linux-kernel - Re: [PATCH v2 0/3] clk: implement remuxing during set

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Message-ID: <CAAG0J991BCziPFddzBrx9vcwRxyRgcCd38A01uKXTDDK9xTnbQ@mail.gmail.com>
Date:	Mon, 13 May 2013 22:30:46 +0100
From:	James Hogan <james.hogan@...tec.com>
To:	Mike Turquette <mturquette@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org,
	LKML <linux-kernel@...r.kernel.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	James Hogan <james.hogan@...tec.com>
Subject: Re: [PATCH v2 0/3] clk: implement remuxing during set_rate

On 13 May 2013 20:57, Mike Turquette <mturquette@...aro.org> wrote:
> Quoting James Hogan (2013-04-19 09:28:21)
>> This patchset adds support for automatic selection of the best parent
>> for a clock mux, i.e. the one which can provide the closest clock rate
>> to that requested. It can be controlled by a new CLK_SET_RATE_REMUX flag
>> so that it doesn't happen unless explicitly allowed.
>>
>
> I'm not sure about the new flag.  One of my regrets during the early
> development of the common struct clk is the CLK_SET_RATE_PARENT flag.
> In hindsight I would have preferred to make parent-propagation of rate
> changes the default behavior, and instead provide a flag to prevent that
> behavior, such as CLK_SET_RATE_NO_PROPAGATE.
>
> One reason for this is the difficulty some have had with setting flags
> from DT bindings.

Could you elaborate on this? I've been adding flags to DT bindings for
this sort of thing, but it feels a bit like it's in that grey area of
not really describing the hardware itself. This information needs to
be specified somehow though.

> Another reason is that a core goal of the framework
> is to remove topology info from drivers; having to set a flag explicitly
> on each clock to propagate rate changes is an impediment to that goal.

(Something that I have found also where CLK_SET_RATE_PARENT doesn't
fit well is when you have a mux where you want the set_rate propagated
to one possible parent (e.g. a dedicated PLL) but not the other (e.g.
the system clock - which happens to be a divider). In this case what I
really want to say is "don't change the rate of this particular
clock", in fact I added a CLK_DIVIDER_READ_ONLY divider flag locally
for this purpose. Was there a particular use case of
CLK_SET_RATE_PARENT where you'd want a clock to be changed from one
child but not another?)

>
> So perhaps CLK_SET_RATE_REMUX is a similar situation.  What do you think
> about making remuxing the default and providing a flag to prevent it?

Yes, fine by me.

> And also we can assume for the immediate future that users of
> .determine_rate WANT to remux, since .round_rate won't do it.  Though
> eventually it might be wise to convert all users of .round_rate over to
> .determine_rate and deprecate .round_rate entirely.

True.

Cheers
James
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