Re: PCIe coherency in spec (was: [RFC PATCH 2/2] drm/ttm: downgrade cached to write_combined when snooping not available)

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jul 04, 2024 at 02:40:16PM +0800, Icenowy Zheng wrote:
> > Nothing in the PCIe specifications that mandates a programming model.
> > Non-coherent DMA is extremely common in lower end devices, and
> > despite
> > all the issues that it causes well supported in Linux.
> > 
> > What are you trying to solve?
> 
> Currently the DRM TTM subsystem (and GPU drivers using it) will assume
> coherency and fail on these non-coherent systems with cryptic error
> messages (like `[drm:amdgpu_ring_test_helper [amdgpu]] *ERROR* ring gfx
> test failed (-110)`) without mentioning coherency issues at all.
> 
> My original patchset tries to solve this problem by make the TTM
> subsystem sensible of coherency status (and prevent CPU-side cached
> mapping when non-coherent), but got argued by TTM maintainer and the
> maintainer says TTM's ignorance on non-coherent systems is intentional.

From the dma mapping subsystem POV all drivers not supporting DMA
incoherent devices are buggy.  But if the drm maintaintainers disagree
(and they have in the past) there is not much I can do, especially
given the DRM is rather special in abuses of all kinds of APIs anyway.