[PATCH 01/52] ARM: introduce atag_offset to replace boot_params

[Nicolas Pitre]

On Fri, 8 Jul 2011, Russell King - ARM Linux wrote:

> On Tue, Jul 05, 2011 at 10:46:46PM -0400, Nicolas Pitre wrote:
> > The boot_params member of the mdesc structure is used to provide a
> > default physical address for the ATAG list.  Since this value is fixed
> > at compile time and often based on ARCH_PHYS_OFFSET, it gets in the way
> > of runtime PHYS_OFFSET usage.
> > 
> > Let's introduce atag_offset which should contains only the relative
> > offset from start of memory instead of an absolute value, in preparation
> > to move all instance of boot_params over to it.
> 
> I don't like this.
> 
> While I understand the issue (the oops when we try to dereference the
> absolute address), the fact is that's how it is.
> 
> Take a moment to think about this.  The ATAG list is normally setup at a
> fixed physical address by the boot loader.  While there are some boot
> loaders which allow you to load the kernel at different physical addresses,
> they still place the ATAG list at that same physical address.

Would they really load the kernel at a different address?

> That means if you change this into an offset from the kernels idea of
> PHYS_OFFSET, it's not going to find the ATAGs.

Is this going to be a real issue in practice?

> So we're probably better off checking whether the platform provided ATAG
> pointer is below our idea of PHYS_OFFSET, and if it is to ignore that.

The code already does that.

Alternatively, I can make the boot_params values into numeric values 
instead of PLAT_PHYS_OFFSET or any other symbolic define I'd like to get 
rid of.  What do you think?


Nicolas