[v3,36/65] clk: versatile: sp810: Add a determine_rate hook

Commit Message

Maxime Ripard April 4, 2023, 10:11 a.m. UTC

The Versatile sp810 "timerclken" clock implements a mux with a
set_parent hook, but doesn't provide a determine_rate implementation.

This is a bit odd, since set_parent() is there to, as its name implies,
change the parent of a clock. However, the most likely candidate to
trigger that parent change is a call to clk_set_rate(), with
determine_rate() figuring out which parent is the best suited for a
given rate.

The other trigger would be a call to clk_set_parent(), but it's far less
used, and it doesn't look like there's any obvious user for that clock.

So, the set_parent hook is effectively unused, possibly because of an
oversight. However, it could also be an explicit decision by the
original author to avoid any reparenting but through an explicit call to
clk_set_parent().

The latter case would be equivalent to setting the flag
CLK_SET_RATE_NO_REPARENT, together with setting our determine_rate hook
to __clk_mux_determine_rate(). Indeed, if no determine_rate
implementation is provided, clk_round_rate() (through
clk_core_round_rate_nolock()) will call itself on the parent if
CLK_SET_RATE_PARENT is set, and will not change the clock rate
otherwise. __clk_mux_determine_rate() has the exact same behavior when
CLK_SET_RATE_NO_REPARENT is set.

And if it was an oversight, then we are at least explicit about our
behavior now and it can be further refined down the line.

Signed-off-by: Maxime Ripard <[e-mail]>
---
 drivers/clk/versatile/clk-sp810.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Comments

Pawel Moll April 6, 2023, 3:21 p.m. UTC | #1

On 04/04/2023 11:11, Maxime Ripard wrote:
> The Versatile sp810 "timerclken" clock implements a mux with a
> set_parent hook, but doesn't provide a determine_rate implementation.
> 
> This is a bit odd, since set_parent() is there to, as its name implies,
> change the parent of a clock. 

Explanation of this mystery is pretty simple - the original patch:

	commit 6e973d2c438502dcf956e76305258ba7d1c7d1d3
	Author: Pawel Moll <[e-mail]>
	Date:   Thu Apr 18 18:23:22 2013 +0100

	    clk: vexpress: Add separate SP810 driver

predates introduction of determine_rate to clk_ops...

	commit 71472c0c06cf9a3d1540762ea205654c584e3bc4
	Author: James Hogan <[e-mail]>
	Date:   Mon Jul 29 12:25:00 2013 +0100

	    clk: add support for clock reparent on set_rate

and clearly no one (the author included ;-) bothered to have another
look at this side of the driver.

> And if it was an oversight, then we are at least explicit about our
> behavior now and it can be further refined down the line.

It's been one hell of a memory lane trip, but my recollection suggest
that the main goal of the driver was simply initialisation of the mux
to select the 1MHz parent, because the other option - 32kHz - just
didn't make any sense whatsoever. And that would be the case on every
single platform using SP810 I know (or at least: knew), so it's seems
to me that making the state permanent, as you're suggesting (or I
think you're suggesting?) it's the right thing to do.

Thanks!

Paweł

Patch

diff --git a/drivers/clk/versatile/clk-sp810.c b/drivers/clk/versatile/clk-sp810.c
index caf0cd2fb5b6..a45b1b7b7d49 100644
--- a/drivers/clk/versatile/clk-sp810.c
+++ b/drivers/clk/versatile/clk-sp810.c
@@ -63,6 +63,7 @@  static int clk_sp810_timerclken_set_parent(struct clk_hw *hw, u8 index)
 }
 
 static const struct clk_ops clk_sp810_timerclken_ops = {
+	.determine_rate = __clk_mux_determine_rate,
 	.get_parent = clk_sp810_timerclken_get_parent,
 	.set_parent = clk_sp810_timerclken_set_parent,
 };
@@ -105,7 +106,7 @@  static void __init clk_sp810_of_setup(struct device_node *node)
 
 	init.name = name;
 	init.ops = &clk_sp810_timerclken_ops;
-	init.flags = 0;
+	init.flags = CLK_SET_RATE_NO_REPARENT;
 	init.parent_names = parent_names;
 	init.num_parents = num;