[PATCH v2 4/4] clk: dt: Introduce always-on clock domain documentation

[Geert Uytterhoeven]

Hi Lee,

On Thu, Feb 19, 2015 at 11:28 AM, Lee Jones <lee.jones at linaro.org> wrote:
> On Thu, 19 Feb 2015, Geert Uytterhoeven wrote:
>> On Thu, Feb 19, 2015 at 11:11 AM, Lee Jones <lee.jones at linaro.org> wrote:
>> > On Thu, 19 Feb 2015, Geert Uytterhoeven wrote:
>> >> On Thu, Feb 19, 2015 at 10:42 AM, Lee Jones <lee.jones at linaro.org> wrote:
>> >> >> What kind of clocks are these? What do they control?
>> >> >> Memory controllers? Bus controllers?
>> >> >>
>> >> >> They must control some device(s), so there should be one or more device
>> >> >> nodes in DT that reference these clocks.
>> >> >> As soon as that information is in DT, support can be added to Linux to
>> >> >> make sure the "critical" clocks stay enabled, either through a real driver,
>> >> >> or through platform code.
>> >> >
>> >> > Some do, some don't.  For instance, we have one clock which controls
>> >> > SPI and I2C that must not be turned off.  We discovered this then when
>> >> > a suspend was attempted and the board refused to resume.  This clock
>> >> > also runs one of the critical interconnects that runs from the a9.  It
>> >> > would be wrong to remove the clk_disable() attempt from the SPI/I2C
>> >> > drivers because the same IP on another board might be controlled by a
>> >> > different clock which is able to be gated.
>> >> >
>> >> > There are also clocks which control other interconnects that are not
>> >> > connected to any device drivers.  If we fail to take references for
>> >> > them before clk_disable_unused() is called, again the board hangs.  We
>> >> > even lose JTAG support.
>> >>
>> >> Interconnects are buses. Can't you represent those buses in the DT
>> >> hierarchy, and give them clocks properties?
>> >
>> > So instead of this nice succinct, simple, cover all bases
>> > (interconnects was just an example, there are bound to be others),
>> > generic framework, you are suggesting to write drivers for devices
>> > which other than "don't turn my clocks off", Linux can't actually see
>> > or control?
>>
>> DT describes the hardware, not behavior.
>
> Okay so ...
>
> /*
>  * ICNs are not visible/controllable in Linux, but references to their
>  * clocks must be obtained and retained or the platform will become
>  * irrecoverably unresponsive.
>  */
> interconnects at 0 {
>        compatible = "always-on-clk-domain";

st,...flexgen...

>        clocks = <&clk_s_c0_flexgen CLK_ICN_SBC>,
>                 <&clk_s_c0_flexgen CLK_ICN_LMI>,
>                 <&clk_s_c0_flexgen CLK_ICN_CPU>,
>                 <&clk_s_c0_flexgen CLK_TX_ICN_DMU>;
> };

And then you can have platform code that binds against st,...flexgen...,
and enables all referenced clocks.

Alternatively, if you have power domains, you can add a reference to
the power domain, and let the power domain driver handle it.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds