Date:   Thu, 26 Jan 2023 22:52:43 -0800
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Christoph Hellwig <hch@lst.de>, Dave Hansen <dave.hansen@intel.com>
CC:     Georgi Djakov <quic_c_gdjako@quicinc.com>, catalin.marinas@arm.com,
        will@kernel.org, dave.hansen@linux.intel.com, luto@kernel.org,
        peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, hch@lst.de, m.szyprowski@samsung.com,
        robin.murphy@arm.com, linux-arm-kernel@lists.infradead.org,
        iommu@lists.linux.dev, linux-kernel@vger.kernel.org,
        djakov@kernel.org
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=2Ede> wrote=
:
>On Thu, Jan 26, 2023 at 10:51:17AM -0800, Dave Hansen wrote:
>>=20
>> Also, what are the practical implications here?  There are obviously an
>> ever decreasing number of 32-bit DMA devices out there=2E  Somebody tha=
t
>> has one and uses this option might be sad because now they're stuck
>> using ZONE_DMA which is quite tiny=2E
>>=20
>> What other ZONE_DMA32 users are left?  Will anyone else care?  There is
>> some DMA32 slab and vmalloc() functionality remaining=2E  Is it impacte=
d?
>
>DMA32 never supported lab=2E  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=2E
>
>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=2E
>
>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=2E

Not to mention all kinds of odd masks like 30, 31, 39, 40, 46, =2E=2E=2E b=
its=2E