linux-kernel - Re: [PATCH 1/2] arm64/mm: check cpu cache line size with non-coherent device

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Date:   Tue, 11 Jun 2019 18:02:47 -0400
From:   Masayoshi Mizuma <msys.mizuma@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        linux-kernel@...r.kernel.org,
        Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
        Zhang Lei <zhang.lei@...fujitsu.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 1/2] arm64/mm: check cpu cache line size with
 non-coherent device

On Tue, Jun 11, 2019 at 07:00:07PM +0100, Catalin Marinas wrote:
> On Tue, Jun 11, 2019 at 11:17:30AM -0400, Masayoshi Mizuma wrote:
> > --- a/arch/arm64/mm/dma-mapping.c
> > +++ b/arch/arm64/mm/dma-mapping.c
> > @@ -91,10 +91,6 @@ static int __swiotlb_mmap_pfn(struct vm_area_struct *vma,
> >  
> >  static int __init arm64_dma_init(void)
> >  {
> > -	WARN_TAINT(ARCH_DMA_MINALIGN < cache_line_size(),
> > -		   TAINT_CPU_OUT_OF_SPEC,
> > -		   "ARCH_DMA_MINALIGN smaller than CTR_EL0.CWG (%d < %d)",
> > -		   ARCH_DMA_MINALIGN, cache_line_size());
> >  	return dma_atomic_pool_init(GFP_DMA32, __pgprot(PROT_NORMAL_NC));
> >  }
> >  arch_initcall(arm64_dma_init);
> > @@ -473,6 +469,11 @@ void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
> >  			const struct iommu_ops *iommu, bool coherent)
> >  {
> >  	dev->dma_coherent = coherent;
> > +
> > +	if (!coherent && (cache_line_size() > ARCH_DMA_MINALIGN))
> > +		dev_WARN(dev, "ARCH_DMA_MINALIGN smaller than CTR_EL0.CWG (%d < %d)",
> > +				ARCH_DMA_MINALIGN, cache_line_size());
> 
> I'm ok in principle with this patch, with the minor issue that since
> commit 7b8c87b297a7 ("arm64: cacheinfo: Update cache_line_size detected
> from DT or PPTT") queued for 5.3 cache_line_size() gets the information
> from DT or ACPI. The reason for this change is that the information is
> used for performance tuning rather than DMA coherency.
> 
> You can go for a direct cache_type_cwg() check in here, unless Robin
> (cc'ed) has a better idea.

Got it, thanks.
I believe coherency_max_size is zero in case of coherent is false,
so I'll modify the patch as following. Does it make sense?

@@ -57,6 +53,11 @@ void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
                        const struct iommu_ops *iommu, bool coherent)
 {
        dev->dma_coherent = coherent;
+
+       if (!coherent && (cache_line_size() > ARCH_DMA_MINALIGN))
+               dev_WARN(dev, "ARCH_DMA_MINALIGN smaller than CTR_EL0.CWG (%d < %d)",
+                               ARCH_DMA_MINALIGN, (4 << cache_type_cwg()));
+
        if (iommu)
                iommu_setup_dma_ops(dev, dma_base, size);

Thanks,
Masa