On Fri, Jun 30, 2017 at 08:47:27AM +0200, Christian König wrote: > Am 30.06.2017 um 04:24 schrieb Michel Dänzer: > > On 29/06/17 07:05 PM, Daniel Vetter wrote: > > > On Thu, Jun 29, 2017 at 06:58:05PM +0900, Michel Dänzer wrote: > > > > On 29/06/17 05:23 PM, Christian König wrote: > > > > > Am 29.06.2017 um 04:35 schrieb Michel Dänzer: > > > > > > On 29/06/17 08:26 AM, John Brooks wrote: > > > > > > > On Wed, Jun 28, 2017 at 03:05:32PM +0200, Christian König wrote: > > > > > > > > > Instead of the flag being set in stone at BO creation, set the flag > > > > > > > > > when a > > > > > > > > > page fault occurs so that it goes somewhere CPU-visible, and clear > > > > > > > > > it when > > > > > > > > > the BO is requested by the GPU. > > > > > > > > > > > > > > > > > > However, clearing the CPU_ACCESS_REQUIRED flag may move BOs in GTT to > > > > > > > > > invisible VRAM, where they may promptly generate another page > > > > > > > > > fault. When > > > > > > > > > BOs are constantly moved back and forth like this, it is highly > > > > > > > > > detrimental > > > > > > > > > to performance. Only clear the flag on CS if: > > > > > > > > > > > > > > > > > > - The BO wasn't page faulted for a certain amount of time > > > > > > > > > (currently 10 > > > > > > > > > seconds), and > > > > > > > > > - its last page fault didn't occur too soon (currently 500ms) after > > > > > > > > > its > > > > > > > > > last CS request, or vice versa. > > > > > > > > > > > > > > > > > > Setting the flag in amdgpu_fault_reserve_notify() also means that > > > > > > > > > we can > > > > > > > > > remove the loop to restrict lpfn to the end of visible VRAM, because > > > > > > > > > amdgpu_ttm_placement_init() will do it for us. > > > > > > > > I'm fine with the general approach, but I'm still absolutely not > > > > > > > > keen about > > > > > > > > clearing the flag when userspace has originally specified it. > > > > > > Is there any specific concern you have about that? > > > > > Yeah, quite a bunch actually. We want to use this flag for P2P buffer > > > > > sharing in the future as well and I don't intent to add another one like > > > > > CPU_ACCESS_REALLY_REQUIRED or something like this. > > > > Won't a BO need to be pinned while it's being shared with another device? > > > That's an artifact of the current kernel implementation, I think we could > > > do better (but for current use-cases where we share a bunch of scanouts > > > and maybe a few pixmaps it's pointless). I wouldn't bet uapi on this never > > > changing. > > Surely there will need to be some kind of transaction though to let the > > driver know when a BO starts/stops being shared with another device? > > Either via the existing dma-buf callbacks, or something similar. We > > can't rely on userspace setting a "CPU access" flag to make sure a BO > > can be shared with other devices? Well I just jumped into the middle of this that it's not entirely out of the question as an idea, but yeah we'd need to rework the dma-buf stuff with probably a callback to evict mappings/stall for outstanding rendering or something like that. > Well, the flag was never intended to be used by userspace. > > See the history was more like we need something in the kernel to place the > BO in CPU accessible VRAM. > > Then the closed source UMD came along and said hey we have the concept of > two different heaps for visible and invisible VRAM, how does that maps to > amdgpu? > > I unfortunately was to tired to push back hard enough on this.... Ehrm, are you saying you have uapi for the closed source stack only? I can help with the push back on this with a revert, no problem :-) -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
