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From:   Martin Blumenstingl <martin.blumenstingl@googlemail.com>
To:     narmstrong@baylibre.com, khilman@baylibre.com,
        linux-amlogic@lists.infradead.org
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-pm@vger.kernel.org,
        Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
        Stefan Agner <stefan@agner.ch>
Subject: [PATCH RFC] soc: amlogic: meson-ee-pwrc: Drop the .shutdown callback from the driver
Date:   Thu, 29 Apr 2021 22:37:23 +0200
Message-Id: <20210429203723.1177082-1-martin.blumenstingl@googlemail.com>
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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>
---
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,
-- 
2.31.1