Messages in this thread | | | Date | Thu, 26 Jan 2023 22:52:43 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [RFC] mm: Allow ZONE_DMA32 to be disabled via kernel command line |
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On January 26, 2023 10:35:55 PM PST, Christoph Hellwig <hch@lst.de> wrote: >On Thu, Jan 26, 2023 at 10:51:17AM -0800, Dave Hansen wrote: >> >> Also, what are the practical implications here? There are obviously an >> ever decreasing number of 32-bit DMA devices out there. Somebody that >> has one and uses this option might be sad because now they're stuck >> using ZONE_DMA which is quite tiny. >> >> What other ZONE_DMA32 users are left? Will anyone else care? There is >> some DMA32 slab and vmalloc() functionality remaining. Is it impacted? > >DMA32 never supported lab. But < 64-bit DMA device are unfortunately >still not uncommon, and configuring out ZONE_DMA32 breaks them pretty >badly as we guarantee that a DMA mask of 32-bit always works. > >So I'm not only very much against this patch, but also the currently >existing way to configure out ZONE_DMA32 on arm64, which needs to >go away. > >If people want ZONE_DMA32 to go away we need something to replace >it first, like a large enough CMA region in the 32-bit addressable >range.
Not to mention all kinds of odd masks like 30, 31, 39, 40, 46, ... bits.
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