diff mbox

[v6,17/17] Documentation: ACPI for ARM64

Message ID 1420368918-5086-18-git-send-email-hanjun.guo@linaro.org (mailing list archive)
State New, archived
Headers show

Commit Message

Hanjun Guo Jan. 4, 2015, 10:55 a.m. UTC
From: Graeme Gregory <graeme.gregory@linaro.org>

Add documentation for the guidelines of how to use ACPI
on ARM64.

Reviewed-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Graeme Gregory <graeme.gregory@linaro.org>
Signed-off-by: Al Stone <al.stone@linaro.org>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
---
 Documentation/arm64/arm-acpi.txt | 327 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 327 insertions(+)
 create mode 100644 Documentation/arm64/arm-acpi.txt

Comments

Christoffer Dall Jan. 19, 2015, 8:33 p.m. UTC | #1
Hi,

On Sun, Jan 04, 2015 at 06:55:18PM +0800, Hanjun Guo wrote:
> From: Graeme Gregory <graeme.gregory@linaro.org>
> 
> Add documentation for the guidelines of how to use ACPI
> on ARM64.
> 
> Reviewed-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
> Signed-off-by: Graeme Gregory <graeme.gregory@linaro.org>
> Signed-off-by: Al Stone <al.stone@linaro.org>
> Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
> ---
>  Documentation/arm64/arm-acpi.txt | 327 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 327 insertions(+)
>  create mode 100644 Documentation/arm64/arm-acpi.txt
> 
> diff --git a/Documentation/arm64/arm-acpi.txt b/Documentation/arm64/arm-acpi.txt
> new file mode 100644
> index 0000000..21e7020
> --- /dev/null
> +++ b/Documentation/arm64/arm-acpi.txt
> @@ -0,0 +1,327 @@
> +ACPI on ARMv8 Servers
> +---------------------
> +ACPI can be used for ARMv8 general purpose servers designed to follow
> +the ARM SBSA (Server Base System Architecture) and SBBR (Server Base
> +Boot Requirements) specifications, currently available to those with
> +an ARM login at http://silver.arm.com.
> +
> +The ARMv8 kernel implements the reduced hardware model of ACPI version
> +5.1 and later.  Links to the specification and all external documents
> +it refers to are managed by the UEFI Forum.  The specification is
> +available at http://www.uefi.org/specifications and external documents
> +can be found via http://www.uefi.org/acpi.
> +
> +If an ARMv8 system does not meet the requirements of the SBSA, or cannot
> +be described using the mechanisms defined in the required ACPI specifications,
> +then it is likely that Device Tree (DT) is more suitable than ACPI for the
> +hardware.
> +
> +
> +Relationship with Device Tree
> +-----------------------------
> +ACPI support in drivers and subsystems for ARMv8 should never be mutually
> +exclusive with DT support at compile time.
> +
> +At boot time the kernel will only use one description method depending on
> +parameters passed from the bootloader (including kernel bootargs).
> +
> +Regardless of whether DT or ACPI is used, the kernel must always be capable
> +of booting with either scheme (in kernels with both schemes enabled at compile
> +time).
> +
> +
> +Booting using ACPI tables
> +-------------------------
> +The only defined method for passing ACPI tables to the kernel on ARMv8
> +is via the UEFI system configuration table.
> +

This is a bit concerning for the approach we are currently taking to
support ACPI on Xen [1].

Background: Xen is a tiny hypervisor which cannot parse the DSDT or any
other non-static table. Xen relies on Linux in Dom0 to manage most
(basically everything except the GIC, serial port, SMMU, and timers)
hardware resources and relies on Dom0 to parse the ACPI tables.

While Xen itself is typically booted by UEFI and finds RSDP through the
UEFI system table, Xen does NOT run another UEFI instance to boot Dom0
nor does it pass through the presence of UEFI to Dom0 in any way.
Instead, it just created a DT (with modifications concerning the
hardware mentioned above) and boots Linux in Dom0 directly (on x86 any
required UEFI call is performed through paravirtualized hypercalls).

