Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 12 Oct 2020 09:47:47 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Ard Biesheuvel <ardb@kernel.org>,
        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>,
        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
Message-ID: <20201012084746.GA9844@gaia>
References: <20201002115541.GC7034@gaia>
 <12f33d487eabd626db4c07ded5a1447795eed355.camel@suse.de>
 <20201009071013.GA12208@lst.de>
 <CAMj1kXG+7Lq=rgUfyU_XS9LrJwpUiC8nKsRPom+R0=phuXioHQ@mail.gmail.com>
 <513833810c15b5efeab7c3cbae1963a78c71a79f.camel@suse.de>
 <CAMj1kXGP_OTKgqMT0-+t3=7EKDY26y9n9xjLodSF1E-mUCe9tg@mail.gmail.com>
 <20201009152433.GA19953@e121166-lin.cambridge.arm.com>
 <CAMj1kXFuqw3qNRAB78OzvMws+t7=B6L8pASA36D2fxXobbvpUA@mail.gmail.com>
 <20201009171051.GL23638@gaia>
 <20201012064715.GA2548@lst.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20201012064715.GA2548@lst.de>
User-Agent: Mutt/1.10.1 (2018-07-13)
Precedence: bulk

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