On Wed, Aug 07, 2019 at 10:48:56AM +0200, Daniel Vetter wrote: > > other drm drivers how do they guarantee addressability without an > > iommu?) > > We use shmem to get at swappable pages. We generally just assume that > the gpu can get at those pages, but things fall apart in fun ways: > - some setups somehow inject bounce buffers. Some drivers just give > up, others try to allocate a pool of pages with dma_alloc_coherent. > - some devices are misdesigned and can't access as much as the cpu. We > allocate using GFP_DMA32 to fix that.
Well, for shmem you can't really call allocators directly, right? One thing I have in my pipeline is a dma_alloc_pages API that allocates pages that are guaranteed to be addressably by the device or otherwise fail. But that doesn't really help with the shmem fs. > Also modern gpu apis pretty much assume you can malloc() and then use > that directly with the gpu. Which is fine as long as the GPU itself supports full 64-bit addressing (or always sits behind an iommu), and the platform doesn't impose addressing limit, which unfortunately some that are shipped right now still do :( But userspace malloc really means dma_map_* anyway, so not really relevant for memory allocations.
