linux-kernel - Re: [PATCH 1/4] of/fdt: Update zone_dma

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Message-ID: <20201012084746.GA9844@gaia>
Date:   Mon, 12 Oct 2020 09:47:47 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Will Deacon <will@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        iommu@...ts.linux-foundation.org, Rob Herring <robh+dt@...nel.org>,
        linux-rpi-kernel@...ts.infradead.org,
        Frank Rowand <frowand.list@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711

On Mon, Oct 12, 2020 at 08:47:15AM +0200, Christoph Hellwig wrote:
> On Fri, Oct 09, 2020 at 06:10:52PM +0100, Catalin Marinas wrote:
> > kdump wants DMA-able memory and,
> 
> DMAable by whom?  The only way to guranteed DMAable memory is to use
> the DMA memory allocator(s) and pass a specific device to them.  Everyting
> else is just fundamentally broken.  Note that even when device is not
> DMAable we can still use swiotlb to access it.

What I meant is that the new kexec'ed kernel needs some memory in the
ZONE_DMA range, currently set to the bottom 30-bit even for platforms
that can cope with the whole 32-bit range (anything other than RPi4).
The memory range available to the kdump kernels is limited to what
reserve_crashkernel() allocated, which may not fit in the lower 1GB.

There are two ongoing threads (complementary):

1. Change the arm64 reserve_crashkernel() similar to x86 which allocates
   memory above 4G with a small block in the ZONE_DMA range.

2. Allow zone_dma_bits to be 32 for arm64 platforms other than RPi4.

The second point also fixes some regressions with CMA reservations that
could no longer fit in the lower 1GB.

-- 
Catalin