Re: [RFC] ACPI Code First ECR: _DSC (Deepest State for Configuration)

["Rafael J. Wysocki" <rafael@kernel.org>]

On Mon, Feb 22, 2021 at 8:23 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On some systems there are devices that should better be left in their
> current power state before attempting to bind drivers to them in order
> to avoid some undesirable side effects of turning the power on (e.g.
> blinking a privacy LED of a camera that may confuse the user to
> believe that the camera sensor is in use while in fact the camera is
> only being probed by a driver).
>
> To support this use case, define a new device PM object called _DSC
> (Deepest State for Configuration) returning the deepest D-state the
> device can be in before evaluating its _CRS, _PRS and _SRS configuration
> objects.
>
> # Title: Introduce _DSC (Deepest State for Configuration)
>
> # Status: Draft
>
> # Document: ACPI Specification 6.4
>
> # License
> SPDX-License Identifier: CC-BY-4.0
>
> # Submitter:
> * Sponsor: Rafael J. Wysocki, Intel
> * Creators/Contributors:
>     * Rafael J. Wysocki, Intel
>     * Sakari Ailus, Intel
>
> # Summary of the Change
> Introduce a new device PM object called _DSC (Deepest State for Configuration)
> returning the deepest D-state the device can be in before evaluating its _CRS,
> _PRS and _SRS configuration objects.
>
> # Benefits of the Change
> It will allow OSPMs to avoid putting devices into D0 in order to check and set
> their resources if _DSC is present and its return value is greater than 0.  In
> particular, if _DSC returns 4 (assuming that _PR3 is present for the given
> device), the device can remain in D3cold during driver probe and the driver
> should be able to gather all of the information needed by it to operate the
> device without changing its power state.
>
> # Impact of the Change
> Enable certain device power management optimizations.
>
> # References
> Support running driver's probe for a device powered off
> <https://lore.kernel.org/linux-acpi/20210210230800.30291-1-sakari.ailus@linux.intel.com/T/#m24e9f51119cbe6eb39f31c097ea3941d00a0baeb>
>
> # Detailed Description of the Change
>
> * Add a new row to Table 7.3 Device Power Management Child Objects:
>
> > _DSC    Object that evaluates to the device's deepest power state for configuration.
>
> * Add new Section 7.3.27:
>
> > 7.3.27. _DSC (Deepest State for Configuration)
> >
> >This optional object evaluates to an integer that conveys to OSPM the deepest
> >D-state the device can be in before evaluating the \_CRS, \_PRS and \_SRS
> >configuration objects for it.
> >
> >If \_DSC is present and the current power state of the device is not deeper
> >than the one represented by its return value, the device's \_CRS, \_PRS (if
> >present) and \_SRS (if present) configuration objects (see :ref:`device-configuration-objects`)
> >can be safely evaluated without putting the device into D0.
> >
> >Therefore, when present, the \_DSC object allows the OSPM to optimize device
> >power management by avoiding an unnecessary change to device power state that
> >would be otherwise made before evaluating configuration objects for the device.
> >
> >**Arguments:**
> >
> >  None
> >
> >**Return Value:**
> >
> >  An Integer representing the deepest D-state the device can be in during
> >  configuration. If OSPM has not indicated that it supports \_PR3 through the OSPM
> >  Platform-Wide Capabilities (see :ref:`platform-wide-ospm-capabilities`), then
> >  the value "3" corresponds to D3. If it has indicated \_PR3 support, the value
> >  "3" represents D3hot and the value "4" represents D3cold.
> >
> >  The \_DSC return value must represent a D-state that is supported by the
> >  device.  In particular, "4" (D3cold) can be returned only if \_PR3 is present.
> >
> >  \_DSC must return the same integer each time it is evaluated.
> >
> >  The \_DSC return value must not represent a D-state shallower then the one
> >  resulting from turning ON all of the power resources listed by \_PRE (if
> >  present) and evaluating \_PSE (if present).

This ECR has been approved as submitted.