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Date:   Fri, 9 Sep 2022 07:47:15 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Will McVicker <willmcvicker@google.com>,
        Jingoo Han <jingoohan1@gmail.com>,
        Gustavo Pimentel <gustavo.pimentel@synopsys.com>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Rob Herring <robh@kernel.org>,
        Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
        Bjorn Helgaas <bhelgaas@google.com>, kernel-team@android.com,
        Vidya Sagar <vidyas@nvidia.com>, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target
 address
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 <5bfd7d4d-d431-6321-89bc-663dcd36e930@arm.com>
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On Fri, Sep 09, 2022 at 02:47:19PM +0100, Robin Murphy wrote:
> On 2022-09-09 14:29, Christoph Hellwig wrote:
> > On Thu, Aug 25, 2022 at 06:50:25PM +0000, Will McVicker wrote:
> > > Since not all devices require a 32-bit MSI address, add support to the
> > > PCIe host driver to allow setting the DMA mask to 64-bits if the 32-bit
> > > allocation fails. This allows kernels to disable ZONE_DMA32 and bounce
> > > buffering (swiotlb) without risking not being able to get a 32-bit address
> > > during DMA allocation.
> > 
> > Umm.  You can't just disable ZONE_DMA32.  Linux absolutely requires a
> > 32-bit dma mask to work, it is in fact the implicit default.
> 
> Eh, it's behind CONFIG_EXPERT, which makes it enough of a "I think I know
> what I'm doing and accept responsibility for picking up the pieces if it
> breaks" thing.

Seem like indeed on arm64 there is a way to disable it.  The x86 model
is to just select it unconditionally, which I think is the right way
if we don't want to get into completely random failures.