Question re DeviceTree addressing

[Florian Fainelli]

On 4/3/2023 7:00 AM, Caleb James DeLisle wrote:
> Hello folks,
> 
> I've been working on trying to port an en7526 and in doing so I'm trying 
> to learn how to at least partially write a DTS file from an old style 
> header full of #defines.
> 
> I ran into a bit of a quandry, I'm comparing the mt7621.dtsi file to an 
> older MT7621 memory map header:
> 
> https://github.com/openwrt/openwrt/blob/master/target/linux/ramips/dts/mt7621.dtsi#L100
> 
> https://github.com/keenetic/kernel-49/blob/master/arch/mips/include/asm/rt2880/rt_mmap.h#L48
> 
> ```
>      palmbus: palmbus at 1e000000 {
>          compatible = "palmbus";
>          reg = <0x1e000000 0x100000>;
>          ranges = <0x0 0x1e000000 0x0fffff>;
> 
> [...]
> 
>          i2c: i2c at 900 {
>              compatible = "mediatek,mt7621-i2c";
>              reg = <0x900 0x100>;
> ```
> 
> For me, means there should be an I2C controller mapped at address 
> 0x1e000900.

Correct.

> 
> But looking at rt_mmap.h I see:
> 
> ```
> #define RALINK_I2C_BASE            0xBE000900
> ```
> 
> And in fact almost everything is based on 0xBE000000, except UART and 
> USB addresses which are "correct". And I see these 0xBE000000 addresses 
> being passed through KSEG1ADDR() so it seems they are physical memory 
> addresses, not virtual.

KSEG1ADDR does a logical or so it would not be altering RALINK_I2C_BASE 
when OR'd with 0xa0000000 (KSEG1) you would still get 0xBE000900.

It seems to me like in places where it may be necessary to pass a 
physical address they used physical addresses such that ioremap() 
ends-up returning a proper kernel virtual address in KSEG1, however 
everywhere else the driver might have just directly de-referenced the 
constant which ends-up working just fine as well.

HTH
-- 
Florian