linux-kernel - Re: [PATCH 2/3] ARM/keystone: move the DMA offset handling under ifdef CONFIG_ARM

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Date:   Fri, 11 Sep 2020 12:27:21 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Santosh Shilimkar <ssantosh@...nel.org>,
        Jim Quinlan <james.quinlan@...adcom.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-acpi@...r.kernel.org, devicetree@...r.kernel.org,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH 2/3] ARM/keystone: move the DMA offset handling under
 ifdef CONFIG_ARM_LPAE

On 2020-09-11 12:15, Russell King - ARM Linux admin wrote:
> On Thu, Sep 10, 2020 at 07:40:37AM +0200, Christoph Hellwig wrote:
>> The DMA offset notifier can only be used if PHYS_OFFSET is at least
>> KEYSTONE_HIGH_PHYS_START, which can't be represented by a 32-bit
>> phys_addr_t.  Currently the code compiles fine despite that, a pending
>> change to the DMA offset handling would create a compiler warning for
>> this case.  Add an ifdef to not compile the code except for LPAE
>> configs.
> 
> However, to have use of the high physical offset, LPAE needs to be
> enabled, which ensures that phys_addr_t is 64-bit.
> 
> I believe that DMA is non-coherent on this platform unless the high
> physical address is used. Or something like that.

Yeah, it's probably not a configuration that anyone would actually want 
to use in anger on Keystone itself, but as long as folks might have 
ARCH_KEYSTONE selected in their non-LPAE multiplatform config we should 
avoid build regressions. I did wonder if Keystone should simply have a 
hard dependency on LPAE, but there does appear to be some explicit 
support for running directly from the non-coherent low 2G alias :/

Robin.