Re: [RFC v1 0/3] driver-core: add asynch module loading support

[Tejun Heo]

Hello, Dmitry.

On Sun, Aug 31, 2014 at 11:28:51AM -0700, Dmitry Torokhov wrote:
> HI Tejun,
> 
> On Sun, Aug 31, 2014 at 06:13:58AM -0400, Tejun Heo wrote:
> > I haven't followed the previous discussions so please let me know if
> > this has been discussed before.  It looks like you're trying to extend
> > the async mechanism and applying them to init functions themselves.
> > That sounds kinda weird to me.  Isn't the root cause of the problem
> > doing device probings along with driver initilaization on module load?
> 
> For my use case it is driver initialization itself (because most of the
> relevant drivers is compiled in). Although, come to think, if we could do

Hmmm... that is a different case from the one I'm aware of - userland
timing out on module probing and sending SIGKILL aborting device
probing, which is taking long but still making progress properly.

> something about resume that would be nice too: then I'd be able to drop all
> stuff in serio that lies about device state and marks it as resumed even though
> mouse/touchpad will be actually reset and operable much later.
> 
> > Wouldn't it be more logical to simply make bus_add_driver() ->
> > driver_attach() invocation asynchronous?  There's no reason to make
> > them parallel either.  We can use an ordered queue for it so that we
> > don't lose the probing order we used to have.
> 
> Sometimes losing probing order is the desired outcome though. Like with my
> beloved touchpad :)

Yeah, at first I was applying the async thing to built-in drivers too
so the ordering was kinda necessary but I don't think it matters at
all for modules.

> > Making things go
> > parallel is the responsibility of each probing function after all and
> > there isn't much to gain by making attach calls go parallel.
> 
> If we make probe function schedule stuff asynchronously, then, in case of
> failures, we'll end up with half-bound driver. Also drivers would have to have
> additional code on removal to make sure probe full done before removing. PM
> methods need to be ready to be called on half-initialized device. It is a mess.

I don't get this.  The relationship between the driver and its devices
is 1:N.  If a driver fails to bind to a device, it should properly
unbind from that device but still remain available so that it can be
used when another device becomes available.  The two have completely
different life cycles.  If you take a look at
drivers/base/dd.c::__driver_attach(), the function ignores error
return from probing.  The return types are pretty misleading due to
historical reasons but we already do not fail module loading after
probing failure, so making probing asynchronous shouldn't really
change anything.

Thanks.

-- 
tejun
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