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? -- Earthling Michel Dänzer | http://www.amd.com Libre software enthusiast | Mesa and X developer
