Re: [PATCH v3 0/6] Introduce Bandwidth OPPs for interconnect paths

Viresh Kumar
2019-07-30T03:02:06Z

On 29-07-19, 13:12, Saravana Kannan wrote:
> On Mon, Jul 29, 2019 at 2:24 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> >
> > On 18-07-19, 21:12, Saravana Kannan wrote:
> > > On Wed, Jul 17, 2019 at 10:37 PM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > > > I would like
> > > > to put this data in the GPU OPP table only. What about putting a
> > > > range in the GPU OPP table for the Bandwidth if it can change so much
> > > > for the same frequency.
> > >
> > > I don't think the range is going to work.
> >
> > Any specific reason for that ?
>
> The next sentence was literally explaining this :) Fine to debate
> that, but ignoring that and asking this question is kinda funny.

Okay, but ...

> > > If a GPU is doing purely
> > > computational work, it's not unreasonable for it to vote for the
> > > lowest bandwidth for any GPU frequency.

... it wasn't clear to me even after reading this sentence again now
:)

I understand that you may have to vote for the lowest bandwidth but
that doesn't explain why a range can't work (sorry if it was just me
who doesn't understood it :)).

> > I think that is fine, but if the GPU is able to find how much
> > bandwidth it needs why can't it just pass that value without needing
> > to have another OPP table for the path ?
>
> You were asking this question in the context of "can the GPU OPP just
> list all the range of bandwidth it might use per GPU frequency". My point
> is that the range would be useless because it would the entire
> available bandwidth range (because purely compute work might not need
> any bandwidth).

If it is useless to have entire range here, then why bother providing
one ? Why can't the GPU request what it needs in exact terms, based on
its calculations ? And then based on these requests, let the
interconnect find what's the best/stable values it really wants to
program the path for (and for that the interconnect can use its own
OPP table, which would be fine).

> Whereas, what the GPU's algorithm actually needs might be the list of
> "useful" bandwidth levels to use.

Hmm, I am not sure GPU's algorithm needs this table AFAIU based on all
the conversations we had until now. It is very capable of finding how
much bandwidth it needs, you just want the GPU driver to finally align
that with a stable bandwidth for the platform later on. And what I am
asking is that it is not required for the end driver to look for
stable values, it just requests what it wants and let the interconnect
core/driver decide the stable values.

Very much like the clock framework, most of the user drivers just ask
for a clk value to be programmed and it is the clock driver which
keeps a table of the stable values and then aligns the requested value
to one of those.

> Also, as we add more ICC request properties, this range idea will not scale.

I am not sure about what kind of properties there can be and where
should they land. My feedback is completely based on what you have
presented in this patchset.

--
viresh