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

[Mostafa Saleh <smostafa@google.com>]

On Thu, Jul 09, 2026 at 10:08:19AM +0100, Catalin Marinas wrote:
> 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>

Thanks! Would you prefer if I send a new version removing
limited_addressing?

Thanks,
Mostafa