[Intel-gfx] [PATCH] drm/i915/tgl: WaDisableGPGPUMidThreadPreemption

[Rafael Antognolli]

On Wed, Mar 04, 2020 at 04:24:13PM +0000, Tvrtko Ursulin wrote:
> 
> On 04/03/2020 16:02, Rafael Antognolli wrote:
> > On Wed, Mar 04, 2020 at 03:31:44PM +0000, Tvrtko Ursulin wrote:
> > > From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > > 
> > > Enable FtrPerCtxtPreemptionGranularityControl bit and select thread-
> > > group as the default preemption level.
> > > 
> > > v2:
> > >   * Remove register whitelisting (Rafael, Tony).
> > > 
> > > Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > > Cc: Michał Winiarski <michal.winiarski at intel.com>
> > > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > > Cc: piotr.zdunowski at intel.com
> > > Cc: michal.mrozek at intel.com
> > > Cc: Tony Ye <tony.ye at intel.com>
> > > Cc: Rafael Antognolli <rafael.antognolli at intel.com>
> > 
> > Thanks for CC'ing me. I also saw a reply from Jason yesterday, but I
> > don't see it in the list now (though my mail client a mess lately).
> 
> I saw nothing from Jason, but there was an email from you asking about
> interface descriptors and whitelisting which is why I copied you.
> 
> > But he asked whether it's possible for Media and OpenCL drivers to
> > also disable mid-thread preemption through the
> > INTERFACE_DESCRIPTOR_DATA, instead of from the kernel side, so we could
> > try to experiment with it in the future.
> > 
> > Also, do you have an idea of how broken it is? Or is it just not tested
> > because no driver is currently implementing it? And do you know if the
> > windows 3D drivers implement it at all? I see code in the driver that
> > seems to me that it's only disabled in certain cases...
> > 
> > To summarize, I think we should either:
> >     1) Disable mid-thread preemption from the kernel and not whitelist
> >     the register (just like you do in this patch); or
> >     2) Not do anything at all from the kernel, and let userspace disable
> >     it if needed.
> > 
> > I think 2) is better, if it's not an issue to the other userspace
> > drivers (OpenCL and Media).
> 
> I know it is somewhat broken like in
> https://gitlab.freedesktop.org/drm/intel/issues/1293.
> 
> And I know OpenCL and Media would prefer i915 to handle it, but that's
> always the case. :) OpenCL and Media folks are on the thread so can comment
> if they are okay with handling this themselves.
> 
> Indeed a blanket ban in i915 means no one can try it out later without
> further kernel changes.

Well, based on your comment from the previous patch:

"General thinking is, since MTP is considered not validated / broken /
dangerous, i915 should default it off. But yes, whitelisting or not on
top is open."

Maybe we should simply ban it and be done. So this patch is:

Acked-by: Rafael Antognolli <rafael.antognolli at intel.com>

> Regards,
> 
> Tvrtko