Hi,

quoted

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 <mwalle@kernel.org>
---
I guess to make it work correctly with the caching of the linux
subsystem a new flag to query the real clock rate is needed. That
way, one could also query the default value without having to turn
the clock and consumer on first. That can be retrofitted later and
the driver could query the firmware capabilities.

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.
---
 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 c5894fc9395e..d73858b5ca7a 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);

Thanks for looking into this Michael. I'm still convinced that it's unusual to
report 0 for a clock rate when the device is powered down. In most cases it's
not actually 0 and is actually just in bypass mode.

Yeah. And you can't query the clock rate you've just set if the
clock is off (and if enabled the clock will have the frequency of
the last set_rate). I still think that is a gap in the firmware
interface.

I was told it's a way to indicate clock status and probably won't be changing
any time soon though. Ignore the fact that we also already have a separate way
to query clock status. :)

This series looks good, but won't quite result in a functional GPU without the
following patch: https://lore.kernel.org/all/20250808232522.1296240-1-rs@ti.com/ (local)

Ahh, I was puzzled why it was working for me. But then I've noticed
that your patch is for the am62p. I'm working with the am67a and
there the ranges are correct for the GPU.

I suppose I'll submit that again on it's own.

Reviewed-by: Randolph Sapp <redacted>

Thanks.

-michael