Re: [PATCH v2] riscv: mm: still create swiotlb buffer for kmalloc() bouncing if required

[Alexandre Ghiti]

Hi Jisheng,

On 02/12/2023 14:42, Jisheng Zhang wrote:
> After commit f51f7a0fc2f4 ("riscv: enable DMA_BOUNCE_UNALIGNED_KMALLOC
> for !dma_coherent"), for non-coherent platforms with less than 4GB
> memory, we rely on users to pass "swiotlb=mmnn,force" kernel parameters
> to enable DMA bouncing for unaligned kmalloc() buffers. Now let's go
> further: If no bouncing needed for ZONE_DMA, let kernel automatically
> allocate 1MB swiotlb buffer per 1GB of RAM for kmalloc() bouncing on
> non-coherent platforms, so that no need to pass "swiotlb=mmnn,force"
> any more.

IIUC, DMA_BOUNCE_UNALIGNED_KMALLOC is enabled for all non-coherent 
platforms, even those with less than 4GB of memory. But the DMA bouncing 
(which is necessary to enable kmalloc-8/16/32/96...) was not enabled 
unless the user specified "swiotlb=mmnn,force" on the kernel command 
line. But does that mean that if the user did not specify 
"swiotlb=mmnn,force", the kmalloc-8/16/32/96 were enabled anyway and the 
behaviour was wrong (by lack of DMA bouncing)?

I'm trying to understand if that's a fix or an enhancement.

Thanks,

Alex


> The math of "1MB swiotlb buffer per 1GB of RAM for kmalloc() bouncing"
> is taken from arm64. Users can still force smaller swiotlb buffer by
> passing "swiotlb=mmnn".
>
> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> ---
>
> since v2:
>   - fix build error if CONFIG_RISCV_DMA_NONCOHERENT=n
>
>   arch/riscv/include/asm/cache.h |  2 +-
>   arch/riscv/mm/init.c           | 16 +++++++++++++++-
>   2 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cache.h b/arch/riscv/include/asm/cache.h
> index 2174fe7bac9a..570e9d8acad1 100644
> --- a/arch/riscv/include/asm/cache.h
> +++ b/arch/riscv/include/asm/cache.h
> @@ -26,8 +26,8 @@
>   
>   #ifndef __ASSEMBLY__
>   
> -#ifdef CONFIG_RISCV_DMA_NONCOHERENT
>   extern int dma_cache_alignment;
> +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
>   #define dma_get_cache_alignment dma_get_cache_alignment
>   static inline int dma_get_cache_alignment(void)
>   {
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 2e011cbddf3a..cbcb9918f721 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -162,11 +162,25 @@ static void print_vm_layout(void) { }
>   
>   void __init mem_init(void)
>   {
> +	bool swiotlb = max_pfn > PFN_DOWN(dma32_phys_limit);
>   #ifdef CONFIG_FLATMEM
>   	BUG_ON(!mem_map);
>   #endif /* CONFIG_FLATMEM */
>   
> -	swiotlb_init(max_pfn > PFN_DOWN(dma32_phys_limit), SWIOTLB_VERBOSE);
> +	if (IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) && !swiotlb &&
> +	    dma_cache_alignment != 1) {
> +		/*
> +		 * If no bouncing needed for ZONE_DMA, allocate 1MB swiotlb
> +		 * buffer per 1GB of RAM for kmalloc() bouncing on
> +		 * non-coherent platforms.
> +		 */
> +		unsigned long size =
> +			DIV_ROUND_UP(memblock_phys_mem_size(), 1024);
> +		swiotlb_adjust_size(min(swiotlb_size_or_default(), size));
> +		swiotlb = true;
> +	}
> +
> +	swiotlb_init(swiotlb, SWIOTLB_VERBOSE);
>   	memblock_free_all();
>   
>   	print_vm_layout();