Re: [Intel-gfx] [PATCH v5 5/7] drm/i915/gt: Create per-tile RC6 sysfs interface

[Andi Shyti <andi.shyti@linux.intel.com>]

Hi Tvrtko,

> > Now tiles have their own sysfs interfaces under the gt/
> > directory. Because RC6 is a property that can be configured on a
> > tile basis, then each tile should have its own interface
> > 
> > The new sysfs structure will have a similar layout for the 4 tile
> > case:
> > 
> > /sys/.../card0
> >           ├── gt
> >           │   ├── gt0
> >           │   │   ├── id
> >           │   │   ├── rc6_enable
> >           │   │   ├── rc6_residency_ms
> >           .   .   .
> >           .   .   .
> >           .   .
> >           │   └── gtN
> >           │       ├── id
> >           │       ├── rc6_enable
> >           │       ├── rc6_residency_ms
> >           │       .
> >           │       .
> >           │
> >           └── power/                -+
> >                ├── rc6_enable        |    Original interface
> >                ├── rc6_residency_ms  +->  kept as existing ABI;
> >                .                     |    it multiplexes over
> >                .                     |    the GTs
> >                                     -+
> > 
> > The existing interfaces have been kept in their original location
> > to preserve the existing ABI. They act on all the GTs: when
> > reading they provide the average value from all the GTs.
> 
> Average feels very odd to me. I'd ask if we can get away providing an errno
> instead? Or tile zero data?

Real multiplexing would be providing something when reading and
when writing. The idea of average came while revieweing with
Chris the write multiplexing. Indeed it makes sense to provide
some common value, but I don't know how useful it can be to the
user (still if the user needs any average).

Joonas, Chris... any idea?

> Case in point, and please correct me if I am wrong, legacy rc6_enable
> returns tile zero, while residency returns average.

As the interface is done now, the rc6_enable is just returning
whether the gpu (i.e. i915, not gt) supports RC6 or not. I think
there is a patch later.

> Even the deprecated message gets logged with every access right?
> 
> Btw is the deperecated message limited to multi-tile platforms (can't see
> that it is) and what is the plan for that?

yes, at this point the message would need to be removed and I
forgot to do it.

> It's a tough problem, no easy answers even after all this time. :D

yeah! quite hard to get it conceptually right!

Thanks Tvrtko,
Andi