Re: [PATCH 2/3] drm/msm: add DRM_MSM_GEM_SYNC_CACHE for non-coherent cache maintenance

[Christoph Hellwig]

On Tue, Oct 13, 2020 at 02:42:38PM +0100, Robin Murphy wrote:
> I still think this situation would be best handled with a variant of
> dma_ops_bypass that also guarantees to bypass SWIOTLB, and can be set
> automatically when attaching to an unmanaged IOMMU domain.

dma_ops_bypass should mostly do the right thing as-is.  swiotlb bouncing
is triggered of two things:

 1) the dma_mask.  This is under control of the driver, and obviously
    if it is too small for a legit reason we can't just proceed
 2) force_dma_unencrypted() - we'd need to do an opt-out here, either
    by a flag or by being smart and looking for an attached iommu on
    the device

> That way the
> device driver can make DMA API calls in the appropriate places that do the
> right thing either way, and only needs logic to decide whether to use the
> returned DMA addresses directly or ignore them if it knows they're
> overridden by its own IOMMU mapping.

I'd be happy to review patches for this.