From: Ard Biesheuvel <ardb@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
<devicetree@vger.kernel.org>,
Frank Rowand <frowand.list@gmail.com>,
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,
Will Deacon <will@kernel.org>, Christoph Hellwig <hch@lst.de>,
Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711
Date: Sat, 10 Oct 2020 12:36:30 +0200 [thread overview]
Message-ID: <CAMj1kXEWeqGVr=QV7zQ+dXBK=t_Lh+W9q1+kLLdBw8=Pj798Ng@mail.gmail.com> (raw)
In-Reply-To: <20201009171051.GL23638@gaia>
On Fri, 9 Oct 2020 at 19:10, Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> 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).
>
One thing that strikes me as odd is that we are applying the same
shifting logic to ZONE_DMA as we are applying to ZONE_DMA32, i.e., if
DRAM starts outside of the zone, it is shifted upwards.
On a typical ARM box, this gives me
[ 0.000000] Zone ranges:
[ 0.000000] DMA [mem 0x0000000080000000-0x00000000bfffffff]
[ 0.000000] DMA32 [mem 0x00000000c0000000-0x00000000ffffffff]
[ 0.000000] Normal [mem 0x0000000100000000-0x0000000fffffffff]
i.e., the 30-bit addressable range has bit 31 set, which is weird.
I wonder if it wouldn't be better (and less problematic in the general
case) to drop this logic for ZONE_DMA, and simply let it remain empty
unless there is really some memory there.
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next prev parent reply other threads:[~2020-10-10 10:36 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-01 16:17 [PATCH 0/4] arm64: Default to 32-bit wide ZONE_DMA Nicolas Saenz Julienne
2020-10-01 16:17 ` [PATCH 1/4] of/fdt: Update zone_dma_bits when running in bcm2711 Nicolas Saenz Julienne
2020-10-01 17:15 ` Catalin Marinas
2020-10-01 17:23 ` Catalin Marinas
2020-10-01 17:31 ` Nicolas Saenz Julienne
2020-10-01 20:02 ` Rob Herring
2020-10-02 11:55 ` Catalin Marinas
2020-10-08 10:05 ` Nicolas Saenz Julienne
2020-10-08 10:13 ` Catalin Marinas
2020-10-08 19:43 ` Ard Biesheuvel
2020-10-09 3:59 ` Jeremy Linton
2020-10-09 8:37 ` Nicolas Saenz Julienne
2020-10-09 7:10 ` Christoph Hellwig
2020-10-09 7:37 ` Ard Biesheuvel
2020-10-09 8:36 ` Nicolas Saenz Julienne
2020-10-09 9:13 ` Ard Biesheuvel
2020-10-09 13:33 ` Nicolas Saenz Julienne
2020-10-09 15:24 ` Lorenzo Pieralisi
2020-10-09 16:23 ` Ard Biesheuvel
2020-10-09 17:10 ` Catalin Marinas
2020-10-10 10:36 ` Ard Biesheuvel [this message]
2020-10-10 10:53 ` Nicolas Saenz Julienne
2020-10-10 12:38 ` Catalin Marinas
2020-10-12 6:47 ` Christoph Hellwig
2020-10-12 8:47 ` Catalin Marinas
2020-10-02 9:05 ` kernel test robot
2020-10-01 16:17 ` [PATCH 2/4] dma-direct: Turn zone_dma_bits default value into a define Nicolas Saenz Julienne
2020-10-01 16:17 ` [PATCH 3/4] arm64: Default to 32-bit ZONE_DMA Nicolas Saenz Julienne
2020-10-01 17:19 ` Catalin Marinas
2020-10-01 16:17 ` [PATCH 4/4] mm: Update DMA zones description Nicolas Saenz Julienne
2020-10-01 17:19 ` Catalin Marinas
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