linux-kernel - Re: [PATCH v4 5/5] clk: qcom: Add Global Clock controller (GCC) driver for SC7180

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Message-Id: <20191108184207.334DD21848@mail.kernel.org>
Date:   Fri, 08 Nov 2019 10:42:06 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Rob Clark <robdclark@...omium.org>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Taniya Das <tdas@...eaurora.org>,
        Michael Turquette <mturquette@...libre.com>,
        David Brown <david.brown@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        linux-soc@...r.kernel.org, linux-clk@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, devicetree@...r.kernel.org,
        robh@...nel.org, Rob Herring <robh+dt@...nel.org>,
        Jordan Crouse <jcrouse@...eaurora.org>,
        Jeykumar Sankaran <jsanka@...eaurora.org>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [PATCH v4 5/5] clk: qcom: Add Global Clock controller (GCC) driver for SC7180

Quoting Rob Clark (2019-11-08 08:54:23)
> On Thu, Nov 7, 2019 at 10:35 PM Stephen Boyd <sboyd@...nel.org> wrote:
> >
> > Quoting Rob Clark (2019-11-07 18:06:19)
> > > On Thu, Nov 7, 2019 at 1:06 PM Stephen Boyd <sboyd@...nel.org> wrote:
> > > >
> > > >
> > > > NULL is a valid clk pointer returned by clk_get(). What is the display
> > > > driver doing that makes it consider NULL an error?
> > > >
> > >
> > > do we not have an iface clk?  I think the driver assumes we should
> > > have one, rather than it being an optional thing.. we could ofc change
> > > that
> >
> > I think some sort of AHB clk is always enabled so the plan is to just
> > hand back NULL to the caller when they call clk_get() on it and nobody
> > should be the wiser when calling clk APIs with a NULL iface clk. The
> > common clk APIs typically just return 0 and move along. Of course, we'll
> > also turn the clk on in the clk driver so that hardware can function
> > properly, but we don't need to expose it as a clk object and all that
> > stuff if we're literally just slamming a bit somewhere and never looking
> > back.
> >
> > But it sounds like we can't return NULL for this clk for some reason? I
> > haven't tried to track it down yet but I think Matthias has found it
> > causes some sort of problem in the display driver.
> >
> 
> ok, I guess we can change the dpu code to allow NULL..  but what would
> the return be, for example on a different SoC where we do have an
> iface clk, but the clk driver isn't enabled?  Would that also return
> NULL?  I guess it would be nice to differentiate between those cases..
> 

So the scenario is DT describes the clk

 dpu_node {
     clocks = <&cc AHB_CLK>;
     clock-names = "iface";
 }

but the &cc node has a driver that doesn't probe?

I believe in this scenario we return -EPROBE_DEFER because we assume we
should wait for the clk driver to probe and provide the iface clk. See
of_clk_get_hw_from_clkspec() and how it looks through a list of clk
providers and tries to match the &cc phandle to some provider.

Once the driver probes, the match will happen and we'll be able to look
up the clk in the provider with __of_clk_get_hw_from_provider(). If
the clk provider decides that there isn't a clk object, it will return
NULL and then eventually clk_hw_create_clk() will turn the NULL return
value into a NULL pointer to return from clk_get().