In the case of ACPI, Xen adds a property to the chosen node and
populates it with the RDSP in Dom0's address space.  Patches have not
been posted yet, but we do have a working prototype of the
implementation and the Linux changes are minimal to support this
approach.  Parth Dixit (cc'ed) can provide more details on this if
needed/wanted.

With the current wording in this patch, it seems Xen is forced to
emulate EFI or run a full UEFI instance for Dom0 which results in a
really heavy boot sequnce for Xen.  Should we be concerned about this
use case at this point and does anyone have suggestions on how to solve
this?

[1]: https://wiki.linaro.org/LEG/Engineering/Virtualization/ACPI_on_Xen

Thanks,
-Christoffer
Hanjun Guo Jan. 21, 2015, 12:37 p.m. UTC | #2
Hi Christoffer,

Sorry for the late reply, I got no answer yet but
with one question below.

On 2015?01?20? 04:33, Christoffer Dall wrote:
> Hi,
>
> On Sun, Jan 04, 2015 at 06:55:18PM +0800, Hanjun Guo wrote:
>> From: Graeme Gregory <graeme.gregory@linaro.org>
>>
>> Add documentation for the guidelines of how to use ACPI
>> on ARM64.
>>
>> Reviewed-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
>> Signed-off-by: Graeme Gregory <graeme.gregory@linaro.org>
>> Signed-off-by: Al Stone <al.stone@linaro.org>
>> Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
>> ---
>>   Documentation/arm64/arm-acpi.txt | 327 +++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 327 insertions(+)
>>   create mode 100644 Documentation/arm64/arm-acpi.txt
>>
>> diff --git a/Documentation/arm64/arm-acpi.txt b/Documentation/arm64/arm-acpi.txt
>> new file mode 100644
>> index 0000000..21e7020
>> --- /dev/null
>> +++ b/Documentation/arm64/arm-acpi.txt
>> @@ -0,0 +1,327 @@
>> +ACPI on ARMv8 Servers
>> +---------------------
>> +ACPI can be used for ARMv8 general purpose servers designed to follow
>> +the ARM SBSA (Server Base System Architecture) and SBBR (Server Base
>> +Boot Requirements) specifications, currently available to those with
>> +an ARM login at http://silver.arm.com.
>> +
>> +The ARMv8 kernel implements the reduced hardware model of ACPI version
>> +5.1 and later.  Links to the specification and all external documents
>> +it refers to are managed by the UEFI Forum.  The specification is
>> +available at http://www.uefi.org/specifications and external documents
>> +can be found via http://www.uefi.org/acpi.
>> +
>> +If an ARMv8 system does not meet the requirements of the SBSA, or cannot
>> +be described using the mechanisms defined in the required ACPI specifications,
>> +then it is likely that Device Tree (DT) is more suitable than ACPI for the
>> +hardware.
>> +
>> +
>> +Relationship with Device Tree
>> +-----------------------------
>> +ACPI support in drivers and subsystems for ARMv8 should never be mutually
>> +exclusive with DT support at compile time.
>> +
>> +At boot time the kernel will only use one description method depending on
>> +parameters passed from the bootloader (including kernel bootargs).
>> +
>> +Regardless of whether DT or ACPI is used, the kernel must always be capable
>> +of booting with either scheme (in kernels with both schemes enabled at compile
>> +time).
>> +
>> +
>> +Booting using ACPI tables
>> +-------------------------
>> +The only defined method for passing ACPI tables to the kernel on ARMv8
>> +is via the UEFI system configuration table.
>> +
>
> This is a bit concerning for the approach we are currently taking to
> support ACPI on Xen [1].
>
> Background: Xen is a tiny hypervisor which cannot parse the DSDT or any
> other non-static table. Xen relies on Linux in Dom0 to manage most
> (basically everything except the GIC, serial port, SMMU, and timers)
> hardware resources and relies on Dom0 to parse the ACPI tables.
>
> While Xen itself is typically booted by UEFI and finds RSDP through the
> UEFI system table, Xen does NOT run another UEFI instance to boot Dom0
> nor does it pass through the presence of UEFI to Dom0 in any way.
> Instead, it just created a DT (with modifications concerning the
> hardware mentioned above) and boots Linux in Dom0 directly (on x86 any
> required UEFI call is performed through paravirtualized hypercalls).
>
> In the case of ACPI, Xen adds a property to the chosen node and
> populates it with the RDSP in Dom0's address space.  Patches have not

