"Kernel logical address" vs "Physical"

[ashish anand]

> I have been working on a PPC platform, running Linux 2.2. This platform has
> a register where you can enable different options and where you can control
> several LED's. The vendor mentions that this register is mapped in memory at
> FA400000.
>
> When I write (in a driver)
>
> char *p=0xFA40000;
>
> *p = ..; (I know I should use writeb to be more portable.)
>
> it seems to work; the leds can be manipulated. But as FA400000 is a physical
> address, I thought that it had to be *p=__va(0xFA40000);
>
> Who can help me out? Where do I find a nice document about this?

actually when you refer to adress mentioned 0xFA400000 in driver this address is already virtual adress hence no need to  use __va macro.
this address was originally a physical adress and unaccessible from device untill earlier in code
a call for ioremap would have taken place.
after ioremapping it is a virtual address and can be used nicely in a driver.
transaction for this address would be valid as processor has now a entry for it and would be decoded by host bridge and forwarded to LED.
without ioremapping if you use you will get a kernel panic.

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