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From:   Rob Clark <robdclark@gmail.com>
Date:   Wed, 13 Nov 2019 21:30:57 -0800
Message-ID: <CAF6AEGv9+Ow=RCXGKmaANfmA2NtR32E07CKwGFKJbeeOJRP9=Q@mail.gmail.com>
Subject: Re: [PATCH v4 5/5] clk: qcom: Add Global Clock controller (GCC)
 driver for SC7180
To:     Matthias Kaehlcke <mka@chromium.org>
Cc:     Rob Clark <robdclark@chromium.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Taniya Das <tdas@codeaurora.org>,
        Michael Turquette <mturquette@baylibre.com>,
        David Brown <david.brown@linaro.org>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        "open list:ARM/QUALCOMM SUPPORT" <linux-soc@vger.kernel.org>,
        linux-clk <linux-clk@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, Rob Herring <robh@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Jordan Crouse <jcrouse@codeaurora.org>,
        Jeykumar Sankaran <jsanka@codeaurora.org>,
        Sean Paul <seanpaul@chromium.org>
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On Wed, Nov 13, 2019 at 5:03 PM Matthias Kaehlcke <mka@chromium.org> wrote:
>
> On Fri, Nov 08, 2019 at 11:40:53AM -0800, Rob Clark wrote:
> > On Fri, Nov 8, 2019 at 10:42 AM Stephen Boyd <sboyd@kernel.org> wrote:
> > >
> > > Quoting Rob Clark (2019-11-08 08:54:23)
> > > > On Thu, Nov 7, 2019 at 10:35 PM Stephen Boyd <sboyd@kernel.org> wrote:
> > > > >
> > > > > Quoting Rob Clark (2019-11-07 18:06:19)
> > > > > > On Thu, Nov 7, 2019 at 1:06 PM Stephen Boyd <sboyd@kernel.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).
>
> The driver appears to be happy with it, at least at probe() time.

Ok, I suppose I should re-send the dpu patch to the appropriate
lists.. does that count as a Tested-by?

BR,
-R