linux-kernel - Re: [PATCH] clk: provide public clk_is

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Date:	Sun, 6 Oct 2013 18:30:11 +0200
From:	Andrew Lunn <andrew@...n.ch>
To:	Andrew Lunn <andrew@...n.ch>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Russell King <linux@....linux.org.uk>,
	Jason Cooper <jason@...edaemon.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>,
	Grant Likely <grant.likely@...aro.org>,
	Mike Turquette <mturquette@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>
Subject: Re: [PATCH] clk: provide public clk_is_enabled function

On Sun, Oct 06, 2013 at 11:06:09AM +0200, Gerhard Sittig wrote:
> On Sat, Oct 05, 2013 at 22:42 +0200, Andrew Lunn wrote:
> > 
> > On Sat, Oct 05, 2013 at 10:24:30PM +0200, Uwe Kleine-König wrote:
> > > On Fri, Oct 04, 2013 at 12:08:30PM +0200, Sebastian Hesselbarth wrote:
> > > > To determine if a clk has been previously enabled, provide a public
> > > > clk_is_enabled function. This is especially helpful to check the state
> > > > of clk-gate without actually changing the state of the gate.
> > > I wonder what you want to do with the return value.
> > > 
> > > When doing
> > > 
> > > 	if (clk_is_enabled(someclk))
> > > 		do_something();
> > > 
> > > you cannot in general know if the clock is still on when you start to
> > > do_something.
> > 
> > Hi Uwe
> > 
> > At least in the use case Sebastian needs it for, we don't need an "in
> > general" solution. It is used early boot time to see if the boot
> > loader left the clock running.
> 
> Wait, unless I'm missing something, the clk_is_enabled() call
> _won't_ determine whether the clock is enabled in hardware
> (whether the boot loader created or left this condition), instead
> it only determines whether clk_enable() was called previously and
> thus the clock _shall_ be enabled.

Nope, you are wrong.

static int clk_gate_is_enabled(struct clk_hw *hw)
{
	u32 reg;
	struct clk_gate *gate = to_clk_gate(hw);

	reg = clk_readl(gate->reg);

	/* if a set bit disables this clk, flip it before masking */
	if (gate->flags & CLK_GATE_SET_TO_DISABLE)
	   reg ^= BIT(gate->bit_idx);

	   reg &= BIT(gate->bit_idx);

	   return reg ? 1 : 0;
}

It reads the hardware state.

> AFAIK the kernel's CCF support is "self contained" and does not
> consider any data or state that was "inherited" from boot staged
> before the kernel.  That's why the "disable unused" step disables
> everything that wasn't acquired _in the kernel_ regardless of
> what the boot loader may have done or what is enabled at reset.

Not quite true. It uses the is_enabled(), which gets the real hardware
state, to turn off clocks which are unused but on. It will not turn
off clock which are already off. So it is inheriting some state from
the boot loader, in that it knows if the bootloader turned it
on. However this is not propagated into prepare/enable status.

> > The other user of the clock is the
> > ethernet driver, which we know cannot change it yet, because driver
> > probing has not started yet.
> 
> I understand that the situation here is, that the ethernet driver
> hasn't probed yet, but the clock driver did.  You are in early setup
> code and want to (check and) fetch data from the hardware which the
> ethernet driver later needs.

Nearly, but not quite. If there is an enabled DT node for the device,
and that node does not have a valid local-mac-address property in the
node, the bootloader should of programmed the MAC address into the
device. If it has done that, the clock should be running, because as
soon as you turn the clock off, it forgets the MAC address. Thus, if
we find an enabled device in DT, without a valid local-mac-address,
and the clock is off, we have a bootloader bug, which we want to
report.

> What's wrong with an explicit enable/disable around the data
> acquisition?

It avoids the CPU locking hard, but will not get us a valid MAC
address, which is the point of the exercise.

	 Andrew
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