linux-kernel - RE: [PATCH v7 5/5] Driver: VMBus: Add Devicetree support

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Message-ID: <BYAPR21MB1688FD8EA30E876F22560645D7B89@BYAPR21MB1688.namprd21.prod.outlook.com>
Date:   Sun, 12 Mar 2023 13:08:02 +0000
From:   "Michael Kelley (LINUX)" <mikelley@...rosoft.com>
To:     Saurabh Singh Sengar <ssengar@...ux.microsoft.com>,
        Wei Liu <wei.liu@...nel.org>
CC:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        "daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "lenb@...nel.org" <lenb@...nel.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>
Subject: RE: [PATCH v7 5/5] Driver: VMBus: Add Devicetree support

From: Saurabh Singh Sengar <ssengar@...ux.microsoft.com> Sent: Thursday, March 9, 2023 9:35 PM
> 
> On Thu, Mar 09, 2023 at 09:16:25PM +0000, Wei Liu wrote:
> > On Thu, Feb 23, 2023 at 03:29:05AM -0800, Saurabh Sengar wrote:

[snip]

> > >
> > >  static int vmbus_platform_driver_probe(struct platform_device *pdev)
> > >  {
> > > +#ifdef CONFIG_ACPI
> > >  	return vmbus_acpi_add(pdev);
> > > +#endif
> >
> > Please use #else here.
> >
> > > +	return vmbus_device_add(pdev);
> >
> > Is there going to be a configuration that ACPI and OF are available at
> > the same time? I don't see they are marked as mutually exclusive in the
> > proposed KConfig.
> 
> Initially, the device tree functions was included in "#else" section after
> the "#ifdef CONFIG_ACPI" section. However, it was subsequently removed to
> increase the coverage for CI builds.
> 
> Ref: https://lkml.org/lkml/2023/2/7/910
> 

I think the point here is that it is possible (and even likely on ARM64?) to
build a kernel where CONFIG_ACPI and CONFIG_OF are both "Y".   So the
code for ACPI and OF is compiled and present in the kernel image.  However,
for a particular Linux boot on a particular hardware or virtual platform,
only one of the two will be enabled.   I specifically mention a particular Linux
kernel boot because there's a kernel boot line option that can force disabling
ACPI.  Ideally, the VMBus code should work if both CONFIG_ACPI and
CONFIG_OF are enabled in the kernel image, and it would determine at
runtime which to use. This approach meets the goals Rob spells out.

There's an exported global variable "acpi_disabled" that is set correctly
depending on CONFIG_ACPI and the kernel boot line option (and perhaps if
ACPI is not detected at runtime during boot -- I didn't check all the details).
So the above could be written as:

	if (!acpi_disabled)
		return vmbus_acpi_add(pdev); 
	else
		return vmbus_device_add(pdev);

This avoids the weird "two return statements in a row" while preferring
ACPI over OF if ACPI is enabled for a particular boot of Linux.

I'm not sure if you'll need a stub for vmbus_acpi_add() when CONFIG_ACPI=n.
In that case, acpi_disabled is #defined to be 1, so the compiler should just
drop the call to vmbus_acpi_add() entirely and no stub will be needed.  But
you'll need to confirm.

Also just confirming, it looks like vmbus_device_add() compiles correctly if
CONFIG_OF=n.  There are enough stubs in places so that you don't need an
#ifdef CONFIG_OF around vmbus_device_add() like is needed for
vmbus_acpi_add().

> > >
> > > +static const __maybe_unused struct of_device_id vmbus_of_match[] = {
> > > +	{
> > > +		.compatible = "microsoft,vmbus",
> > > +	},
> > > +	{
> > > +		/* sentinel */
> > > +	},
> > > +};
> > > +MODULE_DEVICE_TABLE(of, vmbus_of_match);
> > > +
> > > +#ifdef CONFIG_ACPI
> > >  static const struct acpi_device_id vmbus_acpi_device_ids[] = {
> > >  	{"VMBUS", 0},
> > >  	{"VMBus", 0},
> > >  	{"", 0},
> > >  };
> > >  MODULE_DEVICE_TABLE(acpi, vmbus_acpi_device_ids);
> > > +#endif

Couldn't the bracketing #ifdef be dropped and add __maybe_unused, just
as you've done with vmbus_of_match?   ACPI_PTR() is defined to return NULL
if CONFIG_ACPI=n, just like with of_match_ptr() and CONFIG_OF.

> > >
> > >  /*
> > >   * Note: we must use the "no_irq" ops, otherwise hibernation can not work with
> > > @@ -2677,6 +2729,7 @@ static struct platform_driver vmbus_platform_driver = {
> > >  	.driver = {
> > >  		.name = "vmbus",
> > >  		.acpi_match_table = ACPI_PTR(vmbus_acpi_device_ids),
> > > +		.of_match_table = of_match_ptr(vmbus_of_match),
> > >  		.pm = &vmbus_bus_pm,
> > >  		.probe_type = PROBE_FORCE_SYNCHRONOUS,
> > >  	}
> > > --
> > > 2.34.1
> > >