On Thu, Nov 29, 2018 at 12:03 PM Robin Murphy <robin.murphy@xxxxxxx> wrote: > > On 29/11/2018 19:57, Tomasz Figa wrote: > > On Thu, Nov 29, 2018 at 11:40 AM Jordan Crouse <jcrouse@xxxxxxxxxxxxxx> wrote: > >> > >> On Thu, Nov 29, 2018 at 01:48:15PM -0500, Rob Clark wrote: > >>> On Thu, Nov 29, 2018 at 10:54 AM Christoph Hellwig <hch@xxxxxx> wrote: > >>>> > >>>> On Thu, Nov 29, 2018 at 09:42:50AM -0500, Rob Clark wrote: > >>>>> Maybe the thing we need to do is just implement a blacklist of > >>>>> compatible strings for devices which should skip the automatic > >>>>> iommu/dma hookup. Maybe a bit ugly, but it would also solve a problem > >>>>> preventing us from enabling per-process pagetables for a5xx (where we > >>>>> need to control the domain/context-bank that is allocated by the dma > >>>>> api). > >>>> > >>>> You can detach from the dma map attachment using arm_iommu_detach_device, > >>>> which a few drm drivers do, but I don't think this is the problem. > >>> > >>> I think even with detach, we wouldn't end up with the context-bank > >>> that the gpu firmware was hard-coded to expect, and so it would > >>> overwrite the incorrect page table address register. (I could be > >>> mis-remembering that, Jordan spent more time looking at that. But it > >>> was something along those lines.) > >> > >> Right - basically the DMA domain steals context bank 0 and the GPU is hard coded > >> to use that context bank for pagetable switching. > >> > >> I believe the Tegra guys also had a similar problem with a hard coded context > >> bank. > > AIUI, they don't need a specific hardware context, they just need to > know which one they're actually using, which the domain abstraction hides. > > > Wait, if we detach the GPU/display struct device from the default > > domain and attach it to a newly allocated domain, wouldn't the newly > > allocated domain use the context bank we need? Note that we're already > > The arm-smmu driver doesn't, but there's no fundamental reason it > couldn't. That should just need code to refcount domain users and > release hardware contexts for domains with no devices currently attached. > > Robin. > > > doing that, except that we're doing it behind the back of the DMA > > mapping subsystem, so that it keeps using the IOMMU version of the DMA > > ops for the device and doing any mapping operations on the default > > domain. If we ask the DMA mapping to detach, wouldn't it essentially > > solve the problem? Thanks Robin. Still, my point is that the MSM DRM driver attaches the GPU struct device to a new domain it allocates using iommu_domain_alloc() and it seems to work fine, so I believe it's not the problem we're looking into with this patch. Best regards, Tomasz
