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

[Christoph Hellwig]

On Mon, Oct 05, 2020 at 10:35:43AM -0400, Jonathan Marek wrote:
> The cache synchronization doesn't have anything to do with IOMMU (for
> example: cache synchronization would be useful in cases where drm/msm
> doesn't use IOMMU).

It has to do with doing DMA.  And we have two frameworks for doing DMA:
either the DMA API which is for general driver use, and which as part of
the design includes cache maintainance hidden behind the concept of
ownership transfers.  And we have the much more bare bones IOMMU API.

If people want to use the "raw" IOMMU API with not cache coherent
devices we'll need a cache maintainance API that goes along with it.
It could either be formally part of the IOMMU API or be separate.

> What is needed is to call arch_sync_dma_for_{cpu,device} (which is what I
> went with initially, but then decided to re-use drm/msm's
> sync_for_{cpu,device}). But you are also saying those functions aren't for
> driver use, and I doubt IOMMU maintainers will want to add wrappers for
> these functions just to satisfy this "not for driver use" requirement.

arch_sync_dma_for_{cpu,device} are low-level helpers (and not very
great ones at that).  The definitively should not be used by drivers.
They would be very useful buildblocks for a IOMMU cache maintainance
API.

Of course the best outcome would be if we could find a way for the MSM
drm driver to just use DMA API and not deal with the lower level
abstractions.  Do you remember why the driver went for use of the IOMMU
API?