linux-kernel - Re: [PATCH] clk: qcom: gcc-sm8450: set OPS_PARENT_ENABLE on gcc_sdcc2_apps_clk

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Date: Tue, 30 Apr 2024 12:46:52 +0200
From: Konrad Dybcio <konrad.dybcio@...aro.org>
To: Stephen Boyd <sboyd@...nel.org>, Bjorn Andersson <andersson@...nel.org>,
 Michael Turquette <mturquette@...libre.com>, Vinod Koul <vkoul@...nel.org>
Cc: Marijn Suijten <marijn.suijten@...ainline.org>,
 linux-arm-msm@...r.kernel.org, linux-clk@...r.kernel.org,
 linux-kernel@...r.kernel.org, Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Subject: Re: [PATCH] clk: qcom: gcc-sm8450: set OPS_PARENT_ENABLE on
 gcc_sdcc2_apps_clk_src

On 30.04.2024 2:21 AM, Stephen Boyd wrote:
> Quoting Konrad Dybcio (2024-04-27 05:01:07)
>> Similar to how it works on other SoCs, the top frequency of the SDHCI2
>> core clock is generated by a separate PLL (peculiar design choice) that
>> is not guaranteed to be enabled (why does the clock framework not handle
>> this by default?).
>>
>> Add the CLK_OPS_PARENT_ENABLE flag to make sure we're not muxing the
>> RCG input to a dormant source.
> 
> The RCG2 hardware hasn't required the parent to be enabled for clk
> operations besides for the glitch-free source switch. What scenario is
> happening here that's requiring this flag? Is the RCG forcibly enabled
> perhaps because the bootloader has left the root enable bit set
> (CMD_ROOT_EN)? Or are we changing the parent while the clk framework
> thinks the clk is off when it is actually on?
> 
> TL;DR: This is papering over a bigger bug.

Definitely.


Take a look at:

static const struct freq_tbl ftbl_gcc_sdcc2_apps_clk_src[] = {
	F(400000, P_BI_TCXO, 12, 1, 4),
	F(25000000, P_GCC_GPLL0_OUT_EVEN, 12, 0, 0),
	F(50000000, P_GCC_GPLL0_OUT_EVEN, 6, 0, 0),
	F(100000000, P_GCC_GPLL0_OUT_EVEN, 3, 0, 0),
	F(202000000, P_GCC_GPLL9_OUT_MAIN, 4, 0, 0),
	{ }
};

XO and GPLL0 are more or less always on, but GPLL9 is described to only
be used for this specific clock for this specific frequency (perhaps it
feeds something else on the soc but that's besides the point).

Then, the parent input is changed during set_rate, but GPLL9 seems to
never be enabled:


@@ -3272,6 +3274,8 @@ static int gcc_sm8450_probe(struct platform_device *pdev)
        if (IS_ERR(regmap))
                return PTR_ERR(regmap);
 
+       pr_err("GPLL9 is %s at boot\n", trion_pll_is_enabled(&gcc_gpll9, regmap) ? "enabled" : "disabled");
+
        ret = qcom_cc_register_rcg_dfs(regmap, gcc_dfs_clocks,
                                       ARRAY_SIZE(gcc_dfs_clocks));
        if (ret)


(+ cruft to make this callable) results in a:

[    1.637318] GPLL9 is disabled at boot


Konrad