Re: [PATCH 3/6] Documentation: ACPI: Document _DSC object usage for enum power state

[Bingbu Cao <bingbu.cao@linux.intel.com>]

Sakari,

On 10/18/21 8:17 PM, Sakari Ailus wrote:
> Document the use of the _DSC object for setting desirable power state
> during probe.
> 
> Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
> Reviewed-by: Tomasz Figa <tfiga@chromium.org>
> ---
>  Documentation/firmware-guide/acpi/index.rst   |  1 +
>  .../firmware-guide/acpi/non-d0-probe.rst      | 78 +++++++++++++++++++
>  2 files changed, 79 insertions(+)
>  create mode 100644 Documentation/firmware-guide/acpi/non-d0-probe.rst
> 
> diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
> index a99ee402b212b..b053b0c3d6969 100644
> --- a/Documentation/firmware-guide/acpi/index.rst
> +++ b/Documentation/firmware-guide/acpi/index.rst
> @@ -26,5 +26,6 @@ ACPI Support
>     acpi-lid
>     lpit
>     video_extension
> +   non-d0-probe
>     extcon-intel-int3496
>     intel-pmc-mux
> diff --git a/Documentation/firmware-guide/acpi/non-d0-probe.rst b/Documentation/firmware-guide/acpi/non-d0-probe.rst
> new file mode 100644
> index 0000000000000..78781e1ab6a3d
> --- /dev/null
> +++ b/Documentation/firmware-guide/acpi/non-d0-probe.rst
> @@ -0,0 +1,78 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +========================================
> +Probing devices in other D states than 0
> +========================================
> +
> +Introduction
> +============
> +
> +In some cases it may be preferred to leave certain devices powered off for the
> +entire system bootup if powering on these devices has adverse side effects,
> +beyond just powering on the said device.
> +
> +How it works
> +============
> +
> +The _DSC (Device State for Configuration) object that evaluates to an integer
> +may be used to tell Linux the highest allowed D state for a device during
> +probe. The support for _DSC requires support from the kernel bus type if the
> +bus driver normally sets the device in D0 state for probe.
> +
> +The downside of using _DSC is that as the device is not powered on, even if
> +there's a problem with the device, the driver likely probes just fine but the
> +first user will find out the device doesn't work, instead of a failure at probe
> +time. This feature should thus be used sparingly.
> +
> +I²C
> +---
> +
> +If an I²C driver indicates its support for this by setting the
> +I2C_DRV_ACPI_WAIVE_D0_PROBE flag in struct i2c_driver.flags field and the
> +_DSC object evaluates to integer higher than the D state of the device,
> +the device will not be powered on (put in D0 state) for probe.
> +
> +D states
> +--------
> +
> +The D states and thus also the allowed values for _DSC are listed below. Refer
> +to [1] for more information on device power states.
> +
> +.. code-block:: text
> +
> +	Number	State	Description
> +	0	D0	Device fully powered on
> +	1	D1
> +	2	D2
> +	3	D3hot
> +	4	D3cold	Off
> +
> +References
> +==========
> +
> +[1] https://uefi.org/specifications/ACPI/6.4/02_Definition_of_Terms/Definition_of_Terms.html#device-power-state-definitions
> +
> +Example
> +=======
> +
> +An ASL example describing an ACPI device using _DSC object to tell Operating
> +System the device should remain powered off during probe looks like this. Some
> +objects not relevant from the example point of view have been omitted.
> +
> +.. code-block:: text
> +
> +	Device (CAM0)
> +        {
> +		Name (_HID, "SONY319A")
> +		Name (_UID, Zero)
> +		Name (_CRS, ResourceTemplate ()
> +		{
> +			I2cSerialBus(0x0020, ControllerInitiated, 0x00061A80,
> +				     AddressingMode7Bit, "\\_SB.PCI0.I2C0",
> +				     0x00, ResourceConsumer)
> +		})
> +		Name (_DSC, 0, NotSerialized)
> +		{
> +			Return (0x4)
> +                }
One question here:
Is the value of _DSC object evaluated from 'Method' or 'Name' ?

> +	}
> 

-- 
Best regards,
Bingbu Cao