Re: mainline build failure due to 322458c2bb1a ("drm/ttm: Reduce the number of used allocation orders for TTM pages")

[Michael Ellerman <mpe@ellerman.id.au>]

Linus Torvalds <torvalds@linux-foundation.org> writes:
> On Wed, Apr 26, 2023 at 10:44 AM Sudip Mukherjee (Codethink)
> <sudipm.mukherjee@gmail.com> wrote:
>>
>> drivers/gpu/drm/ttm/ttm_pool.c:73:29: error: variably modified 'global_write_combined' at file scope
>>    73 | static struct ttm_pool_type global_write_combined[TTM_DIM_ORDER];
>>       |                             ^~~~~~~~~~~~~~~~~~~~~
>
> Ugh.
>
> This is because we have
>
>   #define TTM_DIM_ORDER (__TTM_DIM_ORDER <= MAX_ORDER ?
> __TTM_DIM_ORDER : MAX_ORDER)
>
> which looks perfectly fine as a constant ("pick the smaller of
> MAX_ORDER and __TTM_DIM_ORDER").
>
> But:
>
>   #define __TTM_DIM_ORDER (TTM_MAX_ORDER + 1)
>   #define TTM_MAX_ORDER (PMD_SHIFT - PAGE_SHIFT)
>
> which still _looks_ fine, but  on 64-bit powerpc, we then have
>
>    #define PTE_INDEX_SIZE  __pte_index_size
>
> so that __TTM_DIM_ORDER constant isn't actually a constant at all.
..
>
> It's a bit sad how that macro that _looks_ like a constant (and is one
> pretty much everywhere else) isn't actually constant on powerpc, but
> looking around it looks fairly unavoidable.

Yeah, it allows us to build a single kernel that can choose at runtime
whether it uses the HPT or Radix MMU. The page table geometry is
different between the MMUs because they support a different sized huge
page for THP.

cheers