linux-kernel - Re: [PATCH] dma-mapping: remove CONFIG_DMA

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Date:   Mon, 28 Feb 2022 12:04:38 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        joro@...tes.org, will@...nel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-mapping: remove CONFIG_DMA_REMAP

On Mon, Feb 28, 2022 at 10:32:54AM +0000, Robin Murphy wrote:
> Is it even possible to hit this case now? From a quick look, all the 
> architectures defining HIGHMEM either have an explicit dependency on MMU or 
> don't allow deselecting it anyway (plus I don't see how HIGHMEM && !MMU 
> could work in general), so I'm pretty sure this whole chunk should go away 
> now.
>
> With that (or if there *is* some subtle wacky case where PageHighmem() can 
> actually return true for !MMU, a comment to remind us in future),

No, you're right - I don't think we can support highmem on !CONFIG_MMU.