Re: [PATCH] arm64: Kconfig: select ZONE_DMA

[Nicolas Saenz Julienne <nsaenzjulienne@suse.de>]

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On Thu, 2020-12-17 at 12:43 +0000, Catalin Marinas wrote:
> On Thu, Dec 17, 2020 at 09:44:31AM +0100, Nicolas Saenz Julienne wrote:
> > On Thu, 2020-12-17 at 16:08 +0800, Peng Fan (OSS) wrote:
> > > From: Peng Fan <peng.fan@nxp.com>
> > > 
> > > ZONE_DMA should not be disabled, otherwise arm64_dma_phys_limit is
> > > left uninitialized and cause swiotlb have IO TLB above 4GB which
> > > might crash some platforms
> 
> Actually, arm64_dma_phys_limit would be zero-initialised. The
> ARCH_LOW_ADDRESS_LIMIT becomes ~0UL, hence it covers the whole address
> space.
> 
> > > Signed-off-by: Peng Fan <peng.fan@nxp.com>
> > > ---
> > > 
> > > Not sure whether need to address code to initialize the variables or
> > > force select ZONE_DMA
> > 
> > What is the cause for the swiotlb related crashes? I assume it's DMA into an
> > address too high for the bus, but it might be something else.
> 
> I think that's the case, swiotlb is not within the low 32-bit of the
> address space.
> 
> > I figure you have a setup with ZONE_DMA32, ZONE_NORMAL and !ZONE_DMA.
> > 
> > First of all, I'd suggest you try arm64's defaults (all zones enabled), the
> > series I mention above should fix most of the issues we've had with
> > ZONE_DMA/ZONE_DMA32 in the past. We now parse DT/ACPI and only create two
> > distinct DMA zones if really needed. Otherwise ZONE_DMA spans the whole 32 bit
> > address space.
> > 
> > That said, IMO we're not doing the right thing in the !ZONE_DMA && ZONE_DMA32
> > case, and this should fix it (I didn't test it):
> > 
> > - #define ARCH_LOW_ADDRESS_LIMIT	(arm64_dma_phys_limit - 1)
> > + #define ARCH_LOW_ADDRESS_LIMIT	(arm64_dma_phys_limit ? : arm64_dma32_phys_limit)
> 
> Does this limit need to be inclusive?

Yes, I'm missing a '- 1'.

I'll send a fix if there is no further push back/comments.

Regards,
Nicolas


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