linux-kernel - Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target address

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Date:   Fri, 9 Sep 2022 16:00:27 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Will McVicker <willmcvicker@...gle.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>, kernel-team@...roid.com,
        Vidya Sagar <vidyas@...dia.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v5 2/2] PCI: dwc: Add support for 64-bit MSI target
 address

On 2022-09-09 15:47, Christoph Hellwig wrote:
> 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.

IIRC there were reasons for wanting as much ZONE_NORMAL memory as 
possible; for the embedded folks who are already typically running with 
"swiotlb=noforce" to save memory because they know their hardware, we 
may as well let them have the footgun. Distros and other general-purpose 
configs should rightly not be going anywhere near this.

Cheers,
Robin.