i'm just trying to understand, what the "[PATCH 0,1,2/2] FLATMEM..." patchset
does. While working through it, it appears to me that
1) Formerly min_low_pfn was never calculated, but left zero-initialized.
2) The patchset now provides proper min_low_pfn calculation from boot_mem_map
(thanks !), and then discards this calculated value:
a) Patch 1/2 prints an info about wasted pagetable-entries and resets
min_low_pfn to zero, retaining the former state.
b) (first part of) Patch 2/2 prints an info about differences between the
the really found (boot_mem_map) and the expected (given by PHYS_OFFSET
through ARCH_PFN_OFFSET) memory-offset, then resets min_low_pfn to
3) Finally the (rather set than) calculated values are completely passed on
to init_bootmem by replacing the former call (mm/bootmem.c)
max_low_pfn = highest;
min_low_pfn = mapstart;
init_bootmem_core(NODE_DATA(0), mapstart, 0, highest);
init_bootmem_core(NODE_DATA(0), mapstart, min_low_pfn, max_low_pfn);
So far, i can't avoid the impression, that this patchset would work perfectly
without using PHYS_OFFSET at all, even better IMHO, since it would use the
*really* detected memory-offset instead of some should-be-value. (Providing
an override-option for critical cases is another topic).
The second part of Patch 2/2 introduces PHYS_OFFSET to __pa/__va and to
virt_to_phys/phys_to_virt (writing "page_offset" shorthand for PAGE_OFFSET
and __pa_page_offset() here):
pa: address - page_offset + PHYS_OFFSET
va: address + page_offset - PHYS_OFFSET
1) Since the relation between kernel virtual address and physical address
is fixed (we agreed about that earlier), introducing PHYS_OFFSET in this
places is a nop at best: (page_offset - PHYS_OFFSET) is constant, whenever
we introduce/change PHYS_OFFSET we must adjust page_offset accordingly
(what's about CKSEG0 in __pa_page_offset() ?).
2) Because of 1) we get the same conversions, if we don't use PHYS_OFFSET
3) The page_offset adjustment may force fixes in other, not yet blown up,
places (pmd_phys() cried out lately...).
Hard to see, what we gain by introducing PHYS_OFFSET here.
What can PHYS_OFFSET achieve here - besides obfuscating ?
Are there future uses for it, that justify the contortions ?
with best regards
On Wed, 10 Jan 2007, Franck Bui-Huu wrote:
> Date: Wed, 10 Jan 2007 09:44:03 +0100
> From: Franck Bui-Huu <firstname.lastname@example.org>
> To: email@example.com
> Cc: firstname.lastname@example.org
> Subject: [PATCH 0/2] FLATMEM: allow memory to start at pfn != 0 [take #2]
> Here's is the second attempt to make this works on your Malta board
> and all other boards that have some data reserved at the start of
> their memories. In these cases the first patchset assumed wrongly that
> the start of the memory was after this reserved area.
> Patch 1/2 should work alone now. the kernel should report that your
> mem config is wasting some memory for tracking reserved pages located
> at the start of the mem.
> Thanks for testing
> arch/mips/kernel/setup.c | 40 +++++++++++++++++++++++++++++++---------
> arch/mips/mm/init.c | 23 +++++++++++------------
> include/asm-mips/dma.h | 1 +
> include/asm-mips/io.h | 4 ++--
> include/asm-mips/page.h | 25 +++++++++++++++++++++----
> 5 files changed, 66 insertions(+), 27 deletions(-)