linux-kernel - Re: [PATCH] arm64: swiotlb: Don’t shrink default buffer when bounce is forced

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Message-ID: <aYOVf066Fdr9L-nw@arm.com>
Date: Wed, 4 Feb 2026 18:52:47 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	iommu@...ts.linux.dev, Will Deacon <will@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>, suzuki.poulose@....com,
	steven.price@....com
Subject: Re: [PATCH] arm64: swiotlb: Don’t
 shrink default buffer when bounce is forced

On Tue, Jan 20, 2026 at 12:31:02PM +0530, Aneesh Kumar K.V (Arm) wrote:
> arm64 reduces the default swiotlb size (for unaligned kmalloc()
> bouncing) when it detects that no swiotlb bouncing is needed.
> 
> If swiotlb bouncing is explicitly forced via the command line
> (swiotlb=force), this heuristic must not apply. Add a swiotlb helper to
> query the forced-bounce state and use it to skip the resize when
> bouncing is forced.

I think the logic you proposed in reply to Robin might work better but
have you actually hit a problem that triggered this patch? Do people
passing swiotlb=force expect a specific size for the buffer?

-- 
Catalin