Re: [PATCH v2 2/3] sparc32: derive phys_base from the PAGE_OFFSET mapping

[Sam Ravnborg]

Hi Magnus.

On Sun, Aug 16, 2026 at 09:50:16AM +0200, Magnus Lindholm wrote:
> setup_arch() computes phys_base as the base of the lowest sp_banks[]
> entry, that is, where RAM starts, and assumes the kernel image was loaded
> there. That holds for the traditional boot path, where SILO places the
> image at physical 0x4000 and PAGE_OFFSET is mapped to physical 0.
> 
> It stops holding once the image no longer fits there. SILO loads a kernel
> between physical 0x4000 and its own text at 0x280000, a window of 2605056
> bytes; a current sparc32 kernel is roughly twice that. The loader must
> then place the image elsewhere in physical memory and map PAGE_OFFSET to
> it, at which point phys_base describes where RAM begins rather than what
> PAGE_OFFSET maps to, and the two disagree.
> 
> phys_base is the offset __pa() and __va() are defined in terms of, so once
> it is wrong every early translation is wrong by the difference, including
> the physical addresses written into page table descriptors. The
> tablewalker then follows pointers into pages that hold nothing while the
> same tables read back correctly through the nocache view. The failure
> surfaces as a hang right after the context table pointer is installed and
> the TLB flushed, with nothing on the console to explain it, since the PROM
> mappings the early console depends on have become just as unreachable.
> 
> Ask the MMU what PAGE_OFFSET actually translates to and adopt that.
> __get_phys() already implements this probe for sun4m and sun4d and returns
> zero elsewhere, so no new low level MMU access is introduced and machines
> without an SRMMU are unaffected.
> 
> Memory below the kernel cannot be reached through the linear map, which
> runs upward from PAGE_OFFSET, so drop the banks that fall below it rather
> than leave entries that __va() would translate to below PAGE_OFFSET.
> 
> With this a 6MB kernel loaded at physical 0x03000000 boots on sun4m: the
> context table lands at its true physical address,
> srmmu_inherit_prom_mappings() preserves the PROM console mappings, and
> srmmu.c needs no change at all, since map_kernel() already handles a
> non-zero phys_base via do_large_mapping().
The patch looks good but I dislike we introduce more code that uses
sp_banks. Can we somehow use memblock for this?

Part of my old grand plan was to replace all uses of sp_banks with
memblock. I have some old patches somewhere in case you are ready to
give this a spin.

The patch is:
Reviewed-by: Sam Ravnborg <sam@xxxxxxxxxxxx>

My sp_banks comment shall not hold it back.
It is anyway Andreas that will handle them.

	Sam