[PATCH] arm64: mm: reduce swiotlb size when dynamic swiotlb enabled

[Will Deacon]

[+Isaac]

On Wed, May 08, 2024 at 09:23:00PM +0800, Kefeng Wang wrote:
> After commit a1e50a82256e ("arm64: Increase the swiotlb buffer size
> 64MB"), the swiotlb buffer size increased to 64M in case of 32-bit only
> devices require many bounce buffering via swiotlb, but with the
> CONFIG_SWIOTLB_DYNAMIC enabled, we could reduce swiotlb size from 64M to
> 4M(MAX_ORDER_NR_PAGES << PAGE_SHIFT) again since swiotlb buffer size could
> be allocated danamicly, and this should save 60M for most platform which
> don't require too much swiotlb buffer.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang at huawei.com>
> ---
>  arch/arm64/mm/init.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 9b5ab6818f7f..425222c13d97 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -370,18 +370,23 @@ void __init bootmem_init(void)
>  void __init mem_init(void)
>  {
>  	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
> +	unsigned long size = 0;
>  
>  	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb) {
>  		/*
>  		 * If no bouncing needed for ZONE_DMA, reduce the swiotlb
>  		 * buffer for kmalloc() bouncing to 1MB per 1GB of RAM.
>  		 */
> -		unsigned long size =
> -			DIV_ROUND_UP(memblock_phys_mem_size(), 1024);
> -		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
> +		size = DIV_ROUND_UP(memblock_phys_mem_size(), 1024);
>  		swiotlb = true;
>  	}
>  
> +	if (IS_ENABLED(CONFIG_SWIOTLB_DYNAMIC) && !size)
> +		size = MAX_ORDER_NR_PAGES << PAGE_SHIFT;
> +
> +	if (size)
> +		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));

If 64MiB is deemed too much, can't you just provide an alternative size
on the kernel cmdline (swiotlb=) instead of adding more heuristics here?

Otherwise, if you really want to change the default, then I think
swiotlb_size_or_default() is the place to do it, not in the arch code.

Will