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

Robin Murphy
2019-06-13T17:10:25Z

On 13/06/2019 16:54, Catalin Marinas wrote:
> 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.
>

Agreed, and I'd say we should keep the taint too, since if this
situation ever was hit the potential crashes would be weird and random
and not obviously DMA-related.

Robin.