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Date:   Thu, 29 Apr 2021 23:08:56 +0200
From:   Stefan Agner <stefan@agner.ch>
To:     Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc:     narmstrong@baylibre.com, khilman@baylibre.com,
        linux-amlogic@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH RFC] soc: amlogic: meson-ee-pwrc: Drop the .shutdown
 callback from the driver
In-Reply-To: <20210429203723.1177082-1-martin.blumenstingl@googlemail.com>
References: <20210429203723.1177082-1-martin.blumenstingl@googlemail.com>
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On 2021-04-29 22:37, Martin Blumenstingl wrote:
> Stefan reports that rebooting his ODROID-N2+ (using a G12B SoC) results
> in the board hanging. His kernel config uses:
>   CONFIG_MESON_EE_PM_DOMAINS=y
>   CONFIG_DRM_MESON=m
> 
> He reports that his kernel config results in the DRM driver's .shutdown
> callback to be executed after the power domain driver's .shutdown
> callback. That's problematic because meson_ee_pwrc_shutdown disables the
> clock which are used by the VPU IP. This causes the board to hang.
> 
> Further he reports that changing from CONFIG_DRM_MESON=m to
> CONFIG_DRM_MESON=y reverses the order in which the DRM and power domain
> driver's shutdown functions are executed, making the reboot successful.
> 
> The reason why we use meson_ee_pwrc_shutdown is because of the VPU power
> domain (which is causing the problem described above). It can be left
> enabled by u-boot. According to the original TOFIX comment in
> meson_ee_pwrc_init_domain we need to be careful because disabling the
> power domain could "cause system errors". As a workaround the clocks
> are manually enabled in meson_ee_pwrc_init_domain and the power domain
> is marked as GENPD_FLAG_ALWAYS_ON (so it can never be turned off).
> 
> Experimenting has shown that the power domain itself can be disabled as
> long as we keep the clocks enabled if u-boot enabled the power domain
> but we don't have any driver enabled for the VPU (CONFIG_DRM_MESON=n).
> 
> Keeping the clocks enabled is the responsibility of the CCF drivers, not
> the power domain driver. Even better: this is already covered as all
> gates in the VPU and VAPB tree on GX an G12 SoCs have the
> CLK_IGNORE_UNUSED flag set, meaning: if the bootloader has previously
> enabled the clock we're not touching it until a driver explicitly asks
> to enable (and then disable) it. In case of CONFIG_DRM_MESON=n we're
> never calling meson_ee_pwrc_on, meaning that we always keep the state of
> the clocks as set by u-boot.
> 
> The original TOFIX comment also mentioned that we need to make sure not
> to mess up the clock's prepare/enable ref-counters. This is the only
> requirement that's left for the meson-ee-pwrc driver that needs to be
> managed for the VPU power domain.
> 
> Three steps can improve this situation:
> - Don't prepare and enable the clocks (just to fix the ref-counting) in
>   meson_ee_pwrc_init_domain if u-boot left that power domain enabled.
>   Instead, remember if the clocks are enabled in meson_ee_pwrc_{on,off}
>   and only disable them if we have previously turned them on ourselves.
> - Drop GENPD_FLAG_ALWAYS_ON as we can always manage the state of the VPU
>   power domain if both the power domain controller and DRM driver are
>   enabled (=m or =y). If the power domain driver is enabled but the DRM
>   driver is disabled we can still use meson_ee_pwrc_off because it's not
>   trying to disable the clocks anymore
> - Drop meson_ee_pwrc_shutdown as it's the responsibility of the genpd
>   framework to call meson_ee_pwrc_off when needed (either when a power
>   domain is being disabled - regardless of whether it's was used by a
>   driver before or not). Now there's also no more shutdown callback
>   ordering dependency between the power domain driver and other drivers
>   anymore.
> 
> Fixes: eef3c2ba0a42a6 ("soc: amlogic: Add support for Everything-Else
> power domains controller")
> Reported-by: Stefan Agner <stefan@agner.ch>
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>

FWIW, I successfully tested this change with CONFIG_DRM_MESON=m and
CONFIG_DRM_MESON=y each with a couple of reboots without any hang.

Tested-by: Stefan Agner <stefan@agner.ch>

Thanks Martin for help debugging and working on this patch!

