[linux-sunxi] Re: [PATCH 4/4] simplefb: add clock handling code

[Geert Uytterhoeven]

Hi Thierry,

On Mon, Sep 29, 2014 at 12:44 PM, Thierry Reding
<thierry.reding at gmail.com> wrote:
>> >> You know that you are going to call that for regulator, reset, power
>> >> domains, just as you would have needed to with the proper API, unless
>> >> that with this kind of solution, you would have to modify *every*
>> >> framework that might interact with any resource involved in getting
>> >> simplefb running?
>> >
>> > We have to add handling for every kind of resource either way. Also if
>> > this evolves into a common pattern we can easily wrap it up in a single
>> > function call.
>>
>> disable_all_power_management(), as this is not limited to clocks.
>
> Right. But it isn't all power management either. It just shouldn't turn
> everything unused off. Clocks, regulators, power domains and so on which
> are used can very well be power managed.

No they can't, as the clock/regulator/PM domain core cannot know if any
of the used ones are also used by a shim driver like simplefb.
Clocks and regulators may be shared. PM domains can contain multiple
hardware blocks. Without more knowledge, the only safe thing is not
disabling anything.

>> >> Plus, speaking more specifically about the clocks, that won't prevent
>> >> your clock to be shut down as a side effect of a later clk_disable
>> >> call from another driver.
>>
>> > Furthermore isn't it a bug for a driver to call clk_disable() before a
>> > preceding clk_enable()? There are patches being worked on that will
>> > enable per-user clocks and as I understand it they will specifically
>> > disallow drivers to disable the hardware clock if other drivers are
>> > still keeping them on via their own referenc.
>>
>> Calling clk_disable() preceding clk_enable() is a bug.
>>
>> Calling clk_disable() after clk_enable() will disable the clock (and
>> its parents)
>> if the clock subsystem thinks there are no other users, which is what will
>> happen here.
>
> Right. I'm not sure this is really applicable to this situation, though.

Yes it is: if all users of a clock/regulator/PM domain are gone, it will
be disabled. Bad luck for simplefb still needing them.

> Either way, if there are other users of a clock then they will just as
> likely want to modify the rate at which point simplefb will break just
> as badly.

BTW, this can also happen for clocks that are properly used.
I guess the clock core code does some arbitration to handle such cases.

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