linux-kernel - Re: [PATCH 0/3] clk: qcom: smd-rpm: Report enable state to framework

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Message-ID: <YYveLqgDUrizgC/Q@ripper>
Date:   Wed, 10 Nov 2021 06:58:54 -0800
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     Shawn Guo <shawn.guo@...aro.org>, Stephen Boyd <sboyd@...nel.org>,
        Loic Poulain <loic.poulain@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] clk: qcom: smd-rpm: Report enable state to framework

On Wed 10 Nov 06:09 PST 2021, Stephan Gerhold wrote:

> On Wed, Nov 10, 2021 at 09:15:11PM +0800, Shawn Guo wrote:
> > On Tue, Nov 09, 2021 at 11:26:21AM +0100, Stephan Gerhold wrote:
> > > On Tue, Nov 09, 2021 at 10:25:55AM +0800, Shawn Guo wrote:
> > > > Currently the enable state of smd-rpm clocks are not properly reported
> > > > back to framework due to missing .is_enabled and .is_prepared hooks.
> > > > This causes a couple of issues.
> > > > 
> > > > - All those unused clocks are not voted for off, because framework has
> > > >   no knowledge that they are unused.  It becomes a problem for vlow
> > > >   power mode support, as we do not have every single RPM clock claimed
> > > >   and voted for off by client devices, and rely on clock framework to
> > > >   disable those unused RPM clocks.
> > > > 
> > > 
> > > I posted a similar patch a bit more than a year ago [1].
> > 
> > Ouch, that's unfortunate!  If your patch landed, I wouldn't have had to
> > spend such a long time to figure out why my platform fails to reach vlow
> > power mode :(
> > 
> 
> Sorry, I was waiting for Stephen to reply and eventually decided to
> shift focus to other things first. :)
> 
> The whole low-power topic is kind of frustrating on older platforms
> because they currently still lack almost everything that is necessary to
> reach those low power states. Even things that you already consider
> natural for newer platforms (such as interconnect) are still very much
> work in progress on all older ones.
> 
> > > Back then one
> > > of the concerns was that we might disable critical clocks just because
> > > they have no driver using it actively. For example, not all of the
> > > platforms using clk-smd-rpm already have an interconnect driver.
> > > Disabling the interconnect related clocks will almost certainly make the
> > > device lock up completely. (I tried it back then, it definitely does...)
> > > 
> > > I proposed adding CLK_IGNORE_UNUSED for the interconnect related clocks
> > > back then [2] which would allow disabling most of the clocks at least.
> > > Stephen Boyd had an alternative proposal to instead move the
> > > interconnect related clocks completely out of clk-smd-rpm [3].
> > > But I'm still unsure how this would work in a backwards compatible way. [4]
> > > 
> > > Since your patches are more or less identical I'm afraid the same
> > > concerns still need to be solved somehow. :)
> > 
> > I do not really understand why smd-rpm clock driver needs to be a special
> > case.  This is a very common issue, mostly in device early support phase
> > where not all clock consumer drivers are ready.  Flag CLK_IGNORE_UNUSED
> > and kernel cmdline 'clk_ignore_unused' are created just for that.  Those
> > "broken" platforms should be booted with 'clk_ignore_unused' until they
> > have related consumer drivers in place.  IMHO, properly reporting enable
> > state to framework is definitely the right thing to do, and should have
> > been done from day one.
> > 
> 
> ... And therefore I think we should be careful with such changes,
> especially if they would prevent devices from booting completely.
> Unfortunately the users trying to make use of old platforms are also
> often the ones who might not be aware that they suddenly need
> "clk_ignore_unused" just to boot a system that was previously working
> (mostly) fine, except for the whole low-power topic.
> 
> I fully agree with you that disabling the unused clocks here is the
> right thing to do, but I think we should try to carefully flag the most
> important clocks in the driver to avoid causing too many regressions.
> 

I don't fancy the idea of forcing everyone to run with specific kernel
command line parameters - in particular not as a means to avoid
"regressions".

I think the only way around this problem is to figure out how to move
the clk disablement to sync_state - probably per clock driver.

Regards,
Bjorn