how to set max-functions in pcie device tree? (about the syntax "max-functions = /bits/ 8 <8>;")

In linux-6.15.68, in Documentation/devicetree/bindings/pci/rockchip-pcie-ep.txt, I see these explanations. (please see the marked line.)

Optional Property:
- num-lanes: number of lanes to use
- max-functions: Maximum number of functions that can be configured (default 1).


pcie0-ep: pcie@f8000000 {
    compatible = "rockchip,rk3399-pcie-ep";
    #address-cells = <3>;
    #size-cells = <2>;
    rockchip,max-outbound-regions = <16>;
    clocks = <&cru ACLK_PCIE>, <&cru ACLK_PERF_PCIE>,
         <&cru PCLK_PCIE>, <&cru SCLK_PCIE_PM>;
    clock-names = "aclk", "aclk-perf",
              "hclk", "pm";
    max-functions = /bits/ 8 <8>;                     // <---- see this line
    num-lanes = <4>;
    reg = <0x0 0xfd000000 0x0 0x1000000>, <0x0 0x80000000 0x0 0x20000>;

<skip>

In the example dts, what does "max-functions = /bins/ 8 <8>;" mean?
I found in Documentation/devicetree/bindings/pci/snps,dw-pcie-ep.yaml, it says

  max-functions:
    $ref: /schemas/types.yaml#/definitions/uint32
    description: maximum number of functions that can be configured

But I don't know how to read the $ref document.

ADD

I found this.

The storage size of an element can be changed using the /bits/ prefix. The /bits/ prefix allows for the creation of 8, 16, 32, and 64-bit elements. The resulting array will not be padded to a multiple of the default 32-bit element size.

e.g. interrupts = /bits/ 8 <17 0xc>; e.g. clock-frequency = /bits/ 64 <0x0000000100000000>;

Does this mean 17 and 0xc are both 8-bit variables and when it is compiled to dtb, it keep the 8-bit format? The linux code will analyze the dtb file, then does the dtb contains the format information too?

Solution

The Device Tree Compiler v1.4.0 onwards supports some extra syntaxes for specifying property values that are not present in The Devicetree Specification up to at least version v0.4-rc1. These extra property value syntaxes are documented in the Device Tree Compiler's Device Tree Source Format and include:

A number in an array between angle brackets can be specified as a character literal such as 'a', '\r' or '\xFF'.
The size of elements in an array between angle brackets can be set using the prefix /bits/ and a bit-width of 8, 16, 32, or 64, defaulting to 32-bit integers.

The binary Flattened Devicetree (DTB) Format contains no explicit information on the type of a property value. A property value is just a string of bytes. Numbers (and character literals) between angle brackets in the source are converted to bytes in big-endian byte order in accordance with the element size in bits divided by 8. For example:

<0x11 'a'> is encoded in the same way as the bytestring [00 00 00 11 00 00 00 61].
/bits/ 8 <17 0xc> is encoded in the same way as the bytestring [11 0c].

It is up to the reader of the property value to "know" what type it is expecting. For example, the Rockchip AXI PCIe endpoint controller driver in the Linux kernel ("drivers/pci/controller/pcie-rockchip-ep.c") "knows" that the "max-functions" property should have been specified as a single byte and attempts to read it using the statement err = of_property_read_u8(dev->of_node, "max-functions", &ep->epc->max_functions);. (It is probably encoded as a single byte property for convenience so that it can be copied directly into the u8 max_functions member of a struct pci_epc.)