linux-kernel - Re: [PATCH] clk: samsung: exynos850: Register clocks early

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Date:   Sat, 23 Oct 2021 11:19:36 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Sam Protsenko <semen.protsenko@...aro.org>,
        Sylwester Nawrocki <s.nawrocki@...sung.com>,
        Tomasz Figa <tomasz.figa@...il.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>
Cc:     Matthias Brugger <matthias.bgg@...il.com>,
        linux-samsung-soc@...r.kernel.org, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: samsung: exynos850: Register clocks early

On 22/10/2021 23:56, Sam Protsenko wrote:
> Some clocks must be registered before init calls. For example MCT clock
> (from CMU_PERI) is needed for MCT timer driver, which is registered
> with TIMER_OF_DECLARE(). By the time we get to core_initcall() used for
> clk-exynos850 platform driver init, it's already too late. Inability to
> get "mct" clock in MCT driver leads to kernel panic, as functions
> registered with *_OF_DECLARE() can't do deferred calls. MCT timer driver
> can't be fixed either, as it's acting as a clock source and it's
> essential to register it in start_kernel() -> time_init().
> 
> Let's register all Exynos850 clocks early, using
> CLK_OF_DECLARE_DRIVER(), and do all stuff relying on "struct dev" object
> (like runtime PM and enabling bus clock) later in platform driver probe.
> Basically CLK_OF_DECLARE_DRIVER() matches CMU compatible, but clears
> OF_POPULATED flag, which allows the same device to be matched again
> later.
> 
> Similar issue was discussed at [1] and addressed in commit 1f7db7bbf031
> ("clk: renesas: cpg-mssr: Add early clock support"), as well as in
> drivers/clk/mediatek/clk-mt2712.c.
> 
> [1] https://patchwork.kernel.org/project/linux-renesas-soc/patch/20180829132954.64862-2-chris.brandt@renesas.com/
> 
> Signed-off-by: Sam Protsenko <semen.protsenko@...aro.org>
> ---
> Notes:
>   - This patch should be applied on top of CMU_APM series
>     (clk: samsung: exynos850: Implement CMU_APM domain)
>   - I considered introducing some macro to reduce the code duplication
>     (actually created one), but decided to go with plain code: this way
>     it's easier to understand and navigate
>   - Also considered registering only *some* clocks early: won't be
>     possible, as leaf clocks (like MCT) depend on the whole clock tree.
>     Registering early only *some* CMUs (like CMU_PERI) looks possible,
>     but I'm not sure what is the best way to implement this, and that
>     won't be consistent with other CMUs (too much code variation as for
>     my taste)
> 

In the long term it is better for entire kernel and SoC to support
deferred probes and register clocks as devices, not as CLK_OF_DECLARE. I
understand that it's not possible now to get rid of CLK_OF_DECLARE
entirely, but we could meet half-way.

The Exynos5433 has the same problem and it registers few core CMUs
early: the TOP, CPUs, memory bus, peripheral buses including PERIS with
MCT. I prefer this approach than yours. The only domains needing
CLK_OF_DECLARE are TOP and PERIS. Maybe also CORE - one would have to
check if GIC and CCI are needed early.


Best regards,
Krzysztof