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SubjectRe: [PATCH v3 08/14] clk: qcom: hfpll: CLK_IGNORE_UNUSED
On Tue 25 Jun 09:47 PDT 2019, 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.

But if CPUfreq has acquired the CPU clock and the hfpll is the currently
selected input, why does the clock framework not know about this clock
being used?

> Notice that in this driver, calls to check whether the clock is
> enabled are routed via the is_enabled callback which queries the
> hardware.
>
> 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@linaro.org>
> Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
> Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>


I can see that we have a real issue in the case where CPUfreq is not
enabled and hence there are no clients, according to Linux. And that I
don't know another way to guard against.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>

Regards,
Bjorn

> ---
> drivers/clk/qcom/hfpll.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/drivers/clk/qcom/hfpll.c b/drivers/clk/qcom/hfpll.c
> index 0ffed0d41c50..d5fd27938e7b 100644
> --- a/drivers/clk/qcom/hfpll.c
> +++ b/drivers/clk/qcom/hfpll.c
> @@ -58,6 +58,13 @@ static int qcom_hfpll_probe(struct platform_device *pdev)
> .parent_names = (const char *[]){ "xo" },
> .num_parents = 1,
> .ops = &clk_ops_hfpll,
> + /*
> + * rather than marking the clock critical and forcing the clock
> + * to be always enabled, we make sure that the clock is not
> + * disabled: the firmware remains responsible of enabling this
> + * clock (for more info check the commit log)
> + */
> + .flags = CLK_IGNORE_UNUSED,
> };
>
> h = devm_kzalloc(dev, sizeof(*h), GFP_KERNEL);
> --
> 2.21.0
>

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