linux-kernel - Re: [PATCH] dma-direct: Optimize get_required

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Date:   Wed, 14 Dec 2022 20:57:24 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Luben Tuikov <luben.tuikov@....com>, iommu@...ts.linux.dev
Cc:     Christoph Hellwig <hch@....de>,
        Alex Deucher <Alexander.Deucher@....com>,
        Christian König <christian.koenig@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma-direct: Optimize get_required_mask

On 2022-12-14 18:00, Luben Tuikov wrote:
> Optimize dma_direct_get_required_mask(), in that we don't need to multiply by
> two if we don't subtract 1 from the exponent. That is,
> 
>      (1 << (n - 1)) * 2 - 1 <==>
>      2^(n-1) * 2^1 - 1       = (by rule of exponents)
>      2^n - 1                 <==>
>      (1 << n) - 1.

...except when n==64 (for the actual code below), in which case the 
result of the shift becomes undefined.

Robin.

> Cc: Christoph Hellwig <hch@....de>
> Cc: Robin Murphy <robin.murphy@....com>
> Cc: Alex Deucher <Alexander.Deucher@....com>
> Cc: Christian König <christian.koenig@....com>
> Cc: iommu@...ts.linux.dev
> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
> Signed-off-by: Luben Tuikov <luben.tuikov@....com>
> ---
>   kernel/dma/direct.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 63859a101ed831..bb416a3949dac0 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -41,7 +41,7 @@ u64 dma_direct_get_required_mask(struct device *dev)
>   	phys_addr_t phys = (phys_addr_t)(max_pfn - 1) << PAGE_SHIFT;
>   	u64 max_dma = phys_to_dma_direct(dev, phys);
>   
> -	return (1ULL << (fls64(max_dma) - 1)) * 2 - 1;
> +	return (1ULL << fls64(max_dma)) - 1;
>   }
>   
>   static gfp_t dma_direct_optimal_gfp_mask(struct device *dev, u64 dma_mask,
> 
> base-commit: e2ca6ba6ba0152361aa4fcbf6067db71b2c7a770