Re: [PATCH V6 09/22] LoongArch: Add boot and setup routines

[Mike Rapoport <rppt@kernel.org>]

Hi,

On Fri, Mar 04, 2022 at 08:43:03PM +0800, Huacai Chen wrote:
> Hi, Mike,
> 
> On Fri, Mar 4, 2022 at 6:49 PM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > Hi,
> >
> >
> > So ideally, the physical memory detection and registration should follow
> > something like:
> >
> > * memblock_reserve() the memory used by firmware, kernel and initrd
> > * detect NUMA topology
> > * add memory regions along with their node ids to memblock.
> >
> > s390::setup_arch() is a good example of doing early reservations:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/s390/kernel/setup.c#n988
> I have a fast reading of S390, and I think we can do some adjust:
> 1, call memblock_set_node(0, ULONG_MAX, &memblock.memory, 0) in
> early_memblock_init().
> 2, move memblock_reserve(PHYS_OFFSET, 0x200000) and
> memblock_reserve(__pa_symbol(&_text), __pa_symbol(&_end) -
> __pa_symbol(&_text)) to early_memblock_init().
> 3, Reserve initrd memory in the first place.
> It is nearly the same as the S390, then.

It does not have to look like the same as s390 :)
The important thing is to reserve all the memory before memblock
allocations are possible. 

> > > > > +early_param("memmap", early_parse_memmap);
> > > >
> > > > The memmap= processing is a hack indented to workaround bugs in firmware
> > > > related to the memory detection. Please don't copy if over unless there is
> > > > really strong reason.
> > >
> > > Hmmm, I have read the documents, most archs only support mem=limit,
> > > but MIPS support mem=limit@base. memmap not only supports
> > > memmap=limit@base, but also a lot of advanced syntax. LoongArch needs
> > > both limit and limit@base syntax. So can we make our code to support
> > > only mem=limit and memmap=limit@base, and remove all other syntax
> > > here?
> >
> > The documentation describes what was there historically and both these
> > options tend not to play well with complex memory layouts.
> >
> > If you must have them it's better to use x86 as an example rather than
> > MIPS, just take into the account that on x86 memory always starts from 0,
> > so they never needed to have a different base.
> >
> > For what use-cases LoongArch needs options?
>
> The use-case of limit@base syntax is kdump, because our kernel is not
> relocatable. I'll use X86 as an example.

I missed that mem= can be used several times, so with MIPS implementation
it's possible to define something like "mem=limit0@base0 mem=limit1@base1"
and this will create two contiguous memory regions.
 
> Huacai

-- 
Sincerely yours,
Mike.