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Date:   Wed, 29 Sep 2021 09:36:30 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
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Subject: Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO
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On Wed, Sep 29, 2021 at 08:48:28AM +0000, Tian, Kevin wrote:

> ARM:
>     - set to snoop format if IOMMU_CACHE
>     - set to nonsnoop format if !IOMMU_CACHE
> (in both cases TLP snoop bit is ignored?)

Where do you see this? I couldn't even find this functionality in the
ARM HW manual??
 
What I saw is ARM linking the IOMMU_CACHE to a IO PTE bit that causes
the cache coherence to be disabled, which is not ignoring no snoop.

> I didn't identify the exact commit for above meaning change.
> 
> Robin, could you help share some thoughts here?

It is this:

static int dma_info_to_prot(enum dma_data_direction dir, bool coherent,
		     unsigned long attrs)
{
	int prot = coherent ? IOMMU_CACHE : 0;

Which sets IOMMU_CACHE based on:

static void *iommu_dma_alloc(struct device *dev, size_t size,
		dma_addr_t *handle, gfp_t gfp, unsigned long attrs)
{
	bool coherent = dev_is_dma_coherent(dev);
	int ioprot = dma_info_to_prot(DMA_BIDIRECTIONAL, coherent, attrs); 

Driving IOMMU_CACHE from dev_is_dma_coherent() has *NOTHING* to do
with no-snoop TLPs and everything to do with the arch cache
maintenance API

Jason