Subject: [PATCH] arm64: mm: only initialize swiotlb when necessary

From: [email-masked] (Jisheng Zhang)
Date: Wed, 8 Jun 2016 16:01:40 +0800

Dear all,

On Wed, 8 Jun 2016 15:53:46 +0800 Jisheng Zhang wrote:

> we only initialize swiotlb when swiotlb_force is true or not all system
> memory is DMA-able, this trivial optimization saves us 64MB when
> swiotlb is not necessary.

another solution is to call swiotlb_free() as ppc does. Either solution can
solve my problem. If maintainers prefer that solution, I can send a v2 patch.

Thanks,
Jisheng

> 
> Signed-off-by: Jisheng Zhang <[email-masked]>
> ---
>  arch/arm64/mm/dma-mapping.c | 15 ++++++++++++++-
>  arch/arm64/mm/init.c        |  3 ++-
>  2 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
> index c566ec8..46a4157 100644
> --- a/arch/arm64/mm/dma-mapping.c
> +++ b/arch/arm64/mm/dma-mapping.c
> @@ -19,6 +19,7 @@
>  
>  #include <linux/gfp.h>
>  #include <linux/acpi.h>
> +#include <linux/bootmem.h>
>  #include <linux/export.h>
>  #include <linux/slab.h>
>  #include <linux/genalloc.h>
> @@ -29,6 +30,8 @@
>  
>  #include <asm/cacheflush.h>
>  
> +static int swiotlb __read_mostly;
> +
>  static pgprot_t __get_dma_pgprot(struct dma_attrs *attrs, pgprot_t prot,
>  				 bool coherent)
>  {
> @@ -341,6 +344,13 @@ static int __swiotlb_get_sgtable(struct device *dev, struct sg_table *sgt,
>  	return ret;
>  }
>  
> +static int __swiotlb_dma_supported(struct device *hwdev, u64 mask)
> +{
> +	if (swiotlb)
> +		return swiotlb_dma_supported(hwdev, mask);
> +	return 1;
> +}
> +
>  static struct dma_map_ops swiotlb_dma_ops = {
>  	.alloc = __dma_alloc,
>  	.free = __dma_free,
> @@ -354,7 +364,7 @@ static struct dma_map_ops swiotlb_dma_ops = {
>  	.sync_single_for_device = __swiotlb_sync_single_for_device,
>  	.sync_sg_for_cpu = __swiotlb_sync_sg_for_cpu,
>  	.sync_sg_for_device = __swiotlb_sync_sg_for_device,
> -	.dma_supported = swiotlb_dma_supported,
> +	.dma_supported = __swiotlb_dma_supported,
>  	.mapping_error = swiotlb_dma_mapping_error,
>  };
>  
> @@ -513,6 +523,9 @@ EXPORT_SYMBOL(dummy_dma_ops);
>  
>  static int __init arm64_dma_init(void)
>  {
> +	if (swiotlb_force || max_pfn > (arm64_dma_phys_limit >> PAGE_SHIFT))
> +		swiotlb = 1;
> +
>  	return atomic_pool_init();
>  }
>  arch_initcall(arm64_dma_init);
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index d45f862..7d25b4d 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -403,7 +403,8 @@ static void __init free_unused_memmap(void)
>   */
>  void __init mem_init(void)
>  {
> -	swiotlb_init(1);
> +	if (swiotlb_force || max_pfn > (arm64_dma_phys_limit >> PAGE_SHIFT))
> +		swiotlb_init(1);
>  
>  	set_max_mapnr(pfn_to_page(max_pfn) - mem_map);
>