linux-kernel - Re: [PATCH v2 01/11] asm-generic: add dma_zone

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Date:   Mon, 26 Aug 2019 09:09:39 +0200
From:   Christoph Hellwig <hch@....de>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     catalin.marinas@....com, hch@....de, wahrenst@....net,
        marc.zyngier@....com, robh+dt@...nel.org,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-arch@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
        Arnd Bergmann <arnd@...db.de>, phill@...pberryi.org,
        f.fainelli@...il.com, will@...nel.org,
        linux-kernel@...r.kernel.org, eric@...olt.net, mbrugger@...e.com,
        linux-rpi-kernel@...ts.infradead.org, akpm@...ux-foundation.org,
        frowand.list@...il.com, m.szyprowski@...sung.com
Subject: Re: [PATCH v2 01/11] asm-generic: add dma_zone_size

On Tue, Aug 20, 2019 at 04:58:09PM +0200, Nicolas Saenz Julienne wrote:
> Some architectures have platform specific DMA addressing limitations.
> This will allow for hardware description code to provide the constraints
> in a generic manner, so as for arch code to properly setup it's memory
> zones and DMA mask.

I know this just spreads the arm code, but I still kinda hate it.

MAX_DMA_ADDRESS is such an oddly defined concepts.  We have the mm
code that uses it to start allocating after the dma zones, but
I think that would better be done using a function returning
1 << max(zone_dma_bits, 32) or so.  Then we have about a handful
of drivers using it that all seem rather bogus, and one of which
I think are usable on arm64.