linux-kernel - Re: [PATCH] clk: add DT test clock consumer driver

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Message-ID: <20130319015425.8663.84827@quantum>
Date:	Mon, 18 Mar 2013 18:54:25 -0700
From:	Mike Turquette <mturquette@...aro.org>
To:	Arnd Bergmann <arnd@...db.de>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Cc:	linux-doc@...r.kernel.org,
	Linus Walleij <linus.walleij@...aro.org>,
	linux-kernel@...r.kernel.org,
	Rob Herring <rob.herring@...xeda.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Landley <rob@...dley.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	devicetree-discuss@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] clk: add DT test clock consumer driver

Quoting Arnd Bergmann (2013-03-16 07:56:54)
> On Saturday 16 March 2013, Sebastian Hesselbarth wrote:
> > This driver adds a DT test clock consumer that exposes debugfs files to
> > enable/disable and set/get rate of the attached programmable clock.
> > During development of a i2c-attached clock generator I found it useful
> > to debug the clock generator's internal pll settings by enforcing clock
> > rates through debugfs.
> > 
> > Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
> 
> It sounds a little clumsy to have a device driver to match a device that
> you create just for matching the driver.
> 
> Would it be possible to separate the debugging logic from the platform
> device logic? I think it may be useful to have a debugfs or sysfs
> inteface for all clocks in the system, even if that is disabled by
> default or only available after manually loading a module implementing
> that functionality.
> 

I agree that a generic approach is needed here.  I have been meaning to
break the existing debugfs stuff out into clk-debug.c.  I'll do that
soon and maybe you can add a new Kconfig entry for
COMMON_CLK_DEBUG_USERSPACE (or something like that) which implements
this?

On the other hand this sort of stuff really scares me.  I know for a
fact that a debug interface to enable/disable clocks and set clock rate
would ship on real devices.  Quite likely some android phones out there
would be controlling hardware clocks from some horrible userspace
utility.

*shudder*

Sebastian, another small nitpick, can you change the "enable" attribute
to be named "prepare_enable"?  This more accurately reflects what is
going on.

I also wonder how simple it would be to add a "parent" attribute here
that allows one to call clk_set_parent from the debugfs interface?  To
make it easy on you, the interface could accept an integer as the index
of the clk->parents[] array.  This is a bad interface design as it
requires the user to look into the code to know which index corresponds
to which parent clock; however I do not want people to use this
interface for anything other than debug/testing, so I am ok with this
interface being a PITA to use.

Regards,
Mike

>         Arnd
> 
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