Re: [PATCH v1 2/9] docs: firmware-guide: ACPI: Clarify ACPI bus concepts

[Rafael J. Wysocki]

On Monday, September 25, 2023 4:48:35 PM CEST Michal Wilczynski wrote:
> Some devices implement ACPI driver as a way to manage devices
> enumerated by the ACPI. This might be confusing as a preferred way to
> implement a driver for devices not connected to any bus is a platform
> driver, as stated in the documentation. Clarify relationships between
> ACPI device, platform device and ACPI entries.
> 
> Suggested-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> Signed-off-by: Michal Wilczynski <michal.wilczynski@xxxxxxxxx>
> ---
>  Documentation/firmware-guide/acpi/enumeration.rst | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/Documentation/firmware-guide/acpi/enumeration.rst b/Documentation/firmware-guide/acpi/enumeration.rst
> index 56d9913a3370..f56cc79a9e83 100644
> --- a/Documentation/firmware-guide/acpi/enumeration.rst
> +++ b/Documentation/firmware-guide/acpi/enumeration.rst
> @@ -64,6 +64,19 @@ If the driver needs to perform more complex initialization like getting and
>  configuring GPIOs it can get its ACPI handle and extract this information
>  from ACPI tables.
>  
> +ACPI bus
> +====================
> +
> +Historically some devices not connected to any bus were represented as ACPI
> +devices, and had to implement ACPI driver. This is not a preferred way for new
> +drivers. As explained above devices not connected to any bus should implement
> +platform driver. ACPI device would be created during enumeration nonetheless,
> +and would be accessible through ACPI_COMPANION() macro, and the ACPI handle would
> +be accessible through ACPI_HANDLE() macro. ACPI device is meant to describe
> +information related to ACPI entry e.g. handle of the ACPI entry. Think -
> +ACPI device interfaces with the FW, and the platform device with the rest of
> +the system.
> +
>  DMA support
>  ===========

I rewrote the above entirely, so here's a new patch to replace this one:

---
From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Subject: [PATCH v2 2/9] ACPI: docs: enumeration: Clarify ACPI bus concepts

In some cases, ACPI drivers are implemented as a way to manage devices
enumerated with the help of the platform firmware through ACPI.

This might be confusing, since the preferred way to implement a driver
for a device that cannot be enumerated natively, is a platform
driver, as stated in the documentation.

Clarify relationships between ACPI device objects, platform devices and
ACPI Namespace entries.

Suggested-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Co-developed-by: Michal Wilczynski <michal.wilczynski@xxxxxxxxx>
Signed-off-by: Michal Wilczynski <michal.wilczynski@xxxxxxxxx>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
---
 Documentation/firmware-guide/acpi/enumeration.rst |   43 ++++++++++++++++++++++
 1 file changed, 43 insertions(+)

Index: linux-pm/Documentation/firmware-guide/acpi/enumeration.rst
===================================================================
--- linux-pm.orig/Documentation/firmware-guide/acpi/enumeration.rst
+++ linux-pm/Documentation/firmware-guide/acpi/enumeration.rst
@@ -64,6 +64,49 @@ If the driver needs to perform more comp
 configuring GPIOs it can get its ACPI handle and extract this information
 from ACPI tables.
 
+ACPI device objects
+===================
+
+Generally speaking, there are two categories of devices in a system in which
+ACPI is used as an interface between the platform firmware and the OS: Devices
+that can be discovered and enumerated natively, through a protocol defined for
+the specific bus that they are on (for example, configuration space in PCI),
+without the platform firmware assistance, and devices that need to be described
+by the platform firmware so that they can be discovered.  Still, for any device
+known to the platform firmware, regardless of which category it falls into,
+there can be a corresponding ACPI device object in the ACPI Namespace in which
+case the Linux kernel will create a struct acpi_device object based on it for
+that device.
+
+Those struct acpi_device objects are never used for binding drivers to natively
+discoverable devices, because they are represented by other types of device
+objects (for example, struct pci_dev for PCI devices) that are bound to by
+device drivers (the corresponding struct acpi_device object is then used as
+an additional source of information on the configuration of the given device).
+Moreover, the core ACPI device enumeration code creates struct platform_device
+objects for the majority of devices that are discovered and enumerated with the
+help of the platform firmware and those platform device objects can be bound to
+by platform drivers in direct analogy with the natively enumerable devices
+case.  Therefore it is logically inconsistent and so generally invalid to bind
+drivers to struct acpi_device objects, including drivers for devices that are
+discovered with the help of the platform firmware.
+
+Historically, ACPI drivers that bound directly to struct acpi_device objects
+were implemented for some devices enumerated with the help of the platform
+firmware, but this is not recommended for any new drivers.  As explained above,
+platform device objects are created for those devices as a rule (with a few
+exceptions that are not relevant here) and so platform drivers should be used
+for handling them, even though the corresponding ACPI device objects are the
+only source of device configuration information in that case.
+
+For every device having a corresponding struct acpi_device object, the pointer
+to it is returned by the ACPI_COMPANION() macro, so it is always possible to
+get to the device configuration information stored in the ACPI device object
+this way.  Accordingly, struct acpi_device can be regarded as a part of the
+interface between the kernel and the ACPI Namespace, whereas device objects of
+other types (for example, struct pci_dev or struct platform_device) are used
+for interacting with the rest of the system.
+
 DMA support
 ===========