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

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Date:   Thu, 11 May 2017 02:58:05 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Sinan Kaya <okaya@...eaurora.org>
Cc:     Lukas Wunner <lukas@...ner.de>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Timur Tabi <timur@...eaurora.org>, Len Brown <lenb@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ACPI / GED: use late init to allow other drivers init

On Thursday, April 27, 2017 10:32:07 PM Sinan Kaya wrote:
> On 4/25/2017 12:24 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@...eaurora.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.
> > 
> > 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. 
> > 
> > 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. 
> > 
> > How is this problem solved for EC as it has the same problem?
> > 
> 
> This recommendation came from Timur. I wanted to see how everybody feels about it.
> 
> When GED driver makes an AML call and the driver on the right side of the picture
> is not present, GED driver gets an ACPI error return code. 

This means that _EVT evaluation failed, right?

How does the _EVT in question look like?  What does make it depend on the
other drivers to be present in particular?

Thanks,
Rafael