linux-kernel - Re: [PATCH RFC RFT 3/3] clk: introduce CLK_ENABLE_HAND

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Date:	Tue, 11 Aug 2015 10:09:24 -0700
From:	Michael Turquette <mturquette@...libre.com>
To:	Maxime Coquelin <maxime.coquelin@...com>,
	"Lee Jones" <lee.jones@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
	sboyd@...eaurora.org, maxime.ripard@...e-electrons.com,
	s.hauer@...gutronix.de, geert@...ux-m68k.org
Subject: Re: [PATCH RFC RFT 3/3] clk: introduce CLK_ENABLE_HAND_OFF flag

Quoting Maxime Coquelin (2015-08-11 03:02:23)
> Hi Mike,
> 
> On 08/11/2015 10:43 AM, Lee Jones wrote:
> > On Mon, 10 Aug 2015, Michael Turquette wrote:
> >
> >>
> >>
> >> ST's driver is an unfortunate case. All of the clock data was shoved
> >> into DT before we had a clue that doing so is a terrible idea.
> 
> I tend to agree, and wouldn't do it this way if we could rewrite the 
> history.
> But now, we have to support it.
> 
> How can we pass CLK_ENABLE_HAND_OFF flag to a specific clock on STi 
> platform?
> 
> Could we imagine having a kind of "clocks-enable-hand-off" property we 
> could use in our clock controller DT node?

Maxime,

Yes. I'm sure that the ST binding isn't the only one that needs
something like this. Furthermore I am sure that there are interesting
users like the FPGA people that would love to dynamically set this flag
from DT based on their hardware description.

So the question is, what does it look like? We've already discussed
doing a clk-conf.c approach, but that is really meant for consumers of a
clock to set their default parameters. I don't think that is the right
way here.

Probably we should list the hand-off clocks directly in the
clock-provider node itself. We can design it as a list (for
clock-controller nodes that expose multiple clocks). In practice for the
st,flexgen binding it will always be a list with one element in it.

In my email to Lee a few minutes ago I asked if ST actually needs to
turn on gated clocks, or if the goal is to prevent already-on clocks
(enabled by default out of reset, or bootloader) from being gated? I
guess that the goal is the latter since we've been discussing "critical"
clocks that will crash the system if disabled.

Regards,
Mike

> 
> Regards,
> Maxime
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