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. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
