RE: [PATCH] clk: imx: add CLK_GET_RATE_NOCACHE flag for i.MX8M composite clock

Hi, Stephen

Best Regards!
Anson Huang

-----Original Message-----
From: Stephen Boyd [mailto:sboyd@kernel.org]
Sent: 2018年12月19日 3:32
To: Anson Huang <redacted>; Lucas Stach
[off-list ref]
Cc: Fabio Estevam <festevam@gmail.com>; s.hauer@pengutronix.de;
linux-kernel@vger.kernel.org; linux-clk@vger.kernel.org; dl-linux-imx
[off-list ref]; kernel@pengutronix.de; Fabio Estevam
[off-list ref]; shawnguo@kernel.org;
mturquette@baylibre.com; linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] clk: imx: add CLK_GET_RATE_NOCACHE flag for i.MX8M
composite clock

Quoting Lucas Stach (2018-12-18 06:02:28)

quoted

Am Dienstag, den 18.12.2018, 13:53 +0000 schrieb Anson Huang:
[...]

quoted

quoted

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Regarding about the over head, yes, the change in common
composite clock register has too many over head for other
clocks, what if I ONLY have dram core clock to pass the
CLK_GET_RATE_NOCACHE flag to register the composite clock?

IMHO marking clocks under TF-A control explicitly as nocache would
be much more acceptable than doing it for every composite clock.
This seems okay for a short term solution.

Still I think that whatever is causing the bus frequency scale to
change should have a way to explicitly invalidate the clock cache
for the affected clocks eventually.

It is because the DDR PLL/clocks can only be changed with strict DDR
freq change flow, and it is done in TF-A, Linux kernel can NOT touch
it in runtime, so we have to mark the child clock of DDR PLL to be
uncached, in V2 patch, I will just add the flag for the DDR PLL
child clocks to be a shorten solution, should be only very few ones,
hope it is acceptable, thanks.

I fully understand why you are doing the frequency change in TF-A and
I agree with the reasoning to do so. I also think that using uncached
for the few clocks under TF-A control is fine for now.

But if/when the bus frequency scaling is actually implemented for
upstream I think the flow should look something like that:

1. Bus freq scaling driver determines that a change is necessary 2.
Scaling driver calls into TF-A to do the change 3. TF-A reconfigures
clock rates 4. Scaling driver calls into clock driver to signal that a
clock change might have happened 5. Clock driver invalidates and
recalculates cached values for the affected clocks

Does any clk consuming driver of the downstream clks that are branched off of
the bus clk managed by firmware care about the frequency? Or do they just
want the clk to be on. If they don't care then it's possible to break the parent
dependency and just not care to tell them what the bus frequency is anymore.

I don't know how you would implement #4 above, besides by having the bus
freq scaling driver use clk_set_rate() to tell the bus clk that it wants a new rate
and then having that clk implementation do #2. That way the rate propagation
works without having to notify clk code somehow.

In our case, the original clock relationship is as below, when DDR freq needs to be scaled, below things are proceeded:

Clock tree:
IMX8MQ_DRAM_PLL2
	IMX8MQ_DRAM_PLL2_DIV
		IMX8MQ_DRAM_PLL2_OUT
			IMX8MQ_DRAM_PLL_OUT
				IMX8MQ_CLK_DRAM_CORE

1. Linux kernel do SMC call into TF-A;
2. in TF-A, we have to scale the IMX8MQ_DRAM_PLL2 to dedicated frequency along with DDR freq scale flow;
3. After TF-A done the DDR freq scale and return back to Linux, all the downstream of IMX8MQ_DRAM_PLL2 clock will
  have mismatch clock rate between clock tree and HW settings since they are cached;
4. Maybe we can call clk_set_rate in Linux for IMX8MQ_DRAM_PLL2 again (although the HW settings are already expected)
  to update the downstream clocks, looks like can fix it, but will the call be skipped by clock framework when clock driver
  re-calculate the PLL rate based on HW setting, as HW setting already equal the rate wants to be set, and clk_propagate_rate_change
  will be skipped too or it will automatically update all child clocks' rate?

Is it correct? if all child clocks' rate can be updated by clk_propagate_rate_change(), then we no need to add any clock flag,
just make sure after TF-A finish the DDR freq scale, make sure calling the clk_set_rate() for the IMX8MQ_DRAM_PLL2, then everything should be correct?

Anson.

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