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

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Message-Id: <20191107210606.E536F21D79@mail.kernel.org>
Date:   Thu, 07 Nov 2019 13:06:06 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Matthias Kaehlcke <mka@...omium.org>,
        Taniya Das <tdas@...eaurora.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        David Brown <david.brown@...aro.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, robh@...nel.org, robh+dt@...nel.org,
        Jordan Crouse <jcrouse@...eaurora.org>,
        Jeykumar Sankaran <jsanka@...eaurora.org>,
        Sean Paul <seanpaul@...omium.org>,
        Rob Clark <robdclark@...omium.org>
Subject: Re: [PATCH v4 5/5] clk: qcom: Add Global Clock controller (GCC) driver for SC7180

Quoting Matthias Kaehlcke (2019-10-31 10:41:49)
> Hi Taniya,
> 
> On Thu, Oct 31, 2019 at 04:59:26PM +0530, Taniya Das wrote:
> > Hi Matthias,
> > 
> > Thanks for your comments.
> > 
> > On 10/29/2019 11:29 PM, Matthias Kaehlcke wrote:
> > > Hi Taniya,
> > > 
> > > On Mon, Oct 14, 2019 at 03:53:08PM +0530, Taniya Das wrote:
> > > > Add support for the global clock controller found on SC7180
> > > > based devices. This should allow most non-multimedia device
> > > > drivers to probe and control their clocks.
> > > > 
> > > > Signed-off-by: Taniya Das <tdas@...eaurora.org>
> > 
> > > 
> > > v3 also had
> > > 
> > > +   [GCC_DISP_AHB_CLK] = &gcc_disp_ahb_clk.clkr,
> > > 
> > > Removing it makes the dpu_mdss driver unhappy:
> > > 
> > > [    2.999855] dpu_mdss_enable+0x2c/0x58->msm_dss_enable_clk: 'iface' is not available
> > > 
> > > because:
> > > 
> > >          mdss: mdss@...0000 {
> > >                     ...
> > > 
> > >   =>             clocks = <&gcc GCC_DISP_AHB_CLK>,
> > >                           <&gcc GCC_DISP_HF_AXI_CLK>,
> > >                           <&dispcc DISP_CC_MDSS_MDP_CLK>;
> > >                  clock-names = "iface", "gcc_bus", "core";
> > >     };
> > > 
> > 
> > The basic idea as you mentioned below was to move the CRITICAL clocks to
> > probe. The clock provider to return NULL in case the clocks are not
> > registered.
> > This was discussed with Stephen on v3. Thus I submitted the below patch.
> > clk: qcom: common: Return NULL from clk_hw OF provider.
> 
> I see. My assumption was that the entire clock hierarchy should be registered,
> but Stephen almost certainly knows better :)
> 
> > Yes it would throw these warnings, but no functional issue is observed from
> > display. I have tested it on the cheza board.
> 
> The driver considers it an error (uses DEV_ERR to log the message) and doesn't
> handle other clocks when one is found missing. I'm not really famililar with
> the dpu_mdss driver, but I imagine this can have some side effects. Added some
> of the authors/contributors to cc.

NULL is a valid clk pointer returned by clk_get(). What is the display
driver doing that makes it consider NULL an error?