Re: [PATCH 1/6] clk: samsung: Enable bus clock on init

Sylwester Nawrocki
2021-09-15T12:52:02Z

Hi,

On 14.09.2021 17:56, Sam Protsenko wrote:
> By default if bus clock has no users its "enable count" value is 0. It
> might be actually running if it's already enabled in bootloader, but
> then in some cases it can be disabled by mistake. For example, such case
> was observed when dw_mci_probe() enabled bus clock, then failed to do
> something and disabled that bus clock on error path. After that even
> attempt to read the 'clk_summary' file in DebugFS freezed forever, as
> CMU bus clock ended up being disabled and it wasn't possible to access
> CMU registers anymore.
>
> To avoid such cases, CMU driver must increment the ref count for that
> bus clock by running clk_prepare_enable(). There is already existing
> '.clk_name' field in struct samsung_cmu_info, exactly for that reason.
> It was added in commit 523d3de41f02 ("clk: samsung: exynos5433: Add
> support for runtime PM"). But the clock is actually enabled only in
> Exynos5433 clock driver. Let's mimic what is done there in generic
> samsung_cmu_register_one() function, so other drivers can benefit from
> that `.clk_name' field. As was described above, it might be helpful not
> only for PM reasons, but also to prevent possible erroneous clock gating
> on error paths.
>
> Another way to workaround that issue would be to use CLOCK_IS_CRITICAL
> flag for corresponding gate clocks. But that might be not very good
> design decision, as we might still want to disable that bus clock, e.g.
> on PM suspend.
>
> Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
> ---
> drivers/clk/samsung/clk.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/drivers/clk/samsung/clk.c b/drivers/clk/samsung/clk.c
> index 1949ae7851b2..da65149fa502 100644
> --- a/drivers/clk/samsung/clk.c
> +++ b/drivers/clk/samsung/clk.c
> @@ -357,6 +357,19 @@ struct samsung_clk_provider * __init samsung_cmu_register_one(
>
> ctx = samsung_clk_init(np, reg_base, cmu->nr_clk_ids);
>
> + /* Keep bus clock running, so it's possible to access CMU registers */
> + if (cmu->clk_name) {
> + struct clk *bus_clk;
> +
> + bus_clk = __clk_lookup(cmu->clk_name);
> + if (bus_clk) {
> + clk_prepare_enable(bus_clk);
> + } else {
> + pr_err("%s: could not find bus clock %s\n", __func__,
> + cmu->clk_name);
> + }
> + }
> +
> if (cmu->pll_clks)
> samsung_clk_register_pll(ctx, cmu->pll_clks, cmu->nr_pll_clks,
> reg_base);

I would suggest to implement runtime PM ops in your driver instead, even though
those would initially only contain single clk enable/disable. Things like
the clk_summary will work then thanks to runtime PM support in the clk core
(see clk_pm_runtime_* calls).
We could also make common runtime PM suspend/resume helpers but I wouldn't focus
on that too much now, it could well be done later.
And please avoid introducing new __clk_lookup() calls.

--
Regards,
Sylwester