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From: Stephan Gerhold <>
To: Niklas Cassel <>
Cc: Bjorn Andersson <>,
	Loic Poulain <>,,
Subject: Re: [PATCH] arch: arm64: dts: apq8016-dbc: Add missing cpu opps
Date: Tue, 26 May 2020 22:54:03 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, May 26, 2020 at 05:54:20PM +0200, Niklas Cassel wrote:
> On Tue, May 26, 2020 at 10:59:48AM +0200, Stephan Gerhold wrote:
> > > Considering that CPR is not an actual power domain, CPR gives
> > > adjustments to VDD_APC, but I don't know of any other device
> > > connected to VDD_APC, other than the CPU, so in hindsight the CPR
> > > driver probably should have been implemented using .target_index(),
> > > rather than as a power domain provider using performance states.
> > 
> > I suppose having CPR, MEMACC etc as power domain providers is a bit
> > overkill, given there is just one consumer. However, at least the
> > "performance state" part fits quite well in my opinion. At the end
> > all these requirements represent some performance state that must be
> > set when the CPU frequency is changed.
> > 
> For MX, it makes sense to model it as a power domain provider, and for
> it to have its own OPP table, since this actually is a power domain.
> For CPR, I think that the target_index() model of just giving an index
> in a frequency table is much better, the OPP library can still be used
> to get the frequencies/frequency_table.
> Since at least for Qualcom CPU's, the corner (opp-level) is defined as
> an increasing number 1,2,3,4, without skips.
> Even if it wasn't always without skips, we could just put opp-level in
> the CPU opp table, and get it from there.
> The only thing that the corner is used for really, is to use it as an
> index the local drv->corner array, which is where the (current) VDD_APC
> voltage is stored for each index/corner.
> For CPR, the .target_index() in cpufreq-dt.c gets called, which is
> supplied with an index, but the index gets converted to a frequency.
> This frequency is then sent to the OPP library, and is then converted
> back to an index of the same value (just increased by one), before
> cpr_set_performance_state() is called (which then has to subtract one).
> In this case, all the extra overhead of going via genpd is totally
> unnecessary.
> This is totally correct when setting a performance state on a power
> domain like MX, since for an actual power domain you might have
> multiple consumers, so you need to go via genpd.
> Considering that CPR is not a power domain, I wish the driver wasn't
> designed around performance states, which, _for the CPR case_,
> is misleading, unnecessary, and adds extra overhead for no reason.
> I realize the irony of me criticizing my own code.
> I simply know better now, and wish I had designed it differently :)

I see what you mean. I'm not sure how much of a problem the "genpd
overhead" really is in practice (although I assume it's called quite
frequently with a dynamic CPU frequency governor). There is also the
argument of it being slightly misleading (because CPR is not actually
a real power domain).

Speaking of the current solution, I also have to say that (IMO) the
device tree binding for "required-opps" is rather confusing
and potentially misleading.

e.g. for VDD_MX scaling I use

	required-opps = <&rpmpd_opp_nom>;

but looking at just the OPP table absolutely nothing tells me this is
supposed to apply to VDD_MX. You actually need to go search for the cpu@
device tree node and then know that some of the power domains there
(in some order) are eventually going to be used for the required-opps
there. The order is only defined by the qcom-nvmem-cpufreq driver.

It took me a few hours to get that right... :)

Nevertheless I guess we need a solution for scaling MEMACC without CPR
for now. :) I'm not sure if rewriting all this is very realistic
(if even possible). So I guess we might be stuck with the genpd approach?


  reply	other threads:[~2020-05-26 20:54 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-01 17:50 [PATCH] arch: arm64: dts: apq8016-dbc: Add missing cpu opps Loic Poulain
2020-04-01 23:46 ` Bjorn Andersson
2020-04-02  8:13 ` Stephan Gerhold
2020-04-02  9:58   ` Loic Poulain
2020-04-03  1:31   ` Bjorn Andersson
2020-04-03 10:09     ` Stephan Gerhold
2020-04-03 18:00       ` Stephan Gerhold
2020-04-23  4:55         ` Bjorn Andersson
2020-04-26 12:31           ` Stephan Gerhold
2020-05-06 21:18             ` Stephan Gerhold
2020-05-07  5:34               ` Bjorn Andersson
2020-05-08 12:08                 ` Ulf Hansson
2020-05-08 13:42                   ` Stephan Gerhold
2020-05-11  5:29                   ` Viresh Kumar
2020-05-07 10:46               ` Stephan Gerhold
2020-05-21 19:18                 ` Stephan Gerhold
2020-05-23 12:08                   ` Stephan Gerhold
2020-05-27 20:47                     ` Georgi Djakov
2020-05-25 15:32           ` Niklas Cassel
2020-05-25 16:36             ` Stephan Gerhold
2020-05-25 19:44               ` Niklas Cassel
2020-05-26  8:59                 ` Stephan Gerhold
2020-05-26 15:54                   ` Niklas Cassel
2020-05-26 20:54                     ` Stephan Gerhold [this message]
2020-05-27 10:39                       ` Niklas Cassel
2020-05-27 12:04                         ` Stephan Gerhold
2020-05-27 12:59                           ` Niklas Cassel
2020-05-27 20:56                             ` Stephan Gerhold
2020-05-27 23:10                               ` Niklas Cassel
2020-05-28 13:32                                 ` Stephan Gerhold
2020-05-28  4:44                           ` Viresh Kumar
2020-04-28 20:04 ` Amit Kucheria

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