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 ===========
