On Tue Dec 30, 2025 at 9:12 PM CET, Nishanth Menon wrote: > On 13:47-20251223, Michael Walle wrote: >> The TISCI firmware will return 0 if the clock or consumer is not >> enabled although there is a stored value in the firmware. IOW a call to >> set rate will work but at get rate will always return 0 if the clock is >> disabled. >> The clk framework will try to cache the clock rate when it's requested >> by a consumer. If the clock or consumer is not enabled at that point, >> the cached value is 0, which is wrong. Thus, disable the cache >> altogether. >> >> Signed-off-by: Michael Walle <[email protected]> >> Reviewed-by: Kevin Hilman <[email protected]> >> Reviewed-by: Randolph Sapp <[email protected]> >> --- >> drivers/clk/keystone/sci-clk.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/drivers/clk/keystone/sci-clk.c b/drivers/clk/keystone/sci-clk.c >> index 9d5071223f4c..0a1565fdbb3b 100644 >> --- a/drivers/clk/keystone/sci-clk.c >> +++ b/drivers/clk/keystone/sci-clk.c >> @@ -333,6 +333,14 @@ static int _sci_clk_build(struct sci_clk_provider >> *provider, >> >> init.ops = &sci_clk_ops; >> init.num_parents = sci_clk->num_parents; >> + >> + /* >> + * A clock rate query to the SCI firmware will return 0 if either the >> + * clock itself is disabled or the attached device/consumer is disabled. >> + * This makes it inherently unsuitable for the caching of the clk >> + * framework. >> + */ >> + init.flags = CLK_GET_RATE_NOCACHE; >> sci_clk->hw.init = &init; >> >> ret = devm_clk_hw_register(provider->dev, &sci_clk->hw); >> -- >> 2.47.3 >> > > Reviewed-by: Nishanth Menon <[email protected]> > > I wish there was a better scheme, but inherently, just like SCMI and > other systems where power management co-processor controls clocks, there > is no real feasible caching scheme I can think of. I wonder if Stephen > or others have a thought on this? > > That said, I wonder if we need fixes tag to this? I am sure there are > other clocks susceptible to this as well. I wonder if > commit 3c13933c6033 ("clk: keystone: sci-clk: add support for > dynamically probing clocks") is the appropriate tag?
>From my previous versions of this patch:
> Regarding a Fixes: tag. I didn't include one because it might have a
> slight performance impact because the firmware has to be queried
> every time now and it doesn't have been a problem for now. OTOH I've
> enabled tracing during boot and there were just a handful
> clock_{get/set}_rate() calls.
I'm still undecided if this needs a Fixes tag or not. Strictly
speaking it would need one. Although, I'm not sure it's the one
you mentioned, because the culprit is the "we return 0 if the clock
or it's consumer is disabled", which then caches the wrong value.
So it is probably the very first commit b745c0794e2f ("clk:
keystone: Add sci-clk driver support").
-michael
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