Re: [PATCH v5 4/5] clk: add CLK_SET_RATE_NO_REPARENT flag

From: James Hogan
Date: Thu Jul 25 2013 - 08:55:27 EST

Hi Sylwester

On 25/07/13 13:34, Sylwester Nawrocki wrote:
> On 06/13/2013 06:06 PM, James Hogan wrote:
>> Add a CLK_SET_RATE_NO_REPARENT clock flag, which will prevent muxes
>> being reparented during clk_set_rate.
>> To avoid breaking existing platforms, all callers of clk_register_mux()
>> are adjusted to pass the new flag. Platform maintainers are encouraged
>> to remove the flag if they wish to allow mux reparenting on set_rate.
> [..]
>> Changes in v3:
>> * rename/invert CLK_SET_RATE_REMUX to CLK_SET_RATE_NO_REPARENT and move
>> to this new patch.
>> * patch 3: add CLK_SET_RATE_NO_REPARENT flag to all callers of
>> clk_register_mux. If you don't mind your clocks being reparented in
>> response to set_rate please let me know and I'll drop the relevant
>> portion of the patch.
> Why is this better to change current behaviour of the clock core
> and modify all drivers instead of having, e.g. CLK_SET_RATE_REPARENT
> set in drivers of hardware that supports clock re-parenting while
> setting clock rate ?

See this message from Mike Turquette which first suggested it:

> Is there intention to just have the automatic clock re-parenting
> as a default feature in the common clock API ?

Yes, that would be the result (except where explicitly disallowed).
Unfortunately where such policy should ideally be defined is still up in
the air.

It's not a property of the hardware, but then it is arguably a property
of the environment the bootloader has configured (like the stuff in the
/chosen device tree node).

Presuming that the usual reason not to reparent a mux is because other
important clocks depend on it, the kernel might know enough to work out
whether it's safe (unless of course there are other cores/threads in the
SoC using the clock that Linux isn't aware of, which brings us back to
it being a bootloader environment thing).

> My apologies if this has already been answered, I haven't been
> following this thread.

No problem :)


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