--
Stefan

> ---
> Neil, I would like you to specifically review (and re-test) the original
> TOFIX part. I *believe* that I understand the original problem and hope
> that my approach also works in that scenario, but I am not 100% sure.
> 
> Kevin, as you're my go-to genpd expert I am also asking you to review
> this to point out any issues you see.
> 
> 
>  drivers/soc/amlogic/meson-ee-pwrc.c | 76 ++++++++++++-----------------
>  1 file changed, 31 insertions(+), 45 deletions(-)
> 
> diff --git a/drivers/soc/amlogic/meson-ee-pwrc.c
> b/drivers/soc/amlogic/meson-ee-pwrc.c
> index 50bf5d2b828b..534c33ba31ce 100644
> --- a/drivers/soc/amlogic/meson-ee-pwrc.c
> +++ b/drivers/soc/amlogic/meson-ee-pwrc.c
> @@ -353,10 +353,26 @@ static int meson_ee_pwrc_off(struct
> generic_pm_domain *domain)
>  
>  	if (pwrc_domain->num_clks) {
>  		msleep(20);
> -		clk_bulk_disable_unprepare(pwrc_domain->num_clks,
> -					   pwrc_domain->clks);
> +
> +		/*
> +		 * We are only allowed to turn off the clocks here if we
> +		 * have previously enabled them ourselves. In other words:
> +		 * for an "unused" power domain (which is not used by any
> +		 * power domain consumer) we have not enabled the clocks
> +		 * previously so we keep them in the state they are.
> +		 * This is relevant for the VPU power domain which may
> +		 * have been enabled by u-boot. If the display driver in
> +		 * Linux is disabled then we need to keep the clocks in
> +		 * the state as u-boot set them, otherwise the board will
> +		 * hang.
> +		 */
> +		if (pwrc_domain->enabled)
> +			clk_bulk_disable_unprepare(pwrc_domain->num_clks,
> +						   pwrc_domain->clks);
>  	}
>  
> +	pwrc_domain->enabled = false;
> +
>  	return 0;
>  }
>  
> @@ -392,8 +408,14 @@ static int meson_ee_pwrc_on(struct
> generic_pm_domain *domain)
>  	if (ret)
>  		return ret;
>  
> -	return clk_bulk_prepare_enable(pwrc_domain->num_clks,
> -				       pwrc_domain->clks);
> +	ret = clk_bulk_prepare_enable(pwrc_domain->num_clks,
> +				      pwrc_domain->clks);
> +	if (ret)
> +		return ret;
> +
> +	pwrc_domain->enabled = true;
> +
> +	return 0;
>  }
>  
>  static int meson_ee_pwrc_init_domain(struct platform_device *pdev,
> @@ -434,33 +456,11 @@ static int meson_ee_pwrc_init_domain(struct
> platform_device *pdev,
>  	dom->base.power_on = meson_ee_pwrc_on;
>  	dom->base.power_off = meson_ee_pwrc_off;
>  
> -	/*
> -         * TOFIX: This is a special case for the VPU power domain, which can
> -	 * be enabled previously by the bootloader. In this case the VPU
> -         * pipeline may be functional but no driver maybe never attach
> -         * to this power domain, and if the domain is disabled it could
> -         * cause system errors. This is why the pm_domain_always_on_gov
> -         * is used here.
> -         * For the same reason, the clocks should be enabled in case
> -         * we need to power the domain off, otherwise the internal clocks
> -         * prepare/enable counters won't be in sync.
> -         */
> -	if (dom->num_clks && dom->desc.get_power && !dom->desc.get_power(dom)) {
> -		ret = clk_bulk_prepare_enable(dom->num_clks, dom->clks);
> -		if (ret)
> -			return ret;
> -
> -		dom->base.flags = GENPD_FLAG_ALWAYS_ON;
> -		ret = pm_genpd_init(&dom->base, NULL, false);
> -		if (ret)
> -			return ret;
> -	} else {
> -		ret = pm_genpd_init(&dom->base, NULL,
> -				    (dom->desc.get_power ?
> -				     dom->desc.get_power(dom) : true));
> -		if (ret)
> -			return ret;
> -	}
> +	ret = pm_genpd_init(&dom->base, NULL,
> +			    (dom->desc.get_power ?
> +			     dom->desc.get_power(dom) : true));
> +	if (ret)
> +		return ret;
>  
>  	return 0;
>  }
> @@ -528,19 +528,6 @@ static int meson_ee_pwrc_probe(struct
> platform_device *pdev)
>  	return of_genpd_add_provider_onecell(pdev->dev.of_node, &pwrc->xlate);
>  }
>  
> -static void meson_ee_pwrc_shutdown(struct platform_device *pdev)
> -{
> -	struct meson_ee_pwrc *pwrc = platform_get_drvdata(pdev);
> -	int i;
> -
> -	for (i = 0 ; i < pwrc->xlate.num_domains ; ++i) {
> -		struct meson_ee_pwrc_domain *dom = &pwrc->domains[i];
> -
> -		if (dom->desc.get_power && !dom->desc.get_power(dom))
> -			meson_ee_pwrc_off(&dom->base);
> -	}
> -}
> -
>  static struct meson_ee_pwrc_domain_data meson_ee_g12a_pwrc_data = {
>  	.count = ARRAY_SIZE(g12a_pwrc_domains),
>  	.domains = g12a_pwrc_domains,
> @@ -606,7 +593,6 @@ MODULE_DEVICE_TABLE(of, meson_ee_pwrc_match_table);
>  
>  static struct platform_driver meson_ee_pwrc_driver = {
>  	.probe = meson_ee_pwrc_probe,
> -	.shutdown = meson_ee_pwrc_shutdown,
>  	.driver = {
>  		.name		= "meson_ee_pwrc",
>  		.of_match_table	= meson_ee_pwrc_match_table,