Syntax of files.* configuration

[Vincent DEFERT <vincent.defert%posteo.net@localhost>]

Hi!

I want to write an I2C driver for the Allwinner Meson target, so I have 
explored the source tree to try and understand how existing drivers worked.
I get how the C code works, but I struggle to understand the various 
build configuration files.
If I take the UART driver as an example, I've found:

- In arch/evbarm/conf/GENERIC64, the following line:

mesonuart*    at fdt? pass 4            # Amlogic Meson UART

- In arch/arm/amlogic/files.meson, the following snippet:

device    mesonuart
attach    mesonuart at fdt with meson_uart
file    arch/arm/amlogic/meson_uart.c        meson_uart

I understand that:

- the 'mesonuart' in GENERIC64 is the same as in files.meson
- the * after mesonuart reflects the fact that there may be several 
instances of the driver
- 'device' declares the new driver, and optionally the interfaces it 
implements, specified after a colon (i2cbus and i2cexec in my case)
- 'attach' means the driver is attached to the matching devices from the 
device tree based on their 'compatible' identifier
- 'file' declares the C module implementing the driver when the optional 
boolean expression after the file name is true

I don't understand:

- what the ? after 'fdt' and 'pass 4' mean
- what the optional 'with meson_uart' means
- where and how the 'meson_uart' boolean is set
- whether there is a relationship between the names in those 
configuration files and the first argument of the CFATTACH_DECL_NEW macro

Is there any documentation explaining this syntax?
Or alternatively, could you please clarify these points?

Thank you in advance!

Vincent