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SubjectRe: [PATCH 1/5] clk: sun6i: Protect CPU clock
Hello Russell,

El 24/02/14 13:30, Russell King - ARM Linux escribió:
> On Mon, Feb 24, 2014 at 05:22:43PM +0100, Maxime Ripard wrote:
>> Right now, AHB is an indirect child clock of the CPU clock. If that happens to
>> change, since the CPU clock has no other consumers declared in Linux, it would
>> be shut down, which is not really a good idea.
>>
>> Prevent this by forcing it enabled.
>>
>> Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
>> ---
>> drivers/clk/sunxi/clk-sunxi.c | 8 ++++++++
>> 1 file changed, 8 insertions(+)
>>
>> diff --git a/drivers/clk/sunxi/clk-sunxi.c b/drivers/clk/sunxi/clk-sunxi.c
>> index 23baad9..cedaf4b 100644
>> --- a/drivers/clk/sunxi/clk-sunxi.c
>> +++ b/drivers/clk/sunxi/clk-sunxi.c
>> @@ -1301,6 +1301,14 @@ static void __init sunxi_clock_protect(void)
>> clk_prepare_enable(clk);
>> clk_put(clk);
>> }
>> +
>> + /* CPU clocks - sun6i */
>> + clk = clk_get(NULL, "cpu");
>> + if (!IS_ERR(clk)) {
>> + clk_prepare_enable(clk);
>> + clk_put(clk);
>> + }
>
> This is broken. I'm not sure what's difficult to grasp about the concept
> of "while a clock is in use, you should keep a reference to that clock".
>
> That implies that if you get a clock, and then enable it, you don't
> put the clock until you've disabled it.

Why is this so? Can't a clock be left enabled while nobody has a
reference to it? I have looked around in Documentation/ (rather quickly
I must say) and have not found any explicit mention that it is required
to keep a reference to the clock while it's enabled. I'd appreciate it
if you could explain this a bit more verbosely or point me to the
relevant documents.

For what it's worth, I've seen this same pattern on
enable/disable_clock() on drivers/base/power/clock_ops.c as well.

Cheers,

Emilio
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