Re: [PATCH v2 5/5] clk: samsung: Introduce Exynos850 clock driver

[Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>]

On 07/10/2021 21:41, Sam Protsenko wrote:
> This is the initial implementation adding only basic clocks like UART,
> MMC, I2C and corresponding parent clocks. Design is influenced by
> Exynos5433 clock driver.
> 
> Bus clock is enabled by default (in probe function) for all CMUs except
> CMU_TOP, the reasoning is as follows. 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 the bus clock, then failed to do something and
> disabled that bus clock on error path. After that, even the 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"), with next mentioning in commit message:
> 
>   > Also for each CMU there is one special parent clock, which has to be
>   > enabled all the time when any access to CMU registers is being done.
> 
> But that clock is actually only enabled in Exynos5433 clock driver right
> now. So the same code is added to exynos850_cmu_probe() function,
> 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>
> ---
> Changes in v2:
>   - Used of_iomap() for the whole CMU range instead of ioremap() in
>     exynos850_init_clocks()
>   - Used readl/writel functions in exynos850_init_clocks() for consistency
>     with other drivers
>   - Added all clock ids
>   - Added CMU_DPU
>   - Implemented platform_driver for all Power Domain capable CMUs
>   - Moved bus clock enablement code here to probe function
>   - Used clk_get() instead of __clk_lookup()
> 
>  drivers/clk/samsung/Makefile        |   1 +
>  drivers/clk/samsung/clk-exynos850.c | 835 ++++++++++++++++++++++++++++
>  2 files changed, 836 insertions(+)
>  create mode 100644 drivers/clk/samsung/clk-exynos850.c
> 

Thanks for the changes, awesome work, I appreciate it.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>

Best regards,
Krzysztof