linux-kernel - Re: [PATCH v1 1/4] clk: samsung: change COMMON_CLK

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Message-ID: <2c8a79f7-711a-b075-745f-ea77b82a1117@canonical.com>
Date:   Tue, 21 Sep 2021 10:35:51 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Will McVicker <willmcvicker@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.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>,
        Lee Jones <lee.jones@...aro.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>
Subject: Re: [PATCH v1 1/4] clk: samsung: change COMMON_CLK_SAMSUNG default
 config logic

On 21/09/2021 09:50, Geert Uytterhoeven wrote:
> On Tue, Sep 21, 2021 at 9:31 AM Krzysztof Kozlowski <krzk@...nel.org> wrote:
>> On 20/09/2021 21:03, Will McVicker wrote:
>>> COMMON_CLK_SAMSUNG is selected by ARCH_EXYNOS which forces this config
>>> to be built-in when ARCH_EXYNOS is enabled. Switch the logic to use a
>>> "default y if ARCH_EXYNOS" to provide flexibilty for vendors to disable
>>> or modularize this driver.
>>
>> The clock drivers are essential, you cannot disable them for a generic
>> kernel supporting ARCH_EXYNOS. Such kernel won't work properly on platforms.
> 
> Obviously it's not gonna work if the clock driver is not enabled
> at all.  But does it work if you make the clock driver modular, and
> put it with all other essential driver modules in initramfs?  Debugging
> would be hard, as the serial console driver also relies on clocks
> and PM Domains etc.

The kernel could boot without clock drivers (default settings from
bootloader), probe clocks from initramfs and proceed with rootfs from
eMMC/SD/net.

In theory.

However I have no reports that it ever worked. If there is such working
upstream configuration, I don't mind here. Just please explain this in
the commit msg.

> 
> If not, this patch should be NAKed, until it works with a modular
> clock driver.
> 
> If yes, perhaps another line should be added (_before_ the other line)?
> 
>   + default m if ARCH_EXYNOS && MODULES
>     default y if ARCH_EXYNOS
> 
> However, many developers may want MODULES=y, but not want to bother
> with an initramfs.  So perhaps we need a new symbol
> MINIMUM_GENERIC_KERNEL or so, protected by EXPERT, and make the
> driver default to m if that is enabled?

Yeah, that's indeed a problem to solve. For most users (and distros)
building kernel for Exynos this should be built-in by default.

Anyway, the option is non-selectable so it cannot be converted to "m" or
disabled. And this is claimed in the commit msg:
"provide flexibilty for vendors to disable or modularize this driver."

The commit does not achieve it.

Best regards,
Krzysztof