On Tuesday, January 8, 2019 7:53:05 AM PST Joonas Lahtinen wrote: > + Ken/Jason for Mesa > Quoting Matt Roper (2019-01-07 21:19:31) > > On Mon, Jan 07, 2019 at 01:23:50PM +0100, Michał Winiarski wrote: > > > On Mon, Jan 07, 2019 at 01:01:16PM +0200, Joonas Lahtinen wrote: > > > > Quoting José Roberto de Souza (2019-01-04 19:37:00) > > > > > According to Workaround database ICL also needs > > > > > WaEnablePreemptionGranularityControlByUMD, to allow userspace to do > > > > > fine-granularity preemptions per-context. > > > > > > > > I must wonder where is the userspace component that needs this, and why > > > > it hasn't been noticed earlier? > > > > > > > > Or is this one more of the cases when no userspace actually uses the > > > > register? > > > > > > It's used: > > > https://gitlab.freedesktop.org/mesa/mesa/blob/master/src/mesa/drivers/dri/i965/brw_state_upload.c#L64 > > > > > > -Michał > > > > Wasn't this just an artificial i915-only workaround that was added to > > prevent breakage of pre-preemption UMD's? Initial gen9 driver releases > > didn't support preemption, so when preemption support did get added to > > i915, the kernel had to force object-level off by default at context > > creation to avoid breaking old userspace that didn't build batch buffers > > with all the necessary preemption workarounds. This CS_CHICKEN1 > > register was then exposed to userspace so that newer, preemption-aware > > userspace could opt back in if it properly supported preemption. > > > > For gen11, there shouldn't be any "old" userspace around that doesn't > > support preemption, so shouldn't the kernel just leave object-level > > preemption enabled by default (meaning there's no need to expose this > > register to userspace to allow it to explicitly opt-in)? > > Makes sense to me. We should have known by know if somebody expects to > control the register, because they would be failing to do so. > > Mesa could also drop the register load for Gen11+ > > Regards, Joonas + Rafael, as he's done all the preemption work in Mesa. That seems reasonable to me. It looks like i965 always enables mid-object preemption (sets CS_CHICKEN1 bit 0) on Gen10+, and never disables it. You can probably safely turn it on by default, and we can stop writing the register altogether. Thanks for the heads up! --Ken
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