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

Masayoshi Mizuma
2019-06-11T22:03:06Z

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