Sorry I'm not familiar with hypervisor, I have question here about
x86 on xen, how ACPI works on XEN for x86?

Thanks
Hanjun
Stefano Stabellini Jan. 21, 2015, 1:03 p.m. UTC | #3
On Wed, 21 Jan 2015, Hanjun Guo wrote:
> Hi Christoffer,
> 
> Sorry for the late reply, I got no answer yet but
> with one question below.
> 
> On 2015?01?20? 04:33, Christoffer Dall wrote:
> > Hi,
> > 
> > On Sun, Jan 04, 2015 at 06:55:18PM +0800, Hanjun Guo wrote:
> > > From: Graeme Gregory <graeme.gregory@linaro.org>
> > > 
> > > Add documentation for the guidelines of how to use ACPI
> > > on ARM64.
> > > 
> > > Reviewed-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
> > > Signed-off-by: Graeme Gregory <graeme.gregory@linaro.org>
> > > Signed-off-by: Al Stone <al.stone@linaro.org>
> > > Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
> > > ---
> > >   Documentation/arm64/arm-acpi.txt | 327
> > > +++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 327 insertions(+)
> > >   create mode 100644 Documentation/arm64/arm-acpi.txt
> > > 
> > > diff --git a/Documentation/arm64/arm-acpi.txt
> > > b/Documentation/arm64/arm-acpi.txt
> > > new file mode 100644
> > > index 0000000..21e7020
> > > --- /dev/null
> > > +++ b/Documentation/arm64/arm-acpi.txt
> > > @@ -0,0 +1,327 @@
> > > +ACPI on ARMv8 Servers
> > > +---------------------
> > > +ACPI can be used for ARMv8 general purpose servers designed to follow
> > > +the ARM SBSA (Server Base System Architecture) and SBBR (Server Base
> > > +Boot Requirements) specifications, currently available to those with
> > > +an ARM login at http://silver.arm.com.
> > > +
> > > +The ARMv8 kernel implements the reduced hardware model of ACPI version
> > > +5.1 and later.  Links to the specification and all external documents
> > > +it refers to are managed by the UEFI Forum.  The specification is
> > > +available at http://www.uefi.org/specifications and external documents
> > > +can be found via http://www.uefi.org/acpi.
> > > +
> > > +If an ARMv8 system does not meet the requirements of the SBSA, or cannot
> > > +be described using the mechanisms defined in the required ACPI
> > > specifications,
> > > +then it is likely that Device Tree (DT) is more suitable than ACPI for
> > > the
> > > +hardware.
> > > +
> > > +
> > > +Relationship with Device Tree
> > > +-----------------------------
> > > +ACPI support in drivers and subsystems for ARMv8 should never be mutually
> > > +exclusive with DT support at compile time.
> > > +
> > > +At boot time the kernel will only use one description method depending on
> > > +parameters passed from the bootloader (including kernel bootargs).
> > > +
> > > +Regardless of whether DT or ACPI is used, the kernel must always be
> > > capable
> > > +of booting with either scheme (in kernels with both schemes enabled at
> > > compile
> > > +time).
> > > +
> > > +
> > > +Booting using ACPI tables
> > > +-------------------------
> > > +The only defined method for passing ACPI tables to the kernel on ARMv8
> > > +is via the UEFI system configuration table.
> > > +
> > 
> > This is a bit concerning for the approach we are currently taking to
> > support ACPI on Xen [1].
> > 
> > Background: Xen is a tiny hypervisor which cannot parse the DSDT or any
> > other non-static table. Xen relies on Linux in Dom0 to manage most
> > (basically everything except the GIC, serial port, SMMU, and timers)
> > hardware resources and relies on Dom0 to parse the ACPI tables.
> > 
> > While Xen itself is typically booted by UEFI and finds RSDP through the
> > UEFI system table, Xen does NOT run another UEFI instance to boot Dom0
> > nor does it pass through the presence of UEFI to Dom0 in any way.
> > Instead, it just created a DT (with modifications concerning the
> > hardware mentioned above) and boots Linux in Dom0 directly (on x86 any
> > required UEFI call is performed through paravirtualized hypercalls).
> > 
> > In the case of ACPI, Xen adds a property to the chosen node and
> > populates it with the RDSP in Dom0's address space.  Patches have not
> 
> Sorry I'm not familiar with hypervisor, I have question here about
> x86 on xen, how ACPI works on XEN for x86?
 
