[PATCH] arm64: swiotlb: Reduce the default size if no ZONE_DMA bouncing needed

[Robin Murphy]

On 2023-10-06 16:32, Catalin Marinas wrote:
> With CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC enabled, the arm64 kernel still
> allocates the default SWIOTLB buffer (64MB) even if ZONE_DMA is disabled
> or all the RAM fits into this zone. However, this potentially wastes a
> non-negligible amount of memory on platforms with little RAM.
> 
> Reduce the SWIOTLB size to 1MB per 1GB of RAM if only needed for
> kmalloc() buffer bouncing.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
> Suggested-by: Ross Burton <ross.burton at arm.com>
> Cc: Ross Burton <ross.burton at arm.com>
> Cc: Robin Murphy <robin.murphy at arm.com>
> Cc: Will Deacon <will at kernel.org>
> ---
>   arch/arm64/mm/init.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 8a0f8604348b..54ee1a4868c2 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -493,8 +493,16 @@ void __init mem_init(void)
>   {
>   	bool swiotlb = max_pfn > PFN_DOWN(arm64_dma_phys_limit);
>   
> -	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC))
> +	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 =
> +			ALIGN(memblock_phys_mem_size(), SZ_1G) >> 10;

Hmm, I wondered if DIV_ROUND_UP(memblock_phys_mem_size(), 1024) might be 
any easier, but by the time I've typed it out it's still just as long :)

Either way then,

Reviewed-by Robin Murphy <robin.murphy at arm.com>

> +		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
>   		swiotlb = true;
> +	}
>   
>   	swiotlb_init(swiotlb, SWIOTLB_VERBOSE);
>