linux-kernel - Re: [PATCH] dma-direct: Skip cache prep for HighMem coherent allocations

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Message-ID: <yq5afr8bdnrj.fsf@kernel.org>
Date: Mon, 12 Jan 2026 17:52:08 +0530
From: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
To: Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Cc: Arnd Bergmann <arnd@...nel.org>, Linus Walleij <linusw@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [PATCH] dma-direct: Skip cache prep for HighMem coherent
 allocations

Marek Szyprowski <m.szyprowski@...sung.com> writes:

> On 09.01.2026 04:15, Aneesh Kumar K.V wrote:
>> Robin Murphy <robin.murphy@....com> writes:
>>> On 2026-01-08 12:41 pm, Marek Szyprowski wrote:
>>>> On 08.01.2026 11:50, Robin Murphy wrote:
>>>>> On 2026-01-02 3:51 pm, Aneesh Kumar K.V (Arm) wrote:
>>>>>> dma_direct_alloc() calls arch_dma_prep_coherent() to clean any dirty
>>>>>> cache lines from the kernel linear alias before creating a coherent
>>>>>> remapping.
>>>>>>
>>>>>> HighMem pages have no kernel alias mapping, so there are no alias cache
>>>>>> lines to clean. Skip arch_dma_prep_coherent() for HighMem allocations.
>>>>> This is assuming that caches are always cleaned when unmapping
>>>>> highmem, and no still-mapped highmem pages are dirty - how is that
>>>>> guaranteed? The fact that they're not in the linear map doesn't mean
>>>>> they don't necessarily have kernel aliases in either vmalloc
>>>>> pagetables or caches.
>>>> Right, so it is better to keep this unconditional
>>>> arch_dma_prep_coherent() call. I will drop it from dma-mapping-fixes then.
>>> Yeah, I think the confusing thing here is that there are architectures
>>> with CONFIG_HIGHMEM that don't actually check for and handle it in their
>>> arch_dma_prep_coherent() as they seemingly should, however I'm not sure
>>> off-hand whether they also support/use highmem CMA in the manner that
>>> could end up being an issue in practice (the lack of any reports of
>>> crashes or DMA corruption over the last however many years suggests not...)
>>>
>> Should we then remove the PageHighMem() check with DMA_ATTR_NO_KERNEL_MAPPING?
>
> Right, this has to be unified.

I had a related question, how do we handle cache flushes required for
architectures that don't implement CONFIG_ARCH_HAS_DMA_PREP_COHERENT (arch/arm)?

-aneesh