The whole thing works differently on x86: on native x86 the OS finds the
RDSP by scanning a couple of memory regions looking for the string "RSD
PTR".

Xen and Dom0 both do that independently (Dom0 can map that memory
region).
Christoffer Dall Jan. 21, 2015, 4:04 p.m. UTC | #4
On Wed, Jan 21, 2015 at 08:37:00PM +0800, Hanjun Guo wrote:
> Hi Christoffer,
> 
> Sorry for the late reply, I got no answer yet but
> with one question below.

no worries, I think the discussion has moved to Stefano's e-mail:
http://lists.infradead.org/pipermail/linux-arm-kernel/2015-January/318335.html

-Christoffer
diff mbox

Patch

diff --git a/Documentation/arm64/arm-acpi.txt b/Documentation/arm64/arm-acpi.txt
new file mode 100644
index 0000000..21e7020
--- /dev/null
+++ b/Documentation/arm64/arm-acpi.txt
@@ -0,0 +1,327 @@ 
+ACPI on ARMv8 Servers
+---------------------
+ACPI can be used for ARMv8 general purpose servers designed to follow
+the ARM SBSA (Server Base System Architecture) and SBBR (Server Base
+Boot Requirements) specifications, currently available to those with
+an ARM login at http://silver.arm.com.
+
+The ARMv8 kernel implements the reduced hardware model of ACPI version
+5.1 and later.  Links to the specification and all external documents
+it refers to are managed by the UEFI Forum.  The specification is
+available at http://www.uefi.org/specifications and external documents
+can be found via http://www.uefi.org/acpi.
+
+If an ARMv8 system does not meet the requirements of the SBSA, or cannot
+be described using the mechanisms defined in the required ACPI specifications,
+then it is likely that Device Tree (DT) is more suitable than ACPI for the
+hardware.
+
+
+Relationship with Device Tree
+-----------------------------
+ACPI support in drivers and subsystems for ARMv8 should never be mutually
+exclusive with DT support at compile time.
+
+At boot time the kernel will only use one description method depending on
+parameters passed from the bootloader (including kernel bootargs).
+
+Regardless of whether DT or ACPI is used, the kernel must always be capable
+of booting with either scheme (in kernels with both schemes enabled at compile
+time).
+
+
+Booting using ACPI tables
+-------------------------
+The only defined method for passing ACPI tables to the kernel on ARMv8
+is via the UEFI system configuration table.
+
+Processing of ACPI tables may be disabled by passing acpi=off on the kernel
+command line; this is the default behavior if both ACPI and DT tables are
+present.  If acpi=force is used, the kernel will ONLY use device configuration
+information contained in the ACPI tables if those tables are available.
+
+In order for the kernel to load and use ACPI tables, the UEFI implementation
+MUST set the ACPI_20_TABLE_GUID to point to the RSDP table (the table with
+the ACPI signature "RSD PTR ").  If this pointer is incorrect and acpi=force
+is used, the kernel will disable ACPI and try to use DT to boot instead.
+
+If the pointer to the RSDP table is correct, the table will be mapped into
+the kernel by the ACPI core, using the address provided by UEFI.
+
+The ACPI core will then locate and map in all other ACPI tables provided by
+using the addresses in the RSDP table to find the XSDT (eXtended System
+Description Table).  The XSDT in turn provides the addresses to all other
+ACPI tables provided by the system firmware; the ACPI core will then traverse
+this table and map in the tables listed.
+
+The ACPI core will ignore any provided RSDT (Root System Description Table).
+RSDTs have been deprecated and are ignored on arm64 since they only allow
+for 32-bit addresses.
+
+Further, the ACPI core will only use the 64-bit address fields in the FADT
+(Fixed ACPI Description Table).  Any 32-bit address fields in the FADT will
+be ignored on arm64.
+
+Hardware reduced mode (see Section 4.1 of the ACPI 5.1 specification) will
+be enforced by the ACPI core on arm64.  Doing so allows the ACPI core to
