linux-kernel - Re: [RFC PATCH] clk: core: refine disable unused clocks

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Message-ID: <5eb12197-330c-4f55-82f7-d13ea458ba43@amlogic.com>
Date: Fri, 8 Nov 2024 19:49:59 +0800
From: Chuan Liu <chuan.liu@...ogic.com>
To: Jerome Brunet <jbrunet@...libre.com>
Cc: Stephen Boyd <sboyd@...nel.org>,
 Neil Armstrong <neil.armstrong@...aro.org>,
 Kevin Hilman <khilman@...libre.com>,
 Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
 linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-amlogic@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH] clk: core: refine disable unused clocks


On 11/8/2024 5:59 PM, Jerome Brunet wrote:
> [ EXTERNAL EMAIL ]
>
> On Fri 08 Nov 2024 at 17:23, Chuan Liu <chuan.liu@...ogic.com> wrote:
>
>>>>> -       if (core->flags & CLK_IGNORE_UNUSED)
>>>>> +       /*
>>>>> +        * If the parent is disabled but the gate is open, we should sanitize
>>>>> +        * the situation. This will avoid an unexpected enable of the clock as
>>>>> +        * soon as the parent is enabled, without control of CCF.
>>>>> +        *
>>>>> +        * Doing so is not possible with a CLK_OPS_PARENT_ENABLE clock without
>>>>> +        * forcefully enabling a whole part of the subtree.  Just let the
>>>>> +        * situation resolve it self on the first enable of the clock
>>>>> +        */
>>>>> +       if (!parent_enabled && (core->flags & CLK_OPS_PARENT_ENABLE))
>> At first, I couldn't grasp the logic behind the 'return' here. Now it's
>> clear. This approach is equivalent to completely giving up on
>> handling clocks with CLK_OPS_PARENT_ENABLE feature in
>> clk_disable_unused_subtree().
>>
> No. It's handled correctly as long as the tree is in coherent state.
>
> What is not done anymore is fixing up an inconsistent tree, by this I
> mean: A clock with CLK_OPS_PARENT_ENABLE, which report enabled from its
> own registers but has its parent disabled.
>
> In that particular case, clk_disable_unused_subtree() won't be turning on
> everything to properly disable that one clock. That is the root cause of
> the problem you reported initially. The clock is disabled anyway.
>
> Every other case are properly handled (at least I think).

name              en_sts            flags
clk_a                1          CLK_IGNORE_UNUSED
     clk_b            0                0
         clk_c        1         CLK_OPS_PARENT_ENABLE

Based on the above case:
1. When 'clk_c' is configured with CLK_OPS_PARENT_ENABLE, disabling
'clk_c' requires enabling 'clk_b' first (disabling 'clk_c' before
disabling 'clk_b'). How can to ensure that during the period of
disabling 'clk_c', 'clk_b' remains enabled?

2. 'clk_c' is not configured with CLK_IGNORE_UNUSED, it should be
disabled later. However, here it goes to a 'goto' statement and then
return 'false', ultimately resulting in 'clk_c' not being disabled?

>>>>>                    goto unlock_out;
>>>>>
>>>>>            /*
>>>>> @@ -1516,8 +1545,7 @@ static void __init clk_disable_unused_subtree(struct clk_core *core)
>>>>>
>>>>>     unlock_out:
>>>>>            clk_enable_unlock(flags);
>>>>> -       if (core->flags & CLK_OPS_PARENT_ENABLE)
>>>>> -               clk_core_disable_unprepare(core->parent);
>>>>> +       return (core->flags & CLK_IGNORE_UNUSED) && enabled;
>>>>>     }
>>>>>
>>>>>     static bool clk_ignore_unused __initdata;
>>>>> @@ -1550,16 +1578,16 @@ static int __init clk_disable_unused(void)
>>>>>            clk_prepare_lock();
>>>>>
>>>>>            hlist_for_each_entry(core, &clk_root_list, child_node)
>>>>> -               clk_disable_unused_subtree(core);
>>>>> +               clk_disable_unused_subtree(core, true);
>>>>>
>>>>>            hlist_for_each_entry(core, &clk_orphan_list, child_node)
>>>>> -               clk_disable_unused_subtree(core);
>>>>> +               clk_disable_unused_subtree(core, true);
>>>>>
>>>>>            hlist_for_each_entry(core, &clk_root_list, child_node)
>>>>> -               clk_unprepare_unused_subtree(core);
>>>>> +               clk_unprepare_unused_subtree(core, true);
>>>>>
>>>>>            hlist_for_each_entry(core, &clk_orphan_list, child_node)
>>>>> -               clk_unprepare_unused_subtree(core);
>>>>> +               clk_unprepare_unused_subtree(core, true);
>>>>>
>>>>>            clk_prepare_unlock();
>>>>>
>>>>> --
>>>>> 2.45.2
>>>>>
>>> --
>>> Jerome
> --
> Jerome