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

[Stephen Boyd <sboyd@kernel.org>]

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@codeaurora.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@ae00000 {
> > >                     ...
> > > 
> > >   =>             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?