+run less complex code since it no longer has to provide support for legacy
+hardware from other architectures.
+
+For the ACPI core to operate properly, and in turn provide the information
+the kernel needs to configure devices, it expects to find the following
+tables (all section numbers refer to the ACPI 5.1 specfication):
+
+    -- RSDP (Root System Description Pointer), section 5.2.5
+
+    -- XSDT (eXtended System Description Table), section 5.2.8
+
+    -- FADT (Fixed ACPI Description Table), section 5.2.9
+
+    -- DSDT (Differentiated System Description Table), section
+       5.2.11.1
+
+    -- MADT (Multiple APIC Description Table), section 5.2.12
+
+    -- GTDT (Generic Timer Description Table), section 5.2.24
+
+    -- If PCI is supported, the MCFG (Memory mapped ConFiGuration
+       Table), section 5.2.6, specifically Table 5-31.
+
+If the above tables are not all present, the kernel may or may not be
+able to boot properly since it may not be able to configure all of the
+devices available.
+
+
+ACPI Detection
+--------------
+Drivers should determine their probe() type by checking for a null
+value for ACPI_HANDLE, or checking .of_node, or other information in
+the device structure.  This is detailed further in the "Driver 
+Recommendations" section.
+
+In non-driver code, if the presence of ACPI needs to be detected at
+runtime, then check the value of acpi_disabled. If CONFIG_ACPI is not
+set, acpi_disabled will always be 1.
+
+
+Device Enumeration
+------------------
+Device descriptions in ACPI should use standard recognized ACPI interfaces.
+These may contain less information than is typically provided via a Device
+Tree description for the same device.  This is also one of the reasons that
+ACPI can be useful -- the driver takes into account that it may have less
+detailed information about the device and uses sensible defaults instead.
+If done properly in the driver, the hardware can change and improve over
+time without the driver having to change at all.
+
+Clocks provide an excellent example.  In DT, clocks need to be specified
+and the drivers need to take them into account.  In ACPI, the assumption
+is that UEFI will leave the device in a reasonable default state, including
+any clock settings.  If for some reason the driver needs to change a clock
+value, this can be done in an ACPI method; all the driver needs to do is
+invoke the method and not concern itself with what the method needs to do
+to change the clock.  Changing the hardware can then take place over time
+by changing what the ACPI method does, and not the driver.
+
+ACPI drivers should only look at one specific ASL object -- the _DSD object
+-- for device driver parameters (known in DT as "bindings", or "Device
+Properties" in ACPI).  DT bindings also will be reviewed before used.  The UEFI
+Forum provides a mechanism for registering such bindings [URL TBD by ASWG]
+so that they may be used on any operating system supporting ACPI.  Device
+properties that have not been registered with the UEFI Forum should not be
+used.
+
+Drivers should look for device properties in the _DSD object ONLY; the _DSD
+object is described in the ACPI specification section 6.2.5, but more
+specifically, use the _DSD Device Properties UUID:
+
+   -- UUID: daffd814-6eba-4d8c-8a91-bc9bbf4aa301
+   
+   -- http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf
+
+The kernel has an interface for looking up device properties in a manner
+independent of whether DT or ACPI is being used and that interface should
+be used; it can eliminate some duplication of code paths in driver probing
+functions and discourage divergence between DT bindings and ACPI device
+properties.
+
+ACPI tables are described with a formal language called ASL, the ACPI
+Source Language (section 19 of the specification).  This means that there
+are always multiple ways to describe the same thing -- including device
