linux-kernel - Re: [PATCH 08/13] clk: qcom: hfpll: CLK_IGNORE

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Date:   Thu, 17 Jan 2019 09:38:16 +0100
From:   Jorge Ramirez <jorge.ramirez-ortiz@...aro.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     robh+dt@...nel.org, mark.rutland@....com, andy.gross@...aro.org,
        david.brown@...aro.org, sboyd@...nel.org, will.deacon@....com,
        mturquette@...libre.com, jassisinghbrar@...il.com,
        vkoul@...nel.org, niklas.cassel@...aro.org, sibis@...eaurora.org,
        georgi.djakov@...aro.org, arnd@...db.de,
        horms+renesas@...ge.net.au, heiko@...ech.de,
        enric.balletbo@...labora.com, jagan@...rulasolutions.com,
        olof@...om.net, amit.kucheria@...aro.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH 08/13] clk: qcom: hfpll: CLK_IGNORE_UNUSED

On 1/17/19 07:33, Bjorn Andersson wrote:
> On Mon 17 Dec 01:46 PST 2018, Jorge Ramirez-Ortiz wrote:
> 
>> When COMMON_CLK_DISABLED_UNUSED is set, in an effort to save power and
>> to keep the software model of the clock in line with reality, the
>> framework transverses the clock tree and disables those clocks that
>> were enabled by the firmware but have not been enabled by any device
>> driver.
>>
>> If CPUFREQ is enabled, early during the system boot, it might attempt
>> to change the CPU frequency ("set_rate"). If the HFPLL is selected as
>> a provider, it will then change the rate for this clock.
>>
>> As boot continues, clk_disable_unused_subtree will run. Since it wont
>> find a valid counter (enable_count) for a clock that is actually
>> enabled it will attempt to disable it which will cause the CPU to
>> stop. Notice that in this driver, calls to check whether the clock is
>> enabled are routed via the is_enabled callback which queries the
>> hardware.
>>
> 
> With the CPUFREQ referencing the CPU clock driver, that has decided to
> run off this clock, why is it not refcounted?

COMMON_CLK_DISABLED_UNUSED relies on the enable_count reference counter
to disable the clocks that were enabled by the firwmare and not by the
drivers.

the cpufreq driver does not enable the cpu clock.

so when clk_change_rate is called, the enable_count counter is not
incremented and therefore it just remains null since this was enabled by
the firmware.

I tried doing:

diff --git a/drivers/cpufreq/cpufreq-dt.c b/drivers/cpufreq/cpufreq-dt.c
index e58bfcb..5a9f83e 100644
--- a/drivers/cpufreq/cpufreq-dt.c
+++ b/drivers/cpufreq/cpufreq-dt.c
@@ -124,6 +124,10 @@ static int resources_available(void)
                return ret;
        }

+ ret = clk_prepare_enable(cpu_clk);
+ if (ret)
+         return ret;
+
        clk_put(cpu_clk);

        name = find_supply_name(cpu_dev);


and that removed the need for CLK_IGNORE_UNUSED. But I am not sure of
the system wide consequences of that change to cpufreq.

maybe Viresh can comment?

> 
> Regards,
> Bjorn
> 
>> The following commit, rather than marking the clock critical and
>> forcing the clock to be always enabled, addresses the above scenario
>> making sure the clock is not disabled but it continues to rely on the
>> firmware to enable the clock.
>>
>> Co-developed-by: Niklas Cassel <niklas.cassel@...aro.org>
>> Signed-off-by: Niklas Cassel <niklas.cassel@...aro.org>
>> Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@...aro.org>
>> ---
>>  drivers/clk/qcom/hfpll.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/clk/qcom/hfpll.c b/drivers/clk/qcom/hfpll.c
>> index 0ffed0d..9d92f5d 100644
>> --- a/drivers/clk/qcom/hfpll.c
>> +++ b/drivers/clk/qcom/hfpll.c
>> @@ -58,6 +58,7 @@ static int qcom_hfpll_probe(struct platform_device *pdev)
>>  		.parent_names = (const char *[]){ "xo" },
>>  		.num_parents = 1,
>>  		.ops = &clk_ops_hfpll,
>> +		.flags = CLK_IGNORE_UNUSED,
>>  	};
>>  
>>  	h = devm_kzalloc(dev, sizeof(*h), GFP_KERNEL);
>> -- 
>> 2.7.4
>>
>