On Fri, 4 Nov 2022 at 14:32, Maxime Ripard maxime@cerno.tech wrote:
The UX500 sysctrl "set_parent" clocks implement 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.
If I recall correctly, that is the use case we did target for these types of clocks. See sound/soc/ux500/ux500_ab85xx.c, for example.
Maybe there are some additional pieces missing from the old down stream kernel, I don't have full picture, sorry.
Anyway, if I am not wrong, this was about supporting a low-power audio use case, which requires us to switch the parent clock (to avoid wasting energy).
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().
Yes, this was the reason.
As a matter of fact, I don't even recall that re-parenting was possible through clk_set_rate() when this clock driver was introduced. But, I might be wrong, it's quite a while ago.
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 maxime@cerno.tech
Seems reasonable to me!
Reviewed-by: Ulf Hansson ulf.hansson@linaro.org
Kind regards Uffe
drivers/clk/ux500/clk-sysctrl.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/drivers/clk/ux500/clk-sysctrl.c b/drivers/clk/ux500/clk-sysctrl.c index 702f2f8b43fa..d36336665b6d 100644 --- a/drivers/clk/ux500/clk-sysctrl.c +++ b/drivers/clk/ux500/clk-sysctrl.c @@ -110,6 +110,7 @@ static const struct clk_ops clk_sysctrl_gate_fixed_rate_ops = { };
static const struct clk_ops clk_sysctrl_set_parent_ops = {
.determine_rate = __clk_mux_determine_rate, .set_parent = clk_sysctrl_set_parent, .get_parent = clk_sysctrl_get_parent,}; @@ -220,6 +221,7 @@ struct clk *clk_reg_sysctrl_set_parent(struct device *dev, unsigned long flags) { return clk_reg_sysctrl(dev, name, parent_names, num_parents,
reg_sel, reg_mask, reg_bits, 0, 0, flags,
reg_sel, reg_mask, reg_bits, 0, 0,flags | CLK_SET_RATE_NO_REPARENT, &clk_sysctrl_set_parent_ops);}
-- b4 0.11.0-dev-99e3a