+properties.  For example, device properties could use an ASL construct
+that looks like this: Name(KEY0, "value0").  An ACPI device driver would
+then retrieve the value of the property by evaluating the KEY0 object.
+However, using Name() this way has multiple problems: (1) ACPI limits
+names ("KEY0") to four characters unlike DT; (2) there is no industry
+wide registry that maintains a list of names, minimzing re-use; (3)
+there is also no registry for the definition of property values ("value0"),
+again making re-use difficult; and (4) how does one maintain backward
+compatibility as new hardware comes out?  The _DSD method was created
+to solve precisely these sorts of problems; Linux drivers should ALWAYS
+use the _DSD method for device properties and nothing else.
+
+The _DSM object (ACPI Section 9.14.1) could also be used for conveying
+device properties to a driver.  Linux drivers should only expect it to
+be used if _DSD cannot represent the data required, and there is no way
+to create a new UUID for the _DSD object.  Note that there is even less
+regulation of the use of _DSM than there is of _DSD.  Drivers that depend
+on the contents of _DSM objects will be more difficult to maintain over
+time because of this.
+
+The _DSD object is a very flexible mechanism in ACPI, as are the registered
+Device Properties.  This flexibility allows _DSD to cover more than just the
+generic server case and care should be taken in device drivers not to expect
+it to replicate highly specific embedded behaviour from DT.
+
+Both DT bindings and ACPI device properties for device drivers have review
+processes.  Use them.  And, before creating new device properties, check to
+be sure that they have not been defined before and either registered in the
+Linux kernel documentation or the UEFI Forum.  If the device drivers supports
+ACPI and DT, please make sure the device properties are consistent in both
+places.
+
+
+Programmable Power Control Resources
+------------------------------------
+Programmable power control resources include such resources as voltage/current
+providers (regulators) and clock sources.
+
+The kernel assumes that power control of these resources is represented with
+Power Resource Objects (ACPI section 7.1).  The ACPI core will then handle
+correctly enabling and disabling resources as they are needed.  In order to
+get that to work, ACPI assumes each device has defined D-states and that these
+can be controlled through the optional ACPI methods _PS0, _PS1, _PS2, and _PS3;
+in ACPI, _PS0 is the method to invoke to turn a device full on, and _PS3 is for
+turning a device full off.
+
+There are two options for using those Power Resources.
+   -- be managed in _PSx routine which gets called on entry to Dx.
+
+   -- be declared separately as power resources with their own _ON and _OFF
+      methods.  They are then tied back to D-states for a particular device
+      via _PRx which specifies which power resources a device needs to be on
+      while in Dx.  Kernel then tracks number of devices using a power resource
+      and calls _ON/_OFF as needed.
+
+The kernel ACPI code will also assume that the _PSx methods follow the normal
+ACPI rules for such methods:
+
+   -- If either _PS0 or _PS3 is implemented, then the other method must also
+      be implemented.
+
+   -- If a device requires usage or setup of a power resource when on, the ASL
+      should organize that it is allocated/enabled using the _PS0 method.
+
+   -- Resources allocated or enabled in the _PS0 method should be disabled
+      or de-allocated in the _PS3 method.
+
+   -- Firmware will leave the resources in a reasonable state before handing
+      over control to the kernel.
+
+Such code in _PSx methods will of course be very platform specific.  But,
+this allows the driver to abstract out the interface for operating the device
