linux-kernel - Re: [PATCH v5 4/5] clk: add CLK_SET_RATE_NO

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Message-ID: <51F1202D.9060403@imgtec.com>
Date:	Thu, 25 Jul 2013 13:55:09 +0100
From:	James Hogan <james.hogan@...tec.com>
To:	Sylwester Nawrocki <s.nawrocki@...sung.com>
CC:	Mike Turquette <mturquette@...aro.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	<linux-kernel@...r.kernel.org>,
	Saravana Kannan <skannan@...eaurora.org>,
	Doug Anderson <dianders@...omium.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	Russell King <linux@....linux.org.uk>,
	Viresh Kumar <viresh.linux@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Chao Xie <xiechao.mail@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Emilio López <emilio@...pez.com.ar>,
	Gregory CLEMENT <gregory.clement@...e-electrons.com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Prashant Gaikwad <pgaikwad@...dia.com>,
	Thierry Reding <thierry.reding@...onic-design.de>,
	Joseph Lo <josephl@...dia.com>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	"Pawel Moll" <pawel.moll@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>
Subject: Re: [PATCH v5 4/5] clk: add CLK_SET_RATE_NO_REPARENT flag

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:

http://marc.info/?l=linux-kernel&m=136847508109344&w=2

> 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 :)

Cheers
James

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