Re: [PATCH v2] arm64/mm: Correct the cache line size warning with non coherent device

[Zhangshaokun]

Hi Masayoshi,

A few trivial comments inline.

On 2019/6/14 21:11, Masayoshi Mizuma wrote:
> From: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
> 
> If the cache line size is greater than ARCH_DMA_MINALIGN (128),
> the warning shows and it's tainted as TAINT_CPU_OUT_OF_SPEC.
> 
> However, it's not good because as discussed in the thread [1], the cpu
> cache line size will be problem only on non-coherent devices.
> 
> Since the coherent flag is already introduced to struct device,
> show the warning only if the device is non-coherent device and
> ARCH_DMA_MINALIGN is smaller than the cpu cache size.
> 
> [1] https://lore.kernel.org/linux-arm-kernel/20180514145703.celnlobzn3uh5tc2@localhost/
> 
> Signed-off-by: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
> Reviewed-by: Hidetoshi Seto <seto.hidetoshi@xxxxxxxxxxxxxx>
> Tested-by: Zhang Lei <zhang.lei@xxxxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/cache.h |  7 +++++++
>  arch/arm64/kernel/cacheinfo.c  |  4 +---
>  arch/arm64/mm/dma-mapping.c    | 14 ++++++++++----
>  3 files changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/cache.h b/arch/arm64/include/asm/cache.h
> index 758af6340314..d24b7c1ecd9b 100644
> --- a/arch/arm64/include/asm/cache.h
> +++ b/arch/arm64/include/asm/cache.h
> @@ -91,6 +91,13 @@ static inline u32 cache_type_cwg(void)
>  
>  #define __read_mostly __attribute__((__section__(".data..read_mostly")))
>  
> +static inline int cache_line_size_of_cpu(void)
> +{
> +	u32 cwg = cache_type_cwg();
> +
> +	return cwg ? 4 << cwg : ARCH_DMA_MINALIGN;
> +}
> +
>  int cache_line_size(void);
>  
>  /*
> diff --git a/arch/arm64/kernel/cacheinfo.c b/arch/arm64/kernel/cacheinfo.c
> index 6eaf1c07aa4e..7fa6828bb488 100644
> --- a/arch/arm64/kernel/cacheinfo.c
> +++ b/arch/arm64/kernel/cacheinfo.c
> @@ -19,12 +19,10 @@
>  
>  int cache_line_size(void)
>  {
> -	u32 cwg = cache_type_cwg();
> -
>  	if (coherency_max_size != 0)
>  		return coherency_max_size;
>  
> -	return cwg ? 4 << cwg : ARCH_DMA_MINALIGN;
> +	return cache_line_size_of_cpu();
>  }

How about simplify it as this?

int cache_line_size(void)
{
        return coherency_max_size ? coherency_max_size :
                cache_line_size_of_cpu();
}

>  EXPORT_SYMBOL_GPL(cache_line_size);
>  
> diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
> index 1669618db08a..379589dc7113 100644
> --- a/arch/arm64/mm/dma-mapping.c
> +++ b/arch/arm64/mm/dma-mapping.c
> @@ -38,10 +38,6 @@ void arch_dma_prep_coherent(struct page *page, size_t size)
>  
>  static int __init arm64_dma_init(void)
>  {
> -	WARN_TAINT(ARCH_DMA_MINALIGN < cache_line_size(),
> -		   TAINT_CPU_OUT_OF_SPEC,
> -		   "ARCH_DMA_MINALIGN smaller than CTR_EL0.CWG (%d < %d)",
> -		   ARCH_DMA_MINALIGN, cache_line_size());
>  	return dma_atomic_pool_init(GFP_DMA32, __pgprot(PROT_NORMAL_NC));
>  }
>  arch_initcall(arm64_dma_init);
> @@ -56,7 +52,17 @@ void arch_teardown_dma_ops(struct device *dev)
>  void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
>  			const struct iommu_ops *iommu, bool coherent)
>  {
> +	int cls = cache_line_size_of_cpu();

whether we need this local variable, how about use cache_line_size_of_cpu
directly in WARN_TAINT just like before.

Thanks,
Shaokun

> +
>  	dev->dma_coherent = coherent;
> +
> +	if (!coherent)
> +		WARN_TAINT(cls > ARCH_DMA_MINALIGN,
> +			TAINT_CPU_OUT_OF_SPEC,
> +			"%s %s: ARCH_DMA_MINALIGN smaller than CTR_EL0.CWG (%d < %d)",
> +			dev_driver_string(dev), dev_name(dev),
> +			ARCH_DMA_MINALIGN, cls);
> +
>  	if (iommu)
>  		iommu_setup_dma_ops(dev, dma_base, size);
>  
>