Question

in my project I need to work with device drivers, but have a hard time to understand the naming, scope and function of the abstraction layers. As I see the main layer is HAL – “hardware abstraction layer”.
What are the clients of HAL, whom is HAL interfacing?

Answer 1

Are you talking about a specific HAL in windows or linux or something or in general?

When accessing registers from a device driver (code that drives a device, doesnt have to be a kernel thing or have an operating system at all) for example I generally recommend to create functions like PUT32(address,data), data=GET32(address). Or writel and readl, whatever you fancy. The point being to avoid creating a pointer with the address and using that pointer directly. There is a performance gain to the pointer type solution, and performance hit to the abstract PUT32(). Why I use it though is because if the code is clean enough that driver can be used as part of a kernel driver for this os, a kernel driver for that os, run standalone embedded, connect to an hdl simulation of the logic, run on a processor on the same chip, or run on a host computer that reaches into the chip via PCI or jtag, etc. One chunk of code reused from the birth of the logic (hdl sim) to the end user kernel driver.

Perhaps more to your question though think about a uart, you want to send some bytes and receive some bytes right? Create a uart_send() function and a uart_recv() function, everything above the abstraction layer uses these two functions, when you target this code to a specific platform then you implement those functions for the specific uart in that specific hardware. later on you can replace that uart with something else, so long as the new uart can send and receive the code above the abstraction layer does not have to change. Even though you have created an abstraction layer with the functions above, I personally would still use PUT8() and GET8() functions in the implementation of uart_send() and uart_recv() for the specific uart, and in a separate file implement PUT8() and GET8().

How many layers of abstraction between the driver and the actual hardware, how and where are often specific to the task and the hardware.