Re: [PATCH 12/20] drm/i915/icl: Check for fused-off VDBOX and VEBOX instances

Chris Wilson
2018-02-17T12:19:05Z

Quoting Sagar Arun Kamble (2018-02-17 12:10:32)
>
>
> On 2/17/2018 2:34 PM, Chris Wilson wrote:
> > Quoting Sagar Arun Kamble (2018-02-17 08:51:44)
> >> Earlier I had thought of calling ASSIGN_FW_DOMAINS_TABLE,
> >> ASSIGN_*_MMIO_VFUNCS before intel_uncore_fw_domains_init
> >> and use I915_READ here for reading the fuse. But that approach seems to
> >> expose the vfuncs and forcewake table before fw domains
> >> are initialized. Although we can get to know invalid access to
> >> read/write accessors before fw_domains get initialized, current ordering
> >> of fw_domains init followed by fw domain table/read/write vfuncs init
> >> seems right. So using fw_domains_get/put as suggested above
> >> should be the way.
> >>
> >> Chris, Tvrtko, Mika, do you agree?
> > What's the complication with the fw_domains? Do the additional powerwell
> > depend on the fused status of the extra engines? Does it matter if the
> > fw_domain are prepped if they are never used?
> Yes. To discover the available VD/VE engines/power domains, fuse needs
> to be read under blitter forcewake as
> RC6 will be enabled by BIOS. We do have usage of forcewake in IVB to
> discover FORCEWAKE_MT availability in fw_domains_init.
> It should not be a problem if they are prepped but never used.
> > Imo, I would have placed the fused discovery in
> > intel_engines_init_mmio() (where we do the setup and can take forcewake).
> > Then adding something like intel_uncore_reinit_mmio() (which would just
> > prune the uncore->fw_domains) after checking fused status with commentary
> > doesn't seem that horrible.
> Yes. This approach looks good too. But, we might want to optimize the
> driver_load to avoid this setup at first place
> instead of pruning later.

How many cycles does it take to run through all domains and set up the
register offsets? No mmio access required right, we are just moving
memory around without even hitting locked instructions?

> Another related setup that can be avoided/pruned is the fw_range for the
> engine fused off as it can improve the fw lookup.

Which is a bsearch on register range, I doubt that's going to be
substantially impacted by removing a few ranges. Where it matters, we
should be looking to precalculate the result anyway.
-Chris