[RFC 1/2] powerpc/swiotlb: Dont free up allocated SWIOTLB slab on POWER

[Michael Ellerman]

Anshuman Khandual <khandual at linux.vnet.ibm.com> writes:
> Even though SWIOTLB slab gets allocated and initialized on powerpc with
> swiotlb_init() called during mem_init(), it gets released away again on
> POWER platform because 'ppc_swiotlb_enable' never gets set. The function
> swiotlb_detect_4g() checks for 4GB memory and then sets the variable
> 'ppc_swiotlb_enable' which prevents freeing up the SWIOTLB slab. Lets
> make POWER platform call swiotlb_detect_4g() during setup_arch() which
> will keep the SWIOTLB slab through out the runtime.
>
> A previous commit cf5621032f ("powerpc/64: Limit ZONE_DMA32 to 4GiB in
> swiotlb_detect_4g()") enforced 4GB limit on ZONE_DMA32 which is is not
> applicable on POWER (CONFIG_PPC_BOOK3S_64) platform. Lets remove this
> unnecessary restriction.

You're using "POWER" to mean Book3S 64-bit, but "POWER" is something
else (the ISA for POWER1/POWER2).

So please just say "Book3S 64-bit" or talk about a specific CPU, eg.
Power9.

> After the patch, SWIOTLB slab does not get released.
>
> [0.410992] software IO TLB [mem 0xfbff0000-0xffff0000] (64MB) mapped
> at [00000000767f6cb3-000000004a10114f]

But we don't want SWIOTLB on any existing Book3S 64-bit platforms, so
leaving it enabled is just wasting memory.

> diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
> index d73ec518ef80..c4db844e0b0d 100644
> --- a/arch/powerpc/kernel/setup-common.c
> +++ b/arch/powerpc/kernel/setup-common.c
> @@ -944,6 +944,7 @@ void __init setup_arch(char **cmdline_p)
>  	/* Initialize the MMU context management stuff. */
>  	mmu_context_init();
>  
> +	swiotlb_detect_4g();

You shouldn't be calling this, your use case has nothing to do with 4GB.

Instead when you detect that you're running under the ultravisor then
you should set ppc_swiotlb_enable = 1.

cheers