Re: [PATCH 3/6] clk: qcom: gdsc: enable optional power domain support

[Bjorn Andersson]

On Wed 30 Jun 15:29 CDT 2021, Dmitry Baryshkov wrote:

> On Wed, 30 Jun 2021 at 20:11, Bjorn Andersson
> <bjorn.andersson@xxxxxxxxxx> wrote:
> >
> > On Wed 30 Jun 10:47 CDT 2021, Dmitry Baryshkov wrote:
> >
> > > Hi,
> > >
> > > On Wed, 30 Jun 2021 at 18:00, Bjorn Andersson
> > > <bjorn.andersson@xxxxxxxxxx> wrote:
> > > >
> > > > On Wed 30 Jun 08:31 CDT 2021, Dmitry Baryshkov wrote:
> > > >
> > > > > On sm8250 dispcc and videocc registers are powered up by the MMCX power
> > > > > domain. Currently we used a regulator to enable this domain on demand,
> > > > > however this has some consequences, as genpd code is not reentrant.
> > > > >
> > > > > Teach Qualcomm clock controller code about setting up power domains and
> > > > > using them for gdsc control.
> > > > >
> > > > > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
> > > >
> > > > There's a proposal to add a generic binding for statically assigning a
> > > > performance states here:
> > > >
> > > > https://lore.kernel.org/linux-arm-msm/1622095949-2014-1-git-send-email-rnayak@xxxxxxxxxxxxxx/
> 
> I checked this thread. It looks like Rajendra will also switch to the
> "required-opps" property. So if that series goes in first, we can drop
> the call to set_performance_state. If this one goes in first, we can
> drop the set_performance_state call after getting Rajendra's work in.
> 
> > > >
> > > >
> > > > But that said, do you really need this?
> > > >
> > > > The requirement for driving MMCX to LOW_SVS on SM8250 (and NOM on
> > > > SM8150/SC8180x) seems to only come from the fact that you push MDP_CLK
> > > > to 460MHz in &mdss.
> > > >
> > > > But then in &mdss_mdp you do the same using an opp-table based on the
> > > > actual MDP_CLK, which per its power-domains will scale MMCX accordingly.
> > >
> > > MDSS and DSI would bump up MMCX performance state requirements on
> > > their own, depending on the frequency being selected.
> > >
> >
> > Right, but as I copied things from the sm8250.dtsi to come up with
> > sm8150/sc8180x.dtsi I concluded that as soon as the assigned-clockrate
> > in &mdss kicks in I need the performance state to be at NOM.
> >
> > So keeping the assigned-clockrate in &mdss means that MMCX will never go
> > below NOM.
> 
> No, because once MDP is fully running, it will lower the clock frequency:
> 
> # grep mdp_clk /sys/kernel/debug/clk/clk_summary
>           disp_cc_mdss_mdp_clk_src       1        1        0
> 150000000          0     0  50000         ?
>              disp_cc_mdss_mdp_clk       2        2        0
> 150000000          0     0  50000         Y
> 

But won't that just lower the performance state requested by the
&mdss_mdp, while the &mdss still votes for NOM - with the outcome being
that we maintain NOM even if the clock goes down?

> >
> > > > So wouldn't it be sufficient to ensure that MDSS_GDSC is parented by
> > > > MMCX and then use opp-tables associated with the devices that scales the
> > > > clock and thereby actually carries the "required-opps".
> > >
> > > Actually no. I set the performance state in the qcom_cc_map, so that
> > > further register access is possible. Initially I was doing this in the
> > > qcom_cc_really_probe() and it was already too late.
> > > Just to remind: this patchset is not about MDSS_GDSC being parented by
> > > MMCX, it is about dispcc/videocc registers being gated with MMCX.
> > >
> >
> > So you're saying that just enabling MMCX isn't enough to touch the
> > dispcc/videocc registers? If that's the case it seems like MMCX's
> > definition of "on" needs to be adjusted - because just specifying MMCX
> > as the power-domain for dispcc/videocc and enabling pm_runtime should
> > ensure that MMCX is enabled when the clock registers are accessed (I
> > don't see anything like that for the GDSC part though).
> 
> No, it is not enough. If I comment out the set_performance_state call,
> the board reboots.
> 
> However I can set the opps as low as RET and register access will work.
> I'll run more experiments and if everything works as expected, I can
> use retention or min_svs level in the next iteration.
> Just note that downstream specifies low_svs as minimum voltage level
> for MMCX regulator.
> 

It doesn't make sense to me that a lone power_on on the power-domain
wouldn't give us enough juice to poke the registers.

But digging into the rpmhpd implementation answers the question, simply
invoking rpmhpd_power_on() is a nop, unless
rpmhpd_set_performance_state() has previously been called, because
pd->corner is 0. So this explains why enable isn't sufficient.

Compare this with the rpmpd implementation that will send an
enable request to the RPM in this case.

> > I thought our problem you had was that you need to set a
> > performance_state in order to clock up some of the clocks - e.g.
> > MDP_CLK.
> 
> No, even register access needs proper perf state.
> 

Per above finding you're right, enabling a rpmhpd power-domain doesn't
do anything. And I don't find this intuitive or even in line with the
expectations of the api...



A quick test booting rb3 and rb5 seems to indicate that it's possible to
initialize pd->corner to 1 (to ensure that enable at least gives us the
lowest level).

set_performance_state(0) will however then result in voting for "off",
rather than the lowest enabled level.


Rajendra, Stephen, is this really how rpmhpd is supposed to work?!

Regards,
Bjorn