linux-kernel - Re: [PATCH] riscv: make ZONE

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Message-ID: <ZvJemFN1qdvi8qo-@infradead.org>
Date: Mon, 23 Sep 2024 23:39:20 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Vladimir Kondratiev <Vladimir.Kondratiev@...ileye.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH] riscv: make ZONE_DMA32 optional

On Sun, Sep 22, 2024 at 10:06:59AM +0000, Vladimir Kondratiev wrote:
> This is the whole point - there are platforms where you can't allocate 32-bit
> dma-able memory.

Ugg.  They will be broke for all kinds of devices (plug in PCIe devices
IP IP blocks).

> Unless there's IOMMU or some platform specific tricks,
> DMA address is same as physical address.

Well, that's where the usual platform specific trick is, as the
platforms with high DRAM either have a mapping between physical
and DMA range (see the dma_ranges concept in the DMA and OF code)
or a mirror of the higher addresses.  Or very rarely (e.g. sparc)
a required IOMMU that is always used.