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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 12 Jul 2022 14:10:56 +0200
Message-ID: <CAK8P3a3bSYZyKV_BywQFE5vi5RE4WoPwspXfTtO1ycV1zQDbmw@mail.gmail.com>
Subject: Re: [PATCH 4/4] sample/reserved_mem: Introduce a sample of struct
 page and dio support to no-map rmem
To:     Li Chen <me@linux.beauty>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Li Chen <lchen@ambarella.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        DTML <devicetree@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>
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On Tue, Jul 12, 2022 at 12:55 PM Li Chen <me@linux.beauty> wrote:
>  >
>  > This is a common limitation that gets handled automatically by setting
>  > the dma_mask of the device through the dma-ranges property in DT.
>  > When the driver does dma_alloc_coherent() or similar to gets its buffer,
>  > it will then allocate pages below this boundary.
>
> Thanks for the tip! I wasn't aware that dma-ranges can be used by devices other than PCIe controllers.

You should actually have dma-ranges listed in the parent of any DMA master
capable device, to list the exact DMA capabilities. Without this, devices
fall back to the default 32-bit address limit, which would be correct for your
video device but is often wrong in other devices.

        Arnd