Re: [PATCH 04/12] mips: Reserve memory for the kernel image resources

Paul Burton
2019-04-24T22:43:57Z

Hi Serge,

On Wed, Apr 24, 2019 at 01:47:40AM +0300, Serge Semin wrote:
> The reserved_end variable had been used by the bootmem_init() code
> to find a lowest limit of memory available for memmap blob. The original
> code just tried to find a free memory space higher than kernel was placed.
> This limitation seems justified for the memmap ragion search process, but
> I can't see any obvious reason to reserve the unused space below kernel
> seeing some platforms place it much higher than standard 1MB.

There are 2 reasons I'm aware of:

1) Older systems generally had something like an ISA bus which used
addresses below the kernel, and bootloaders like YAMON left behind
functions that could be called right at the start of RAM. This sort
of thing should be accounted for by /memreserve/ in DT or similar
platform-specific reservations though rather than generically, and
at least Malta & SEAD-3 DTs already have /memreserve/ entries for
it. So this part I think is OK. Some other older platforms might
need updating, but that's fine.

2) trap_init() only allocates memory for the exception vector if using
a vectored interrupt mode. In other cases it just uses CAC_BASE
which currently gets reserved as part of this region between
PHYS_OFFSET & _text.

I think this behavior is bogus, and we should instead:

- Allocate the exception vector memory using memblock_alloc() for
CPUs implementing MIPSr2 or higher (ie. CPUs with a programmable
EBase register). If we're not using vectored interrupts then
allocating one page will do, and we already have the size
calculation for if we are.

- Otherwise use CAC_BASE but call memblock_reserve() on the first
page.

I think we should make that change before this one goes in. I can
try to get to it tomorrow, but feel free to beat me to it.

Thanks,
Paul