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

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Message-ID: <yq5a8qd6iep6.fsf@kernel.org>
Date: Fri, 06 Feb 2026 11:41:33 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
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

Catalin Marinas <catalin.marinas@....com> writes:

> 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?
>

This issue was observed while implementing swiotlb for a trusted device.
I was testing the protected swiotlb space using the swiotlb=force
option, which causes the device to use swiotlb even in protected mode.
As per Robin, an end user using the swiotlb=force option will also
specify a custom swiotlb size

-aneesh