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

[Sakari Ailus]

Hi Bingbu,

On Mon, Oct 25, 2021 at 07:01:41PM +0800, Bingbu Cao wrote:
> 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@xxxxxxxxxxxxxxx>
> > Reviewed-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
> > ---
> >  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' ?

"Method". This is a bug in the documentation. Thanks!

Rafael: could you fix this while applying?

-- 
Sakari Ailus