Re: [PATCH 1/3] clk: bcm: rpi: Turn firmware clock on/off when preparing/unpreparing

["Maíra Canal" <mcanal@igalia.com>]

Hi Maxime,

On 29/07/25 04:27, Maxime Ripard wrote:
> Hi Maíra,
> 
> On Mon, Jul 28, 2025 at 09:35:38AM -0300, Maíra Canal wrote:
>> Currently, when we prepare or unprepare RPi's clocks, we don't actually
>> enable/disable the firmware clock. This means that
>> `clk_disable_unprepare()` doesn't actually change the clock state at
>> all, nor does it lowers the clock rate.
>>
>>  From the Mailbox Property Interface documentation [1], we can see that
>> we should use `RPI_FIRMWARE_SET_CLOCK_STATE` to set the clock state
>> off/on. Therefore, use `RPI_FIRMWARE_SET_CLOCK_STATE` to create a
>> prepare and an unprepare hook for RPi's firmware clock.
>>
>> As now the clocks are actually turned off, some of them are now marked
>> with CLK_IGNORE_UNUSED or CLK_IS_CRITICAL, as those are required since
>> early boot or are required during reboot.
> 
> What difference is there between the CLK_IGNORE_UNUSED and
> CLK_IS_CRITICAL clocks?

 From my understanding, CLK_IGNORE_UNUSED will prevent the clock to be
gated during boot (on "clk: Disabling unused clocks"), but after it, the
clock can be gated.

With CLK_IS_CRITICAL, the clock will never be disabled.

For example, RPI_FIRMWARE_M2MC_CLK_ID is used by vc4. It needs to be
enabled at boot (I tested; if not enabled, it won't boot). However,
after vc4 is probed, we would like vc4 to have control of it and be able
to unprepare it in `vc4_hdmi_runtime_suspend()`. If I set it as
CLK_IS_CRITICAL, vc4 won't be able to unprepare it.

I only set RPI_FIRMWARE_PIXEL_BVB_CLK_ID as critical, as, otherwise, the
RPi won't reboot.

Best Regards,
- Maíra

> 
> I'm asking, because CLK_IGNORE_UNUSED is mostly useless, and
> CLK_IS_CRITICAL is probably what you're looking for for all of them.
> 
> Maxime