Stephen Boyd писал(а) 11.01.2022 01:14:
> Quoting Nikita Travkin (2022-01-07 23:25:19)
>> Hi,
>>
>> Stephen Boyd писал(а) 08.01.2022 05:52:
>> > Quoting Nikita Travkin (2021-12-09 08:37:17)
>> I'm adding this error here primarily to bring attention of the
>> user (e.g. developer enabling some peripheral that needs
>> duty cycle control) who might have to change their clock tree
>> to make this control effective. So, assuming that if someone
>> sets the duty cycle to 50% then they might set it to some other
>> value later, it makes sense to fail the first call anyway.
>>
>> If you think there are some other possibilities for this call
>> to happen specifically with 50% duty cycle (e.g. some
>> preparations or cleanups in the clk subsystem or some drivers
>> that I'm not aware of) then I can make an exemption in the check
>> for that.
>>
>
> I don't see anywhere in clk_set_duty_cycle() where it would bail out
> early if the duty cycle was set to what it already is. The default for
> these clks is 50%, so I worry that some driver may try to set the duty
> cycle to 50% and then fail now. Either we need to check the duty cycle
> in the core before calling down into the driver or we need to check it
> here in the driver. Can you send a patch to check the current duty cycle
> in the core before calling down into the clk ops?
Hi, sorry for a rather delayed response,
I spent a bit of time looking at how to make the clk core be
careful with ineffective duty-cycle calls and I can't find a
nice way to do this... My idea was something like this:
static int clk_core_set_duty_cycle_nolock(struct clk_core *core,
struct clk_duty *duty)
{ /* ... */
/* Update core->duty values */
clk_core_update_duty_cycle_nolock(core);
if ( /* duty doesn't match core->duty */ ) {
ret = core->ops->set_duty_cycle(core->hw, duty);
/* ... */
}
However there seem to be drawbacks to any variant of the
comparison that I could come up with:
Naive one would be to do
if (duty->num != core->duty->num || duty->den != core->duty->den)
but it won't correctly compare e.g. 1/2 and 10/20.
Other idea was to do
if (duty->den / duty->num != core->duty->den / core->duty->num)
but it will likely fail with very close values (e.g. 100/500 and 101/500)
I briefly thought of some more sophisticated math but I don't
like the idea of complicating this too far.
I briefly grepped the kernel sources for duty-cycle related methods
and I saw only one user of the clk_set_duty_cycle:
sound/soc/meson/axg-tdm-interface.c
Notably it sets the cycle to 1/2 in some cases, though it seems to
be tied to the drivers/clk/meson/sclk-div.c clock driver by being
the blocks of the same SoC.
Thinking of it a bit more, I saw another approach to the problem
I want to solve: Since I just want to make developers aware of the
hardware quirk, maybe I don't need to fail the set but just put a
WARN or even WARN_ONCE there? This way the behavior will be unchanged.
Thanks,
Nikita
