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Upper Half and Lower Half Drivers

Monolithic Drivers

When one thinks about device drivers in an OS, one thinks of a single thing, a single block in a block diagram with these two primary interfaces:

• The device monolithic driver exposes a single, standard device driver interface. With the Virtual File System (VFS), this provides the application user interface to the driver functionality. And
• A low-level interface to the hardware that is managed by the device driver.

Upper Half and Lower Half Drivers

NuttX supports many, many different MCU platforms, each with many similar but distinct built-in peripherals.
Certainly we could imagine a realization where each such peripheral is supported by monolithic driver as described in the preceding paragraph.
That would involve a lot code duplication, however.
The MCU peripherals may be unique at a low, register-level interface.
However, the peripherals are really very similar at a higher level of abstraction.

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• The lower-half side of the interface to the the upper0half driver, and
• The low-level interface to the hardware that is managed by the lower half device driver.

One to Many: Encapsulation and Polymorphism

These modular upper- and lower-half drivers have certain properties that you would associate with an object oriented design: Encapsulation, data abstraction, and polymorphism certainly.
Because of this encapsulation, the upper-half driver is complete unaware of any implementation details within the lower-half driver.
Everything needed for the upper- and lower-half drivers to integrate is provided by the defined interface between between those two things.
In fact, a single upper-half driver may service many lower-half driver instances in a one-to-many relationship.

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