[PATCH 13/46] davinci: support re-parenting a clock in the clock framework

[Russell King - ARM Linux]

On Fri, Oct 16, 2009 at 12:09:17PM -0700, Kevin Hilman wrote:
> From: Sekhar Nori <nsekhar at ti.com>
> 
> The clk_set_parent() API is implemented to enable re-parenting
> clocks in the clock tree.
> 
> This is useful in DVFS and helps by shifting clocks to an asynchronous
> domain where supported by hardware
> 
> Signed-off-by: Sekhar Nori <nsekhar at ti.com>
> Signed-off-by: Kevin Hilman <khilman at deeprootsystems.com>
> ---
>  arch/arm/mach-davinci/clock.c |   23 +++++++++++++++++++++++
>  1 files changed, 23 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm/mach-davinci/clock.c b/arch/arm/mach-davinci/clock.c
> index 09e0e1c..12ceeea 100644
> --- a/arch/arm/mach-davinci/clock.c
> +++ b/arch/arm/mach-davinci/clock.c
> @@ -141,6 +141,29 @@ int clk_set_rate(struct clk *clk, unsigned long rate)
>  }
>  EXPORT_SYMBOL(clk_set_rate);
>  
> +int clk_set_parent(struct clk *clk, struct clk *parent)
> +{
> +	unsigned long flags;
> +
> +	if (clk == NULL || IS_ERR(clk))
> +		return -EINVAL;
> +
> +	mutex_lock(&clocks_mutex);
> +	clk->parent = parent;
> +	list_del_init(&clk->childnode);
> +	list_add(&clk->childnode, &clk->parent->children);
> +	mutex_unlock(&clocks_mutex);

This is bad, and is suffering from precisely the same problem which
OMAP suffered from:

1. it takes no notice of whether the clk is in use.
2. it takes no notice of whether the parent clocks are being used.

The result is that using clk_set_parent() on a clk_enable()'d clock,
you totally destroy the usecounting of the parent clocks, leading to
the clock tree usecount becoming totally buggered up.

Either refuse to change the parent of an already clk_enable()'d clock,
or do proper usecount fixups when changing the parent.