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

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Message-ID: <CAJs_Fx6KCirGMtQxE=xA-A=bd5LeuYWviee0+KqO5OtGT9GKEw@mail.gmail.com>
Date:   Fri, 8 Nov 2019 11:40:53 -0800
From:   Rob Clark <robdclark@...omium.org>
To:     Stephen Boyd <sboyd@...nel.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

On Fri, Nov 8, 2019 at 10:42 AM Stephen Boyd <sboyd@...nel.org> wrote:
>
> 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().
>

ok, that was the scenario I was worried about (since unclk'd register
access tends to be insta-reboot and hard to debug)..  so I think it
should be ok to make dpu just ignore NULL clks.

>From a quick look, I think something like the attached (untested).
(Sorry, I'd just paste it inline but gmail somehow eats all the
whitespace when I do that :-/)

View attachment "0001-drm-msm-dpu-ignore-NULL-clocks.patch" of type "text/x-patch" (2061 bytes)