linux-kernel - Re: [PATCH 05/11] spi/pxa2xx: make clock rate configurable from platform data

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Date:	Wed, 9 Jan 2013 12:51:07 +0200
From:	Mika Westerberg <mika.westerberg@...ux.intel.com>
To:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-kernel@...r.kernel.org, grant.likely@...retlab.ca,
	linus.walleij@...aro.org, eric.y.miao@...il.com,
	linux@....linux.org.uk, haojian.zhuang@...il.com,
	chao.bi@...el.com, "Rafael J. Wysocki" <rjw@...k.pl>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: Re: [PATCH 05/11] spi/pxa2xx: make clock rate configurable from
 platform data

On Tue, Jan 08, 2013 at 10:33:55PM +0100, Rafael J. Wysocki wrote:
> On 1/8/2013 2:10 PM, Mark Brown wrote:
> >On Tue, Jan 08, 2013 at 02:41:53PM +0200, Mika Westerberg wrote:
> >>On Tue, Jan 08, 2013 at 11:02:28AM +0000, Mark Brown wrote:
> >>>No, the way to do this is to fix x86 to enable the clock API there.  The
> >>>x86 maintainers couldn't be bothered when I submitted a patch and
> >>>getting anyone to take a patch to make it available by default appears
> >>>to be unreasonably hard but perhaps if someone from Intel tries the x86
> >>>maintainers might take a patch...
> >>Do you mean enabling CONFIG_COMMON_CLK on x86?
> >Yes.
> 
> Why so?  x86 doesn't have a notion of direct clock control, at least
> not on the ACPI systems.
> 
> >>>We shouldn't be adding special case code to every driver that might need
> >>>a clock that gets used on an Intel system.
> >>I agree and it is cleaner to have the same API for all arches. However, I'm
> >>not sure how do we create the fixed clocks then? There is nothing in ACPI
> >>namespace (or in the ACPI 5.0 spec) that allows you to describe clocks and
> >>their relationships.
> >I'm sure it's not beyond the bounds of possibility that we could solve
> >this problem...
> 
> No, it isn't.  Any suggestions?

I have one suggestion.

Since even on x86 we are now starting to see peripherals that exists
normally on traditional SoCs, like the SPI controller, and the drivers
expect to have clock for these.

What if we make drivers/clk/clk-x86.c that initializes necessary clocks
like the LPSS 100MHz fixed clock and registers this to all the LPSS
devices? Something along the lines of:

        clk = clk_register_fixed_rate(NULL, "lpss_iclk", NULL, CLK_IS_ROOT,
				      100000000);

	...

	clk_register_clkdev(clk, NULL, "INT33C0:00");

These are clocks that you really can't control (enable/disable) but they
allow one to get the clock rate using the standard clk API.

One problem (among other things) with this is that now we have these clocks
created and registered on every single x86 system. Other problem is that
this setup needs manual maintenance as we can't get the configuration from
ACPI.
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