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Message-ID: <519AFBB6.90700@codeaurora.org>
Date: Mon, 20 May 2013 21:44:38 -0700
From: Saravana Kannan <skannan@...eaurora.org>
To: James Hogan <james.hogan@...tec.com>
CC: Mike Turquette <mturquette@...aro.org>,
linux-arm-kernel@...ts.infradead.org,
Stephen Boyd <sboyd@...eaurora.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/5] clk: clk-mux: implement remuxing on set_rate
On 05/20/2013 06:28 AM, James Hogan wrote:
> Implement clk-mux remuxing if the CLK_SET_RATE_NO_REPARENT flag isn't
> set. This implements determine_rate for clk-mux to propagate to each
> parent and to choose the best one (like clk-divider this chooses the
> parent which provides the fastest rate <= the requested rate).
>
> The determine_rate op is implemented as a core helper function so that
> it can be easily used by more complex clocks which incorporate muxes.
>
> Signed-off-by: James Hogan <james.hogan@...tec.com>
> Cc: Mike Turquette <mturquette@...aro.org>
> Cc: linux-arm-kernel@...ts.infradead.org
> ---
> Changes in v4:
>
> * never pass NULL to determine_rate's best_parent_clk parameter.
>
> Changes in v3:
>
> * rename/invert CLK_SET_RATE_REMUX to CLK_SET_RATE_NO_REPARENT and move
> to patch 3.
>
> drivers/clk/clk-mux.c | 1 +
> drivers/clk/clk.c | 49 ++++++++++++++++++++++++++++++++++++++++++++
> include/linux/clk-provider.h | 3 +++
> 3 files changed, 53 insertions(+)
>
> diff --git a/drivers/clk/clk-mux.c b/drivers/clk/clk-mux.c
> index 25b1734..cecfa01 100644
> --- a/drivers/clk/clk-mux.c
> +++ b/drivers/clk/clk-mux.c
> @@ -100,6 +100,7 @@ static int clk_mux_set_parent(struct clk_hw *hw, u8 index)
> const struct clk_ops clk_mux_ops = {
> .get_parent = clk_mux_get_parent,
> .set_parent = clk_mux_set_parent,
> + .determine_rate = __clk_mux_determine_rate,
> };
> EXPORT_SYMBOL_GPL(clk_mux_ops);
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index 3ce4810..85b661d 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -692,6 +692,55 @@ struct clk *__clk_lookup(const char *name)
> return NULL;
> }
>
> +/*
> + * Helper for finding best parent to provide a given frequency. This can be used
> + * directly as a determine_rate callback (e.g. for a mux), or from a more
> + * complex clock that may combine a mux with other operations.
> + */
> +long __clk_mux_determine_rate(struct clk_hw *hw, unsigned long rate,
> + unsigned long *best_parent_rate,
> + struct clk **best_parent_p)
> +{
> + struct clk *clk = hw->clk, *parent, *best_parent = NULL;
> + int i, num_parents;
> + unsigned long parent_rate, best = 0;
> +
> + /* if NO_REPARENT flag set, pass through to current parent */
> + if (clk->flags & CLK_SET_RATE_NO_REPARENT) {
> + parent = clk->parent;
> + if (clk->flags & CLK_SET_RATE_PARENT)
> + best = __clk_round_rate(parent, rate);
> + else if (parent)
> + best = __clk_get_rate(parent);
> + else
> + best = __clk_get_rate(clk);
> + goto out;
> + }
> +
> + /* find the parent that can provide the fastest rate <= rate */
> + num_parents = clk->num_parents;
> + for (i = 0; i < num_parents; i++) {
While writing a similar code for our internal tree, I quickly came to
the realization that, "all parents are equal, but some are more equal
than others". The simplest example is a clock tree like this:
Source -> Divider -> Mux
Source -> Mux
A rate of Y can be achieved for Mux by either running Source at Y and
picking that input or running Source at Y * 2 and Divider set to div-2
and picking the Divider input.
The solution seems to be a priority list of parents. I'm sure there
would be other reason (jitter, clock quality, etc) for a mux to pick one
parent vs. another when both of them can provide the required rate.
I think this loop should loop over parents based on their priority
order. So, parents should become a struct of { clk, index } and have the
parents listed in the order of priority. Well, at least in that long run
that would be better to avoid messing up parent/index values. But for
now, you could just have a priority array of index or parents.
It might not fit 100% of the cases where two parents can provide the
same rate, but it should fit most use cases.
> + parent = clk_get_parent_by_index(clk, i);
> + if (!parent)
> + continue;
> + if (clk->flags & CLK_SET_RATE_PARENT)
> + parent_rate = __clk_round_rate(parent, rate);
> + else
> + parent_rate = __clk_get_rate(parent);
> + if (parent_rate <= rate && parent_rate > best) {
> + best_parent = parent;
> + best = parent_rate;
> + }
> + }
> +
> +out:
> + if (best_parent)
> + *best_parent_p = best_parent;
> + *best_parent_rate = best;
> +
> + return best;
> +}
> +
Thanks,
Saravana
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