Re: [PATCH 1/2] drm/i915/oa: Reconfigure contexts on the fly

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Chris Wilson (2019-07-08 14:39:35)
> Quoting Tvrtko Ursulin (2019-07-08 13:24:39)
> > 
> > On 08/07/2019 13:16, Chris Wilson wrote:
> > > Quoting Tvrtko Ursulin (2019-07-08 13:10:34)
> > >>
> > >> On 08/07/2019 12:19, Chris Wilson wrote:
> > >>> Avoid a global idle barrier by reconfiguring each context by rewriting
> > >>> them with MI_STORE_DWORD from the kernel context.
> > >>>
> > >>> v2: We only need to determine the desired register values once, they are
> > >>> the same for all contexts.
> > >>>
> > >>> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > >>> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> > >>> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> > >>> Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> > >>> ---
> > >>>    drivers/gpu/drm/i915/gem/i915_gem_context.c |   2 +
> > >>>    drivers/gpu/drm/i915/gt/intel_lrc.c         |   7 +-
> > >>>    drivers/gpu/drm/i915/i915_perf.c            | 248 +++++++++++++++-----
> > >>>    3 files changed, 195 insertions(+), 62 deletions(-)
> > >>>
> > >>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > >>> index e367dce2a696..1f0d10bb88c1 100644
> > >>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > >>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > >>> @@ -624,7 +624,9 @@ i915_gem_context_create_kernel(struct drm_i915_private *i915, int prio)
> > >>>        ctx->sched.priority = I915_USER_PRIORITY(prio);
> > >>>        ctx->ring_size = PAGE_SIZE;
> > >>>    
> > >>> +     /* Isolate the kernel context from prying eyes and sticky fingers */
> > >>>        GEM_BUG_ON(!i915_gem_context_is_kernel(ctx));
> > >>> +     list_del_init(&ctx->link);
> > >>
> > >> Why exactly?
> > > 
> > > Otherwise we recursively try to modify the context.
> > 
> >  From gen8_configure_all_contexts, twice, or really recursively? If 
> > former isn't that solvable by simply skipping kernel contexts in the 
> > first loop?
> > 
> > >> Any repercussions for i915_sysfs/i915_l3_write? debugfs I gather you
> > >> won't care about?
> > > 
> > > No, because what matters for those is user contexts.
> > 
> > There isn't some time cost associated with l3_remap calls when switching 
> > contexts?
> 
> No, it's even weirder than that as it is not a context register (at
> least on hsw where we support it). I guess, we should just unlazy and emit
> a request from the sysfs.

Actually, I am wrong here. The L3 remapping registers do appear to be
saved in the context image. At least that is what my testing indicates.

I can still argue that they don't apply to the kernel context. But my
plan to remove the walk over GEM contexts and the lazy update fails.
-Chris
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