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

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Date:   Wed, 10 Nov 2021 05:48:10 -0800
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Shawn Guo <shawn.guo@...aro.org>
Cc:     Stephan Gerhold <stephan@...hold.net>,
        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 05:15 PST 2021, Shawn Guo wrote:

> Hi Stephan,
> 
> On Tue, Nov 09, 2021 at 11:26:21AM +0100, Stephan Gerhold wrote:
> > Hi Shawn,
> > 
> > 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 :(
> 
> > 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.

Afaict we still have the problem that if the interconnect driver is
compiled as a module, or for other reasons doesn't probe until after
late_initcall() clk-smd-rpm will turn off these clocks and we never will
get a chance to probe the interconnect provider.

I believe the way to handle that is to rely on sync_state, but there
seems to be a lot of corner cases here.

But with that in place, I agree that we should handle this temporarily
during bringup by the use of clk_ignore_unused.

> IMHO, properly reporting enable state to framework is definitely the
> right thing to do, and should have been done from day one.
> 

I always thought is_enabled() should reflect the hardware state - in
particular for clk_summary. The particular concern being that by
initializing the is_enabled() state to either true or false, we're
making an assumption about the hardware state. And if something where to
do if (enabled) disable (or if (disabled) enable), we might skip a
critical operation just because we tricked the logic.

So, do you need it for anything other than clk_disable_unused()?

I have a clock in the MDP with similar issue, where we don't have
is_enabled() but I need it to be disabled by clk_disable_unused(),
because the next iteration turns off the parent and locks up the still
"active" rcg.
So far I've not received any feedback on this though...
https://lore.kernel.org/all/20210707043859.195870-1-bjorn.andersson@linaro.org/

With this approach we don't make any assumptions about the hardware
state, beyond the fact that we will issue a disable in
clk_disable_unused() if no one has yet enabled the clock - which at
worst turns off a clock that's already is off.

Regards,
Bjorn

> Shawn
> 
> > [1]: https://lore.kernel.org/linux-arm-msm/20200817140908.185976-1-stephan@gerhold.net/
> > [2]: https://lore.kernel.org/linux-arm-msm/20200818080738.GA46574@gerhold.net/
> > [3]: https://lore.kernel.org/linux-arm-msm/159796605593.334488.8355244657387381953@swboyd.mtv.corp.google.com/
> > [4]: https://lore.kernel.org/linux-arm-msm/20200821064857.GA905@gerhold.net/