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ACPI / enumeration: Document the rules regarding the PRP0001 device ID
Document how the ACPI device enumeration code uses the special PRP0001 device ID. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com> Reviewed-by: Hanjun Guo <hanjun.guo@linaro.org> Reviewed-by: Darren Hart <dvhart@linux.intel.com>
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@ -359,3 +359,54 @@ the id should be set like:
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The ACPI id "XYZ0001" is then used to lookup an ACPI device directly under
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the MFD device and if found, that ACPI companion device is bound to the
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resulting child platform device.
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Device Tree namespace link device ID
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The Device Tree protocol uses device indentification based on the "compatible"
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property whose value is a string or an array of strings recognized as device
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identifiers by drivers and the driver core. The set of all those strings may be
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regarded as a device indentification namespace analogous to the ACPI/PNP device
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ID namespace. Consequently, in principle it should not be necessary to allocate
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a new (and arguably redundant) ACPI/PNP device ID for a devices with an existing
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identification string in the Device Tree (DT) namespace, especially if that ID
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is only needed to indicate that a given device is compatible with another one,
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presumably having a matching driver in the kernel already.
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In ACPI, the device identification object called _CID (Compatible ID) is used to
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list the IDs of devices the given one is compatible with, but those IDs must
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belong to one of the namespaces prescribed by the ACPI specification (see
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Section 6.1.2 of ACPI 6.0 for details) and the DT namespace is not one of them.
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Moreover, the specification mandates that either a _HID or an _ADR identificaion
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object be present for all ACPI objects representing devices (Section 6.1 of ACPI
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6.0). For non-enumerable bus types that object must be _HID and its value must
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be a device ID from one of the namespaces prescribed by the specification too.
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The special DT namespace link device ID, PRP0001, provides a means to use the
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existing DT-compatible device identification in ACPI and to satisfy the above
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requirements following from the ACPI specification at the same time. Namely,
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if PRP0001 is returned by _HID, the ACPI subsystem will look for the
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"compatible" property in the device object's _DSD and will use the value of that
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property to identify the corresponding device in analogy with the original DT
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device identification algorithm. If the "compatible" property is not present
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or its value is not valid, the device will not be enumerated by the ACPI
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subsystem. Otherwise, it will be enumerated automatically as a platform device
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(except when an I2C or SPI link from the device to its parent is present, in
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which case the ACPI core will leave the device enumeration to the parent's
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driver) and the identification strings from the "compatible" property value will
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be used to find a driver for the device along with the device IDs listed by _CID
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(if present).
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Analogously, if PRP0001 is present in the list of device IDs returned by _CID,
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the identification strings listed by the "compatible" property value (if present
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and valid) will be used to look for a driver matching the device, but in that
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case their relative priority with respect to the other device IDs listed by
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_HID and _CID depends on the position of PRP0001 in the _CID return package.
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Specifically, the device IDs returned by _HID and preceding PRP0001 in the _CID
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return package will be checked first. Also in that case the bus type the device
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will be enumerated to depends on the device ID returned by _HID.
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It is valid to define device objects with a _HID returning PRP0001 and without
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the "compatible" property in the _DSD or a _CID as long as one of their
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ancestors provides a _DSD with a valid "compatible" property. Such device
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objects are then simply regarded as additional "blocks" providing hierarchical
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configuration information to the driver of the composite ancestor device.
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