Re: [PATCH 5/8] drm/i915/guc: Move lrc desc setup to where it is needed

[John Harrison <john.c.harrison@xxxxxxxxx>]

On 2/22/2022 17:12, Ceraolo Spurio, Daniele wrote:
> On 2/17/2022 3:52 PM, John.C.Harrison@xxxxxxxxx wrote:
> > From: John Harrison <John.C.Harrison@xxxxxxxxx>
> >
> > The LRC descriptor was being initialised early on in the context
> > registration sequence. It could then be determined that the actual
> > registration needs to be delayed and the descriptor would be wiped
> > out. This is inefficient, so move the setup to later in the process
> > after the point of no return.
> >
> > Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
> > ---
> >   drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c | 11 +++++++++--
> >   1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c 
> > b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > index 0ab2d1a24bf6..aa74ec74194a 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > @@ -2153,6 +2153,8 @@ static int __guc_action_register_context(struct 
> > intel_guc *guc,
> >                            0, loop);
> >   }
> >   +static void prepare_context_registration_info(struct intel_context 
> > *ce);
> > +
> >   static int register_context(struct intel_context *ce, bool loop)
> >   {
> >       struct intel_guc *guc = ce_to_guc(ce);
> > @@ -2163,6 +2165,8 @@ static int register_context(struct 
> > intel_context *ce, bool loop)
> >       GEM_BUG_ON(intel_context_is_child(ce));
> >       trace_intel_context_register(ce);
> >   +    prepare_context_registration_info(ce);
> > +
> >       if (intel_context_is_parent(ce))
> >           ret = __guc_action_register_multi_lrc(guc, ce, ce->guc_id.id,
> >                                 offset, loop);
> > @@ -2246,7 +2250,6 @@ static void 
> > prepare_context_registration_info(struct intel_context *ce)
> >       struct intel_context *child;
> >         GEM_BUG_ON(!engine->mask);
> > -    GEM_BUG_ON(!sched_state_is_init(ce));
> >         /*
> >        * Ensure LRC + CT vmas are is same region as write barrier is 
> > done
> > @@ -2314,9 +2317,13 @@ static int try_context_registration(struct 
> > intel_context *ce, bool loop)
> >       bool context_registered;
> >       int ret = 0;
> >   +    GEM_BUG_ON(!sched_state_is_init(ce));
> > +
> >       context_registered = ctx_id_mapped(guc, desc_idx);
> >   -    prepare_context_registration_info(ce);
> > +    if (context_registered)
> > +        clr_ctx_id_mapping(guc, desc_idx);
> > +    set_ctx_id_mapping(guc, desc_idx, ce);
>
> I think we can do the clr unconditionally. Also, should we drop the 
> clr/set pair in prepare_context_registration_info? it shouldn't be 
> needed, unless I'm missing a path where we don;t pass through here.
>
> Daniele
I don't believe so.

The point is that the context id might have changed (it got stolen, 
re-used, etc. - all the state machine code below can cause aborts and 
retries and such like if something is pending and the register needs to 
be delayed). So we need to clear out the old mapping and add a new one 
to be safe. Also, I'm not sure if it is safe to do a xa_store to an 
already used entry as an update or if you are supposed to clear it 
first? But that's what the code did before and I'm trying to not change 
any actual behaviour here.

John.

> >         /*
> >        * The context_lookup xarray is used to determine if the hardware