On Tue, Feb 03, 2015 at 12:28:14PM +0000, Russell King - ARM Linux wrote: > On Tue, Feb 03, 2015 at 08:48:56AM +0100, Daniel Vetter wrote: > > On Mon, Feb 02, 2015 at 03:30:21PM -0500, Rob Clark wrote: > > > On Mon, Feb 2, 2015 at 11:54 AM, Daniel Vetter <dan...@ffwll.ch> wrote: > > > >> My initial thought is for dma-buf to not try to prevent something than > > > >> an exporter can actually do.. I think the scenario you describe could > > > >> be handled by two sg-lists, if the exporter was clever enough. > > > > > > > > That's already needed, each attachment has it's own sg-list. After all > > > > there's no array of dma_addr_t in the sg tables, so you can't use one sg > > > > for more than one mapping. And due to different iommu different devices > > > > can easily end up with different addresses. > > > > > > > > > Well, to be fair it may not be explicitly stated, but currently one > > > should assume the dma_addr_t's in the dmabuf sglist are bogus. With > > > gpu's that implement per-process/context page tables, I'm not really > > > sure that there is a sane way to actually do anything else.. > > > > Hm, what does per-process/context page tables have to do here? At least on > > i915 we have a two levels of page tables: > > - first level for vm/device isolation, used through dma api > > - 2nd level for per-gpu-context isolation and context switching, handled > > internally. > > > > Since atm the dma api doesn't have any context of contexts or different > > pagetables, I don't see who you could use that at all. > > What I've found with *my* etnaviv drm implementation (not Christian's - I > found it impossible to work with Christian, especially with the endless > "msm doesn't do it that way, so we shouldn't" responses and his attitude > towards cherry-picking my development work [*]) is that it's much easier to > keep the GPU MMU local to the GPU and under the control of the DRM MM code, > rather than attaching the IOMMU to the DMA API and handling it that way. > > There are several reasons for that: > > 1. DRM has a better idea about when the memory needs to be mapped to the > GPU, and it can more effectively manage the GPU MMU. > > 2. The GPU MMU may have TLBs which can only be flushed via a command in > the GPU command stream, so it's fundamentally necessary for the MMU to > be managed by the GPU driver so that it knows when (and how) to insert > the flushes.
3. Switching between different address spaces (for per gpu context isolation) often requires intricate knowledge about the gpu and close coordination. Well maybe just a part of 2 really, but an important one. Fully agree and tbh I'm not sure whether the current push in arm to expose all gpu mmus as iommus is solid. Even for pasid (per-context iommu tables) which is a big official standard there's still a lot of open questions about how to do it properly. And it requires strict hw support so that the hw always knows which pasid it should use for a given dma access. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
