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Date:   Fri, 9 Oct 2020 18:10:52 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, Will Deacon <will@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        iommu@lists.linux-foundation.org, Rob Herring <robh+dt@kernel.org>,
        linux-rpi-kernel@lists.infradead.org,
        Frank Rowand <frowand.list@gmail.com>,
        Christoph Hellwig <hch@lst.de>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711
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On Fri, Oct 09, 2020 at 06:23:06PM +0200, Ard Biesheuvel wrote:
> On Fri, 9 Oct 2020 at 17:24, Lorenzo Pieralisi
> <lorenzo.pieralisi@arm.com> wrote:
> > We can move this check to IORT code and call it from arm64 if it
> > can be made to work.
> 
> Finding the smallest value in the IORT, and assigning it to
> zone_dma_bits if it is < 32 should be easy. But as I understand it,
> having these separate DMA and DMA32 zones is what breaks kdump, no? So
> how is this going to fix the underlying issue?

If zone_dma_bits is 32, ZONE_DMA32 disappears into ZONE_DMA (GFP_DMA32
allocations fall back to ZONE_DMA).

kdump wants DMA-able memory and, without a 30-bit ZONE_DMA, that would
be the bottom 32-bit. With the introduction of ZONE_DMA, this suddenly
became 1GB. We could change kdump to allocate ZONE_DMA32 but this one
may also be small as it lost 1GB to ZONE_DMA. However, the kdump kernel
would need to be rebuilt without ZONE_DMA since it won't have any. IIRC
(it's been a while since I looked), the kdump allocation couldn't span
multiple zones.

In a separate thread, we try to fix kdump to use allocations above 4G as
a fallback but this only fixes platforms with enough RAM (and maybe it's
only those platforms that care about kdump).

-- 
Catalin