linux-kernel - Re: [PATCH v2 3/4] clk: Provide an always-on clock domain framework

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Date:	Wed, 25 Feb 2015 15:48:08 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Rob Herring <robherring2@...il.com>
Cc:	Mike Turquette <mturquette@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stephen Boyd <sboyd@...eaurora.org>, kernel@...inux.com,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] clk: Provide an always-on clock domain framework

On Wed, 25 Feb 2015, Rob Herring wrote:

> On Mon, Feb 23, 2015 at 11:23 AM, Mike Turquette <mturquette@...aro.org> wrote:
> > Quoting Lee Jones (2015-02-18 08:15:00)
> >> Much h/w contain clocks which if turned off would prove fatal.  The
> >> only way to recover is to restart the board(s).  This driver takes
> >> references to clocks which are required to be always-on in order to
> >> prevent the common clk framework from trying to turn them off during
> >> the clk_disabled_unused() procedure.
> 
> [...]
> 
> >> +static int ao_clock_domain_probe(struct platform_device *pdev)
> >> +{
> >> +       struct device_node *np = pdev->dev.of_node;
> >> +       int nclks, i;
> >> +
> >> +       nclks = of_count_phandle_with_args(np, "clocks", "#clock-cells");
> >
> > Minor nitpick: please use of_clk_get_parent_count. I spent a solid 5
> > minutes writing that function and I need people to use it so I can get a
> > return on my investment.
> >
> > Otherwise the patch looks good. I believe that this method is targeting
> > always-on clock in a production environment, which is different from the
> > CLK_IGNORE_UNUSED stuff which typically is helpful while bringing up new
> > hardware or dealing with a platform that has incomplete driver support.
> 
> There is also the usecase of keep clocks on until I load a module that
> properly handles my hardware (e.g simplefb). We have a simplefb node
> with clocks and the simplefb driver jumps thru some hoops to hand-off
> clocks to the real driver. I don't really like it and don't want to
> see more examples. And there is the case of I thought I would never
> manage this clock, but kernel subsystems evolve and now I want to
> manage a clock. This should not require a DT update to do so.
> 
> Neither of these may be Lee's usecase, but I want to see them covered
> by the binding.
> 
> > I wonder if there is a clever way for existing clock providers
> > (expressed in DT) to use this without having to create a separate node
> > of clocks with the "always-on-clk-domain" flag. Possibly the common
> > clock binding could declare some always-on flag that is standardized?
> > Then the framework core could use this code easily. Not sure if that is
> > a good idea though...
> 
> I would prefer to see the always on clocks just listed within the
> clock controller's node rather than creating made up nodes with clock
> properties.

> This should be always-on until claimed IMO, but that
> aspect is the OS's problem, not a DT problem.

I disagree with this point.  There are likely to be many unclaimed,
but perfectly gateable clocks in a system, which will consume power
unnecessarily.  The clk framework does the right thing by turning all
unclaimed clocks off IMHO.  This only leaves a small use-case where we
need to artificially claim some which must not be gated.

The other way to do is, as you mentioned is list the clocks which must
stay on in the clock source node, but this will still require a
binding.  It will also require a much more complicated framework
driver.

    clkprovider@...xxxxx {
            always-on-clks = <1, 2, 4, 5, 7>;
    };
-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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