[PATCH 1/2] Add a common struct clk

On Fri, Jan 07, 2011 at 08:10:20AM +0800, Jeremy Kerr wrote:

Hi Richard,

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+struct clk {
+	const struct clk_ops	*ops;
+	unsigned int		enable_count;
+	int			flags;
+	union {
+		struct mutex	mutex;
+		spinlock_t	spinlock;
+	} lock;
+};

Here you have a "polymorphic" lock, where the clock instance knows
which type it is supposed to be.  I got flak from David Miller and

others trying to do the same thing with the mdio_bus:
   http://kerneltrap.org/mailarchive/linux-netdev/2010/7/6/6280618

The criticism, applied to your case, is that the clk_enable() caller
cannot know whether it is safe to make the call or not. I was told,
"there has got to be a better way."

Note that this is not "new".  Currently there is no convention available
if clk_enable sleeps or not.  See e.g.
http://thread.gmane.org/gmane.linux.ports.arm.kernel/100744

As Uwe says, the common clock does not change these semantics; I would prefer 
to keep the driver API changes at a minimum with these patches.

But yes, it would be a good idea to:

 * introduce clk_enable_atomic, which requires clk->flags & CLK_ATOMIC

 * add might_sleep to clk_enable(), encouraging clk uses in atomic contexts
   to switch to clk_enable_atomic.

We'd still be able to handle CLK_ATOMIC clocks in clk_enable(), so the 
enforcement only needs to be one-way.

I think the atomic stuff should be the norm through and through - otherwise
we're going to end up with problems in drivers where they use the _atomic()
stuff, but the clocks behind are coded to sleep.

I hate the GPIO APIs for doing this _cansleep crap as the decision of
whether to use the _cansleep or normal APIs normally can't be made at
the time when the API is used, but sometime later.  Many people just use
the non-_cansleep versions irrespective of the context they're in -
which is unnecessarily restrictive - consider what happens if you then
have that driver use a GPIO on an I2C peripheral...

By inventing two interfaces, you're asking for the same thing to happen
with clocks.