark Brown wrote:
> > On Thu, May 09, 2013 at 03:14:45PM +0100, Russell King - ARM Linux wrote:
> > > On Thu, May 09, 2013 at 02:50:17PM +0100, Mark Brown wrote:
> > 
> > > > Even if the driver copes fine it can still be desirable to avoid the
> > > > power down/up cycle if it involves some user visible effect - things
> > > > like blinking the display off then on for example.  That said I am a
> > > > little suspicious about this approach, it doesn't feel as robust as it
> > > > should to go round individual callers.
> > 
> > > What if the driver for something like your display is a module which
> > > needs to be loaded from userland?
> > 
> > That's clearly a "don't do that then" sort of thing; while we don't want
> > to be unhelpful there's no guarantees with this approach.
> 
> That's not a "don't do that then" thing, because in this case it's
> unreasonable to say that.  The display subsystems like fbdev and
> DRM represent quite a sizable chunk:
> 
> - Base DRM is around 200k.
> - DRM drivers typically around 100k each.
> - Base FBdev is around 100k.
> 
> It won't take long before you're into the territory of having a
> significant portion of your kernel being display drivers of one
> type or other, much of which won't be usable on any one specific
> platform.  So to say "don't build your display drivers as modules"
> is an unreasonable position to take.
> 
> > Yes, exactly - all we're trying to do here is cover the 90% case, not
> > solve all the possible problems since as you say that's not achievable.  
> > There's a clear and reasonable desire to turn off resources we know
> > aren't in use at the current time, but equally well doing so as soon as
> > we start controlling the resources is pretty much guranteed to introduce
> > user visible issues on some systems so it's a question of picking some
> > reasonable point after that.
> 
> I beg to differ on whether it's possible to solve it completely.
> 
> > Another option here which is more in tune with deferred probing and
> > hotplugging would be to switch the delay over to be time based rather
> > than initcall based; do the shutdown at some point based on the time the
> > last resource was registered.  That still won't cover everything
> > though we could make the delay tunable.
> 
> Yuck.  That's crap design.  Really, time based stuff is crap.  I've seen
> this too many times with the gnome crap in Ubuntu 12.04 - where if you
> boot this off SD card it will complain that some applets fail to start
> (and sure enough, half your panel is missing.)  Boot it off eSATA and
> it works 100% reliably.
> 
> Time based stuff to guess when stuff has finished is never a good thing
> and can never be reliable.
> 
> A better solution may be to avoid the problem in kernel space altogether.
> That's already done in the past with the scsi_wait_scan module.  Make the
> the shutdown of stuff a separate loadable module which userspace can load
> at the appropriate time to trigger the shutdown of unused resources.  Or
> provide a different method for userspace to trigger that action.
> 
> With that kind of solution, it is possible to know that the system has
> finished booting (many userspace implementations already do this with
> stuff like not permitting login via network until after the system has
> finished booting despite sshd et.al. already being started.)
I agree with Russell, your bootlaoder setup the splash screen and you did not
load the framebuffer or DRM driver. if you shutdown the clock you loose the
splashscreen

It's a crap way to do

Best Regards,
J.
> 
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