+and avoid having to read special non-standard values from ACPI tables. Further,
+abstracting the use of these resources allows the hardware to change over time
+without requiring updates to the driver.
+
+
+Clocks
+------
+ACPI makes the assumption that clocks are initialized by the firmware -- 
+UEFI, in this case -- to some working value before control is handed over
+to the kernel.  This has implications for devices such as UARTs, or SoC-driven
+LCD displays, for example.
+
+When the kernel boots, the clock is assumed to be set to a reasonable
+working value.  If for some reason the frequency needs to change -- e.g.,
+throttling for power management -- the device driver should expect that 
+process to be abstracted out into some ACPI method that can be invoked 
+(please see the ACPI specification for further recommendations on standard
+methods to be expected) except CPU clocks where CPPC provides a much richer
+interface instead of some method.  If it is not, there is no direct way for
+ACPI to control the clocks.
+
+
+Driver Recommendations
+----------------------
+DO NOT remove any DT handling when adding ACPI support for a driver.  The
+same device may be used on many different systems.
+
+DO try to structure the driver so that it is data-driven.  That is, set up
+a struct containing internal per-device state based on defaults and whatever
+else must be discovered by the driver probe function.  Then, have the rest
+of the driver operate off of the contents of that struct.  Doing so should
+allow most divergence between ACPI and DT functionality to be kept local to
+the probe function instead of being scattered throughout the driver.  For
+example:
+
+static int device_probe_dt(struct platform_device *pdev)
+{
+	/* DT specific functionality */
+	...
+}
+
+static int device_probe_acpi(struct platform_device *pdev)
+{
+	/* ACPI specific functionality */
+	...
+}
+
+static int device_probe(stuct platform_device *pdev)
+{
+	...
+	struct device_node node = pdev->dev.of_node;
+	...
+
+	if (node)
+		ret = device_probe_dt(pdev);
+	else if (ACPI_HANDLE(&pdev->dev))
+		ret = device_probe_acpi(pdev);
+	else
+		/* other initialization */
+		...
+	/* Continue with any generic probe operations */
+	...
+}
+
+DO keep the MODULE_DEVICE_TABLE entries together in the driver to make it
+clear the different names the driver is probed for, both from DT and from
+ACPI:
+
+static struct of_device_id virtio_mmio_match[] = {
+        { .compatible = "virtio,mmio", },
+        { }
+};
+MODULE_DEVICE_TABLE(of, virtio_mmio_match);
+
+static const struct acpi_device_id virtio_mmio_acpi_match[] = {
+        { "LNRO0005", },
+        { }
+};
+MODULE_DEVICE_TABLE(acpi, virtio_mmio_acpi_match);
+
+
+ASWG
+----
+The following areas are not yet fully defined for ARM in the 5.1 version
+of the ACPI specification and are expected to be worked through in the 
+UEFI ACPI Specification Working Group (ASWG):
+
+   -- ACPI based CPU topology
+   -- ACPI based CPU idle control
+   -- ACPI based SMMU and its IO topology
+   -- ITS support for GIC in MADT
+
+Participation in this group is open to all UEFI members.  Please see
+http://www.uefi.org/workinggroup for details on group membership.
+
+It is the intent of the ARMv8 ACPI kernel code to follow the ACPI specification
+as closely as possible, and to only implement functionality that complies with
+the released standards from UEFI ASWG.  As a practical matter, there will be
+vendors that provide bad ACPI tables or violate the standards in some way.
+If this is because of errors, quirks and fixups may be necessary, but will
+be avoided if possible.  If there are features missing from ACPI that preclude
+it from being used on a platform, ECRs (Engineering Change Requests) should be
+submitted to ASWG and go through the normal approval process; for those that
+are not UEFI members, many other members of the Linux community are and would
+likely be willing to assist in submitting ECRs.