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List:       linux-spi
Subject:    Re: [PATCH v2 2/4] ACPI / property: Support Apple _DSM properties
From:       Mika Westerberg <mika.westerberg () linux ! intel ! com>
Date:       2017-06-29 7:45:38
Message-ID: 20170629074538.GA629 () lahna ! fi ! intel ! com
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On Wed, Jun 28, 2017 at 07:20:19PM +0200, Lukas Wunner wrote:
> While the rest of the world has standardized on _DSD as the way to store
> device properties in AML (introduced with ACPI 5.1 in 2014), Apple has
> been using a custom _DSM to achieve the same for much longer (ever since
> they switched from DeviceTree-based PowerPC to Intel in 2005, verified
> with MacOS X 10.4.11).
> 
> The theory of operation on macOS is as follows:  AppleACPIPlatform.kext
> invokes mergeEFIproperties() and mergeDSMproperties() for each device to
> merge properties conveyed by EFI drivers as well as properties stored in
> AML into the I/O Kit registry from which they can be retrieved by
> drivers.  We've been supporting EFI properties since commit 58c5475aba67
> ("x86/efi: Retrieve and assign Apple device properties").  The present
> commit adds support for _DSM properties, thereby completing our support
> for Apple device properties.  The _DSM properties are made available
> under the primary fwnode, the EFI properties under the secondary fwnode.
> So for devices which possess both property types, they can all be
> elegantly accessed with the uniform API in <linux/property.h>.
> 
> Until recently we had no need to support _DSM properties, they contained
> only uninteresting garbage.  The situation has changed with MacBooks and
> MacBook Pros introduced since 2015:  Their keyboard is attached with SPI
> instead of USB and the _CRS data which is necessary to initialize the
> spi driver only contains valid information if OSPM responds "false" to
> _OSI("Darwin").  If OSPM responds "true", _CRS is empty and the spi
> driver fails to initialize.  The rationale is very simple, Apple only
> cares about macOS and Windows:  On Windows, _CRS contains valid data,
> whereas on macOS it is empty.  Instead, macOS gleans the necessary data
> from the _DSM properties.
> 
> Since Linux deliberately defaults to responding "true" to _OSI("Darwin"),
> we need to emulate macOS' behaviour by initializing the spi driver with
> data returned by the _DSM.
> 
> An out-of-tree driver for the SPI keyboard exists which currently binds
> to the ACPI device, invokes the _DSM, parses the returned package and
> instantiates an SPI device with the data gleaned from the _DSM:
> https://github.com/cb22/macbook12-spi-driver/commit/9a416d699ef4
> https://github.com/cb22/macbook12-spi-driver/commit/0c34936ed9a1
> 
> By adding support for Apple's _DSM properties in generic ACPI code, the
> out-of-tree driver will be able to register as a regular SPI device,
> significantly reducing its amount of code and improving its chances to
> be mainlined.
> 
> The SPI keyboard will not be the only user of this commit:  E.g. on the
> MacBook8,1, the UART-attached Bluetooth device likewise returns empty
> _CRS data if OSPM returns "true" to _OSI("Darwin").
> 
> The _DSM returns a Package whose format unfortunately deviates slightly
> from the _DSD spec:  The properties are marshalled up in a single Package
> as alternating key/value elements, unlike _DSD which stores them as a
> Package of 2-element Packages.  The present commit therefore converts
> the Package to _DSD format and the ACPI core can then treat the data as
> if Apple would follow the standard.
> 
> Well, except for one small annoyance:  The properties returned by the
> _DSM only ever have one of two types, Integer or Buffer.  The former is
> retrievable as usual with device_property_read_u64(), but the latter is
> not part of the _DSD spec and it is not possible to retrieve Buffer
> properties with the device_property_read_*() functions due to the type
> checking performed in drivers/acpi/property.c.  It is however possible
> to retrieve them with acpi_dev_get_property().  Apple is using the
> Buffer type somewhat sloppily to store null-terminated strings but also
> integers.  The real data type is not distinguishable by the ACPI core
> and the onus is on the caller to use the contents of the Buffer in an
> appropriate way.
> 
> In case Apple moves to _DSD in the future, this commit first checks for
> _DSD and falls back to _DSM only if _DSD is not found.
> 
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: Mika Westerberg <mika.westerberg@linux.intel.com>

Andy had some comments which I think you should consider. Regardless of
that this looks good to me. Also I have one of those new Macs with SPI
connected keyboard so happy to see the support getting into mainline ;-)

Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
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