[PATCH/RFC v7] ARM: boot: Obtain start of physical memory from DTB

[Ard Biesheuvel]

On Fri, 14 Aug 2020 at 16:03, Linus Walleij <linus.walleij at linaro.org> wrote:
>
> On Thu, Jul 23, 2020 at 3:19 AM Stephen Boyd <sboyd at kernel.org> wrote:
>
> > > > textofs-$(CONFIG_ARCH_IPQ40XX) := 0x00208000
> > > > textofs-$(CONFIG_ARCH_MSM8X60) := 0x00208000
> > > > textofs-$(CONFIG_ARCH_MSM8960) := 0x00208000
> > >
> > > But what on earth is this? I just deleted this and the platform
> > > boots just as well.
> >
> > We need to shift the kernel text to start 2MB beyond the start of memory
> > because there is the shared memory region used to communicate with other
> > processors in the SoC there. It took a while for us to convince other OS
> > folks in the company to put shared memory somewhere else besides the
> > start of RAM, but eventually we won that battle.
> >
> > Does your booted kernel have its text section at the start of RAM or is
> > it offset by 2MB without this change? Check out /proc/iomem to see where
> > the kernel text is in relation to the start of RAM.
>
> The memory on this machine starts at 0x40200000 since the effect
> of the current code is to take pc &= 0xf8000000 and that results in
> 0x40000000 and then this adds textofs 0x00208000 to that
> resulting in 0x40208000 for the kernel physical RAM. Which
> is what we want to achieve since the RAM starts at
> 0x40200000.
>
> But TEXT_OFFSET is also used inside the kernel to offset the
> virtual memory. This means that when we set up the virtual
> memory split, the kernel virtual memory is also bumped by
> these 2 MB so the virtual memory starts at 0xC0208000
> instead of 0xC0008000 as is normal.
>
> It looks weird to me but maybe someone can explain how
> logical that is?
>

The ARM mm code assumes that the relative alignment between PA and VA
is 16 MB, so if we skip 2 MB in the physical space, we must do the
same in the virtual space.