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

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Date:   Wed, 14 Dec 2022 13:00:25 -0500
From:   Luben Tuikov <luben.tuikov@....com>
To:     <iommu@...ts.linux.dev>
CC:     Luben Tuikov <luben.tuikov@....com>,
        Christoph Hellwig <hch@....de>,
        "Robin Murphy" <robin.murphy@....com>,
        Alex Deucher <Alexander.Deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: [PATCH] dma-direct: Optimize get_required_mask

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.

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
-- 
2.39.0