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

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

On 06/10/2021 12:46, Sam Protsenko wrote:
> On Wed, 15 Sept 2021 at 11:21, Krzysztof Kozlowski
> <krzysztof.kozlowski@canonical.com> wrote:
>>
>> 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);
>>> +             }
>>> +     }
>>> +
>>
>> Solving this problem in generic way makes sense but your solution is
>> insufficient. You skipped suspend/resume paths and in such case you
>> should remove the Exynos5433-specific code.
>>
> 
> Keeping core bus clocks always running seems like a separate
> independent feature to me (not related to suspend/resume). It's
> mentioned in commit 523d3de41f02 ("clk: samsung: exynos5433: Add
> support for runtime PM") this way:
> 
>     "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."
> 
> Why do you think suspend/resume paths have to be implemented along
> with it? Btw, I didn't add PM ops in clk-exynos850, as PM is not
> implemented on my board yet and I can't test it.

You can skip the runtime PM, so keep your patch almost like it is now
(in respect to Sylwester's comment about __clk_lookup). However now the
Exynos5433 will enable the clk_name twice: here and in
exynos5433_cmu_probe().

If you keep this approach, you need to remove duplicated part in
exynos5433_cmu_probe()...

> 
> If you are suggesting moving all stuff from exynos5433_cmu_probe()
> into samsung_cmu_register_one(), it would take passing platform_device
> there, and implementing all PM related operations. I guess it's not a
> super easy task, as it would require converting clk-exynos7 to
> platform_driver for instance, and re-testing everything on exynos5433
> and exynos7 boards (which I don't have).
> 
> What do you say if I pull that code to clk-exynos850.c instead for v2?
> Refactoring (merging stuff from exynos5433_cmu_probe() into
> samsung_cmu_register_one() ) can be done later, when I add PM ops into
> clk-exynos850.
> 
>> Best regards,
>> Krzysztof


Best regards,
Krzysztof