linux-kernel - Re: [PATCH v2 2/4] clk: keystone: don't cache clock rate

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Message-ID: <20260109120728.2wku6akxof2ddn4h@tastiness>
Date: Fri, 9 Jan 2026 06:07:28 -0600
From: Nishanth Menon <nm@...com>
To: Michael Walle <mwalle@...nel.org>, Stephen Boyd <sboyd@...nel.org>
CC: Frank Binns <frank.binns@...tec.com>, Matt Coster
	<matt.coster@...tec.com>, Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>, Rob Herring <robh@...nel.org>, "Krzysztof
 Kozlowski" <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, "Vignesh
 Raghavendra" <vigneshr@...com>, Tero Kristo <kristo@...nel.org>, Andrew Davis
	<afd@...com>, Santosh Shilimkar <ssantosh@...nel.org>, Michael Turquette
	<mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, Kevin Hilman
	<khilman@...libre.com>, Randolph Sapp <rs@...com>,
	<linux-clk@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/4] clk: keystone: don't cache clock rate

On 08:55-20260102, Michael Walle wrote:
> 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 <mwalle@...nel.org>
> >> Reviewed-by: Kevin Hilman <khilman@...libre.com>
> >> Reviewed-by: Randolph Sapp <rs@...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 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 <nm@...com>
> >
> > 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").

Stephen: Can you pick the patch or suggest improvements?


-- 
Regards,
Nishanth Menon
Key (0xDDB5849D1736249D) / Fingerprint: F8A2 8693 54EB 8232 17A3  1A34 DDB5 849D 1736 249D
https://ti.com/opensource