Re: [PATCH 1/3] arm64/mm: Simplify SWIOTLB setup in arch_mm_preinit()

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Jun 03, 2026 at 11:05:20AM +0000, Mostafa Saleh wrote:
> At the moment, arch_mm_preinit() checks if the system has limited
> addressing or is running under CCA to enable SWIOTLB, only after to
> be forced to true anyway if it was false due to
> CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC being unconditionally true for
> arm64.
> 
> Simplify this logic, by making it clear that SWIOTLB is always used
> but its size depends on the address layout of the system.
> 
> Signed-off-by: Mostafa Saleh <smostafa@google.com>
> ---
>  arch/arm64/mm/init.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 97987f850a33..fcc9c05a8fe6 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -336,25 +336,22 @@ void __init arch_setup_zero_pages(void)
>  void __init arch_mm_preinit(void)
>  {
>  	unsigned int flags = SWIOTLB_VERBOSE;
> -	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
> +	bool limited_addressing = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
>  
>  	if (is_realm_world()) {
> -		swiotlb = true;
>  		flags |= SWIOTLB_FORCE;
> -	}
> -
> -	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb) {
> +	} else if (!limited_addressing) {

I'm fine with implementing Mike's comment on doing the check inline.

I recall some people still comment out the dma kmalloc bounce option:

https://github.com/raspberrypi/linux/commit/a07564753639fe5ffb3f59d7364f14e6e68e3e9e

(there was no way I could convince them it's worse memory wastage
without the option)

So happy to make it slightly harder for them ;).

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>