device_attach(9) and driver initialization

[Konstantin Belousov]

On Sat, Apr 07, 2012 at 10:04:59AM -0500, Nathan Whitehorn wrote:
> On 04/07/12 07:50, Konstantin Belousov wrote:
> >Hello,
> >there seems to be a problem with device attach sequence offered by newbus.
> >Basically, when device attach method is executing, device is not fully
> >initialized yet. Also the device state in the newbus part of the world
> >is DS_ALIVE. There is definitely no shattering news in the statements,
> >but drivers that e.g. create devfs node to communicate with consumers
> >are prone to a race.
> >
> >If /dev node is created inside device attach method, then usermode
> >can start calling cdevsw methods before device fully initialized itself.
> >Even more, if device tries to use newbus helpers in cdevsw methods,
> >like device_busy(9), then panic occurs "called for unatteched device".
> >I get reports from users about this issues, to it is not something
> >that only could happen.
> >
> >I propose to add DEVICE_AFTER_ATTACH() driver method, to be called
> >from newbus right after device attach finished and newbus considers
> >the device fully initialized. Driver then could create devfs node
> >in the after_attach method instead of attach. Please see the patch below.
> >
> 
> Something like this would also be very useful for drivers that need to 
> interact across the device tree, if newbus called it only after all 
> drivers have been attached. Drivers that need to see other potentially 
> attached drivers now need to do some hacks with SYSINIT. Would it be 
> possible to do this? I don't think it changes the functionality you need.

I am definitely fine with postponing a call further, but I am not sure
how to define that point and how to implement your proposal. I am mostly
thinking of the case of kld being loaded, since for compiled-in drivers,
there is simply no usermode to make the havoc during attach.

For the boot time attachments, I am not sure when to declare the end.
E.g. USB does asynchronous device discovery.

Could you prototype a change that would do what you propose ?
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