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Diffstat (limited to 'Documentation/firmware-guide')
| -rw-r--r-- | Documentation/firmware-guide/acpi/enumeration.rst | 43 |
1 files changed, 43 insertions, 0 deletions
diff --git a/Documentation/firmware-guide/acpi/enumeration.rst b/Documentation/firmware-guide/acpi/enumeration.rst index 56d9913a33..d79f693909 100644 --- a/Documentation/firmware-guide/acpi/enumeration.rst +++ b/Documentation/firmware-guide/acpi/enumeration.rst @@ -64,6 +64,49 @@ 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 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 =========== |