Michael Walle <mwa...@kernel.org> writes:
> 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.
Hmm, it also seems wrong to me that the clock framework would cache a
clock rate when it's disabled. On platforms with clocks that may have
shared management (eg. TISCI or other platforms using SCMI) it's
entirely possible that when Linux has disabled a clock, some other
entity may have changed it.
Could another solution here be to have the clk framework only cache when
clocks are enabled?
> Thus, disable the cache altogether.
>
> Signed-off-by: Michael Walle <mwa...@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.
The performance hit is not just about boot time, it's for *every*
[get|set]_rate call. Since TISCI is relatively slow (involves RPC,
mailbox, etc. to remote core), this may have a performance impact
elsewhere too. That being said, I'm hoping it's unlikely that
[get|set]_rate calls are in the fast path.
All of that being said, I think the impacts of this patch are pretty
minimal, so I don't have any real objections.
Reviewed-by: Kevin Hilman <khil...@baylibre.com>
> ---
> 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);
> --
> 2.39.5
