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

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Date:   Wed, 15 Sep 2021 14:51:54 +0200
From:   Sylwester Nawrocki <s.nawrocki@...sung.com>
To:     Sam Protsenko <semen.protsenko@...aro.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Paweł Chmiel <pawel.mikolaj.chmiel@...il.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Tomasz Figa <tomasz.figa@...il.com>
Cc:     Ryu Euiyoul <ryu.real@...sung.com>, Tom Gall <tom.gall@...aro.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        John Stultz <john.stultz@...aro.org>,
        Amit Pundir <amit.pundir@...aro.org>,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Herring <robh+dt@...nel.org>
Subject: Re: [PATCH 1/6] clk: samsung: Enable bus clock on init

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@...aro.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