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

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Date:   Thu, 13 Jun 2019 16:54:35 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Masayoshi Mizuma <msys.mizuma@...il.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 06:02:47PM -0400, Masayoshi Mizuma wrote:
> 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?

The coherency_max_size gives you the largest cache line in the system,
independent of whether a device is coherent or not. You may have a
device that does not snoop L1/L2 but there is a transparent L3 (system
cache) with a larger cache line that the device may be able to snoop.
The coherency_max_size and therefore cache_line_size() would give you
this L3 value but the device would work fine since CWG <=
ARCH_DMA_MINALIGN.

> 
> @@ -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);

I think the easiest here is to add a local variable:

	int cls = 4 << cache_type_cwg();

and check it against ARCH_DMA_MINALIGN.

-- 
Catalin