100 lines
3.8 KiB
Plaintext
100 lines
3.8 KiB
Plaintext
Platform Devices and Drivers
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Platform devices
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~~~~~~~~~~~~~~~~
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Platform devices are devices that typically appear as autonomous
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entities in the system. This includes legacy port-based devices and
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host bridges to peripheral buses.
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Platform drivers
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~~~~~~~~~~~~~~~~
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Drivers for platform devices are typically very simple and
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unstructured. Either the device was present at a particular I/O port
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and the driver was loaded, or it was not. There was no possibility
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of hotplugging or alternative discovery besides probing at a specific
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I/O address and expecting a specific response.
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Other Architectures, Modern Firmware, and new Platforms
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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These devices are not always at the legacy I/O ports. This is true on
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other architectures and on some modern architectures. In most cases,
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the drivers are modified to discover the devices at other well-known
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ports for the given platform. However, the firmware in these systems
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does usually know where exactly these devices reside, and in some
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cases, it's the only way of discovering them.
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The Platform Bus
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~~~~~~~~~~~~~~~~
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A platform bus has been created to deal with these issues. First and
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foremost, it groups all the legacy devices under a common bus, and
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gives them a common parent if they don't already have one.
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But, besides the organizational benefits, the platform bus can also
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accommodate firmware-based enumeration.
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Device Discovery
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~~~~~~~~~~~~~~~~
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The platform bus has no concept of probing for devices. Devices
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discovery is left up to either the legacy drivers or the
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firmware. These entities are expected to notify the platform of
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devices that it discovers via the bus's add() callback:
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platform_bus.add(parent,bus_id).
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Bus IDs
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~~~~~~~
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Bus IDs are the canonical names for the devices. There is no globally
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standard addressing mechanism for legacy devices. In the IA-32 world,
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we have Pnp IDs to use, as well as the legacy I/O ports. However,
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neither tell what the device really is or have any meaning on other
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platforms.
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Since both PnP IDs and the legacy I/O ports (and other standard I/O
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ports for specific devices) have a 1:1 mapping, we map the
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platform-specific name or identifier to a generic name (at least
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within the scope of the kernel).
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For example, a serial driver might find a device at I/O 0x3f8. The
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ACPI firmware might also discover a device with PnP ID (_HID)
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PNP0501. Both correspond to the same device and should be mapped to the
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canonical name 'serial'.
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The bus_id field should be a concatenation of the canonical name and
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the instance of that type of device. For example, the device at I/O
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port 0x3f8 should have a bus_id of "serial0". This places the
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responsibility of enumerating devices of a particular type up to the
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discovery mechanism. But, they are the entity that should know best
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(as opposed to the platform bus driver).
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Drivers
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~~~~~~~
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Drivers for platform devices should have a name that is the same as
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the canonical name of the devices they support. This allows the
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platform bus driver to do simple matching with the basic data
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structures to determine if a driver supports a certain device.
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For example, a legacy serial driver should have a name of 'serial' and
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register itself with the platform bus.
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Driver Binding
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~~~~~~~~~~~~~~
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Legacy drivers assume they are bound to the device once they start up
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and probe an I/O port. Divorcing them from this will be a difficult
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process. However, that shouldn't prevent us from implementing
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firmware-based enumeration.
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The firmware should notify the platform bus about devices before the
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legacy drivers have had a chance to load. Once the drivers are loaded,
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they driver model core will attempt to bind the driver to any
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previously-discovered devices. Once that has happened, it will be free
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to discover any other devices it pleases.
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