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

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Message-ID: <8a872e0c-f13e-4ff1-a939-bc6814b3ec3e@samsung.com>
Date: Fri, 16 Jan 2026 11:29:37 +0100
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>, 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

On 12.01.2026 13:22, Aneesh Kumar K.V wrote:
> 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)?

ARM 32bit architecture provides arch_dma_alloc(), which handles cache 
management internally.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland