Subject: Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma_coherent()

From: Jason Gunthorpe <[email-masked]>
Date: Wed, 6 Apr 2022 11:24:32 -0300

On Wed, Apr 06, 2022 at 02:56:56PM +0100, Robin Murphy wrote:
> On 2022-04-05 17:16, Jason Gunthorpe wrote:
> > vdpa and usnic are trying to test if IOMMU_CACHE is supported. The correct
> > way to do this is via dev_is_dma_coherent()
> 
> Not necessarily...
> 
> Disregarding the complete disaster of PCIe No Snoop on Arm-Based systems,
> there's the more interesting effectively-opposite scenario where an SMMU
> bridges non-coherent devices to a coherent interconnect. It's not something
> we take advantage of yet in Linux, and it can only be properly described in
> ACPI, but there do exist situations where IOMMU_CACHE is capable of making
> the device's traffic snoop, but dev_is_dma_coherent() - and
> device_get_dma_attr() for external users - would still say non-coherent
> because they can't assume that the SMMU is enabled and programmed in just
> the right way.

Oh, I didn't know about device_get_dma_attr()..

Considering your future issue, maybe this:

/*
 * true if the given domain supports IOMMU_CACHE and when dev is attached to
 * that domain it will have coherent DMA and require no cache
 * maintenance when IOMMU_CACHE is used.
 */
bool iommu_domain_supports_coherent_dma(struct iommu_domain *domain, struct device *dev)
{
	return device_get_dma_attr(dev) == DEV_DMA_COHERENT;
}

? In future it could become a domain op and the SMMU driver could
figure out the situation you described?

Jason