Re: [PATCH] ACPI / enumeration: Document the rules regarding the PRP0001 device ID

[Hanjun Guo]

On 2015/6/12 8:28, Rafael J. Wysocki wrote:
> On Thursday, June 11, 2015 12:41:29 PM Mika Westerberg wrote:
>> On Thu, Jun 11, 2015 at 03:24:02AM +0200, Rafael J. Wysocki wrote:
>>> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>>>
>>> Document how the ACPI device enumeration code uses the special
>>> PRP0001 device ID.
>>>
>>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>> At last the long needed documentation regarding PRP0001 :-) Thank you
>> for doing this,
>>
>> Acked-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
>>
>> One typo found, see below.
> Thanks!
>
> I've fixed this one and added a clarification about enumerating I2C and SPI
> clients with PRP0001 and "compatible".
>
> Updated patch is below.
>
> ---
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Subject: 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@xxxxxxxxx>
> Acked-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> ---
>  Documentation/acpi/enumeration.txt |   51 +++++++++++++++++++++++++++++++++++++
>  1 file changed, 51 insertions(+)
>
> Index: linux-pm/Documentation/acpi/enumeration.txt
> ===================================================================
> --- linux-pm.orig/Documentation/acpi/enumeration.txt
> +++ linux-pm/Documentation/acpi/enumeration.txt
> @@ -359,3 +359,54 @@ the id should be set like:
>  The ACPI id "XYZ0001" is then used to lookup an ACPI device directly under
>  the MFD device and if found, that ACPI companion device is bound to the
>  resulting child platform device.
> +
> +Device Tree namespace link device ID
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +The Device Tree protocol uses device indentification based on the "compatible"
> +property whose value is a string or an array of strings recognized as device
> +identifiers by drivers and the driver core.  The set of all those strings may be
> +regarded as a device indentification namespace analogous to the ACPI/PNP device
> +ID namespace.  Consequently, in principle it should not be necessary to allocate
> +a new (and arguably redundant) ACPI/PNP device ID for a devices with an existing
> +identification string in the Device Tree (DT) namespace, especially if that ID
> +is only needed to indicate that a given device is compatible with another one,
> +presumably having a matching driver in the kernel already.
> +
> +In ACPI, the device identification object called _CID (Compatible ID) is used to
> +list the IDs of devices the given one is compatible with, but those IDs must
> +belong to one of the namespaces prescribed by the ACPI specification (see
> +Section 6.1.2 of ACPI 6.0 for details) and the DT namespace is not one of them.
> +Moreover, the specification mandates that either a _HID or an _ADR identificaion
> +object be present for all ACPI objects representing devices (Section 6.1 of ACPI
> +6.0).  For non-enumerable bus types that object must be _HID and its value must
> +be a device ID from one of the namespaces prescribed by the specification too.
> +
> +The special DT namespace link device ID, PRP0001, provides a means to use the
> +existing DT-compatible device identification in ACPI and to satisfy the above
> +requirements following from the ACPI specification at the same time.  Namely,
> +if PRP0001 is returned by _HID, the ACPI subsystem will look for the
> +"compatible" property in the device object's _DSD and will use the value of that
> +property to identify the corresponding device in analogy with the original DT
> +device identification algorithm.  If the "compatible" property is not present
> +or its value is not valid, the device will not be enumerated by the ACPI
> +subsystem.  Otherwise, it will be enumerated automatically as a platform device
> +(except when an I2C or SPI link from the device to its parent is present, in
> +which case the ACPI core will leave the device enumeration to the parent's
> +driver) and the identification strings from the "compatible" property value will
> +be used to find a driver for the device along with the device IDs listed by _CID
> +(if present).
> +
> +Analogously, if PRP0001 is present in the list of device IDs returned by _CID,
> +the identification strings listed by the "compatible" property value (if present
> +and valid) will be used to look for a driver matching the device, but in that
> +case their relative priority with respect to the other device IDs listed by
> +_HID and _CID depends on the position of PRP0001 in the _CID return package.
> +Specifically, the device IDs returned by _HID and preceding PRP0001 in the _CID
> +return package will be checked first.  Also in that case the bus type the device
> +will be enumerated to depends on the device ID returned by _HID.
> +
> +It is valid to define device objects with a _HID returning PRP0001 and without
> +the "compatible" property in the _DSD or a _CID as long as one of their
> +ancestors provides a _DSD with a valid "compatible" property.  Such device
> +objects are then simply regarded as additional "blocks" providing hierarchical
> +configuration information to the driver of the composite ancestor device.
>

Thanks for documenting this, it's very useful :)

Reviewed-by: Hanjun Guo <hanjun.guo@xxxxxxxxxx>

Thanks
Hanjun

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