linux-kernel - Re: [PATCH v2] dma-direct: set decrypted flag for remapped DMA allocations

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Message-Id: <bb62cb70-77ba-4867-b202-2947d383cf15@app.fastmail.com>
Date: Wed, 07 Jan 2026 22:23:09 +0100
From: "Arnd Bergmann" <arnd@...nel.org>
To: "Linus Walleij" <linusw@...nel.org>
Cc: "Jason Gunthorpe" <jgg@...pe.ca>,
 "Aneesh Kumar K.V (Arm)" <aneesh.kumar@...nel.org>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org, "Marek Szyprowski" <m.szyprowski@...sung.com>,
 "Robin Murphy" <robin.murphy@....com>,
 "Matthew Wilcox" <willy@...radead.org>,
 "Suzuki K Poulose" <suzuki.poulose@....com>,
 "Dmitry Osipenko" <digetx@...il.com>, "Thierry Reding" <treding@...dia.com>
Subject: Re: [PATCH v2] dma-direct: set decrypted flag for remapped DMA allocations

On Wed, Jan 7, 2026, at 18:41, Linus Walleij wrote:
> I did a grep 'alloc-ranges' but now I'm a bit slow in the head
> so cannot really figure out if these will end up outside the
> linear map or not.

The way I see it, we have two ways of doing it:

1GB of RAM, CMA in the last 256MB:
arch/arm/boot/dts/nvidia/tegra20-acer-a500-picasso.dts:
        memory@0 {
                reg = <0x00000000 0x40000000>;
        };
        reserved-memory {
                #address-cells = <1>;
                #size-cells = <1>;
                ranges;
                linux,cma@...00000 {
                        compatible = "shared-dma-pool";
                        alloc-ranges = <0x30000000 0x10000000>;
                        size = <0x10000000>; /* 256MiB */
                        linux,cma-default;
                        reusable;
                };
        };

2GB of RAM, CMA in the first 768MB:
arch/arm/boot/dts/nvidia/tegra30-asus-p1801-t.dts
        memory@...00000 {
                reg = <0x80000000 0x80000000>;
        };
        reserved-memory {
                #address-cells = <1>;
                #size-cells = <1>;
                ranges;
                linux,cma@...00000 {
                        compatible = "shared-dma-pool";
                        alloc-ranges = <0x80000000 0x30000000>;
                        size = <0x10000000>;            /* 256MiB */
                        linux,cma-default;
                        reusable;
                };
         };

With CONFIG_VMSPLIT_3G, the first example spans all of highmem,
i.e. 0x30000000-0x3fffffff, the second example spans all of
lowmem at 0x80000000-0xafffffff.

      Arnd