Re: [PATCH v4 2/5] dma: swiotlb: Centralize minimal pool sizing

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

On Mon, Sep 14, 2026 at 12:31:31PM +0530, Aneesh Kumar K.V (Arm) wrote:
> NOTE: We lose the RISC-V SWIOTLB_ANY setting in this patch. It will be
> reinstated in a follow-up patch where we make SWIOTLB_ANY redundant.

[...]

> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index e0e4c103d603..734712230f35 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -165,31 +165,11 @@ static void print_vm_layout(void) { }
>  
>  void __init arch_mm_preinit(void)
>  {
> -	bool swiotlb = max_pfn > PFN_DOWN(dma32_phys_limit) &&
> -		       memblock_start_of_DRAM() < dma32_phys_limit;
>  	unsigned int swiotlb_flags = SWIOTLB_VERBOSE;
>  #ifdef CONFIG_FLATMEM
>  	BUG_ON(!mem_map);
>  #endif /* CONFIG_FLATMEM */
>  
> -	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb &&
> -	    dma_cache_alignment != 1) {
> -		/*
> -		 * No 32-bit DMA bouncing needed (either all DRAM is within
> -		 * the 32-bit limit, or it all starts above it), but
> -		 * kmalloc() buffers whose sizes are not cache-line-aligned
> -		 * still require bouncing for non-coherent DMA.  Use
> -		 * SWIOTLB_ANY so that the buffer can be allocated from high
> -		 * memory when DRAM starts above dma32_phys_limit.  Allocate
> -		 * ~1 MB per 1 GB of RAM.
> -		 */
> -		unsigned long size =
> -			DIV_ROUND_UP(memblock_phys_mem_size(), 1024);
> -		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
> -		swiotlb = true;
> -		swiotlb_flags |= SWIOTLB_ANY;
> -	}

Does it break the bisectability for risc-v? IIUC this undoes commit
cfca5a48b03f until patch 5. Basically if there's no low memory and no
SWIOTLB_ANY passed, memblock_alloc_low() will fail.

I think we could preserve SWIOTLB_ANY until patch 5. Or it might be
simpler to move patches 2 and 3 at the end (I haven't tried).

> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index aa19e81ae544..90e0bfd0715c 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -304,6 +304,11 @@ unsigned long swiotlb_size_or_default(void)
>  	return default_nslabs << IO_TLB_SHIFT;
>  }
>  
> +static bool __init swiotlb_default_size_changed(void)
> +{
> +	return default_nslabs != IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT;
> +}

I mentioned before in passing
(https://lore.kernel.org/all/ann6noRzoPLL9kDY@arm.com/), if one sets
swiotlb= to the default size, it will go unnoticed. I know it's an
existing issue but we might as well fix it now with a bool that tells us
the swiotlb size was forced/changed.

-- 
Catalin