Re: [PATCH] ACPI / GED: use late init to allow other drivers init

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

Sorry for the delay.

On Tuesday, April 25, 2017 12:24:19 PM Sinan Kaya wrote:
> On 4/25/2017 3:01 AM, Lukas Wunner wrote:
> > On Sat, Apr 22, 2017 at 12:48 AM, Sinan Kaya <okaya@codeaurora.org> wrote:
> >> On 4/21/2017 6:43 PM, Rafael J. Wysocki wrote:
> >>> +late_initcall(ged_init);
> >>> Does this fix the problem?
> >>>
> >>> What about if the module in question is loaded after running
> >>> late_initcalls?
> >>
> >> This fixed the issue for me where I had dependencies for QUP I2C driver
> >> and GHES drivers. Both of them are modules and get probed via normal
> >> module execution path.
> >>
> >> However, I'm open to improvements.  Do you have a better suggestion?
> >> I can try to add some _DEP stuff if it is present, but I remember Linux
> >> doesn't like _DEP stuff too much.
> > 
> > Would it be possible to solve this by just returning -EPROBE_DEFER from the
> > ->probe hook if the devices you depend on are not bound yet?
> > 
> 
> I'm not sure. 
> 
> > Alternatively, would it be possible to solve it with a struct device_link?
> 
> I wasn't aware of device_link concept. This is something that I will keep in
> my mind when I'm dealing with producer/consumer problems with known device
> driver instances. It looked very useful.
> 
> Here is how the overall relationship between drivers.
> 
> | GED | <--->  | Platform specific ACPI AML | <----> Vendor GPIO
>                                               <----> Vendor I2C
>                                               <----> ACPI GHES
> 					      <----> Some other driver
> 
> The problem with Generic Event Device (GED) is that it produces event
> notification facility to the platform specific AML code and GED doesn't
> have any idea about the consumers of these interrupts or what platform AML
> does. 
> 
> GED only sees the interrupts that it needs to register and
> knows the ASL code it needs to execute when that interrupt happens.
> 
> It is possible for AML code not to use any of these drivers or require
> some arbitrary driver as well as vendor specific drivers. It is totally
> up to the firmware to decide what to do with this event.
> 
> My proposal was to require platform AML code to indicate the dependencies
> between GED and drivers on the right side of the picture via _DEP as this
> cannot be done via normal kernel mechanisms.

Something like _DEP would be needed.

However, _DEP as specified is only about operation region dependencies, which
doesn't seem to be applicable here.

That said, _DEP is used for general dependecies by firmware already, but it
would at least be good to send a proposal for a spec update regarding that
before mandating using _DEP for GED.

> This approach might work in general. However, it also has its own caveats.
> 
> All of these drivers on the right side are unrelated to each other. Some
> operating system can implement a subset of these drivers.
> 
> If I include the dependencies, GED will never load for partial driver situations.
> This is also a deal breaker. 

_DEP doesn't mean a hard dependency AFAICS.  It is about ordering, not about
presence, at least as specified currently.

> Why would you break some other feature if your OS doesn't support RAS as an
> example?
> 
> Given all these lose bindings and no driver association, where do we go
> from here?
> 
> I consider GED as a light version of Embedded controller (EC) implementation. 

No, it is not.

It is more of a generalization of the GPE/SCI mechanism in order to make it
possible to cover things different from GPIO (which already is covered by
_AEI).

> How is this problem solved for EC as it has the same problem?

It doesn't.  The EC relies on the GPE/SCI mechanism to be there and that is
always present.

Thanks,
Rafael