On Tue, Feb 3, 2015 at 7:28 AM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> 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 <daniel@xxxxxxxx> 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. > If gpu mmu needs some/all updates to happen from command-stream then probably better to handle it internally.. That is a slightly different scenario from msm, where we have many instances of the same iommu[*] scattered through the SoC in front of various different devices. BR, -R [*] at least from iommu register layout, same driver is used for all instances.. but maybe the tlb+walker are maybe more tightly integrated to the gpu, but that is just speculation on implementation details based on some paper I found along the way > > * - as a direct result of that, I've stopped all further development of > etnaviv drm, and I'm intending to strip it out from my Xorg DDX driver > as the etnaviv drm API which Christian wants is completely incompatible > with the non-etnaviv drm, and that just creates far too much pain in the > DDX driver. > > -- > FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up > according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
