linux-kernel - [PATCH 1/2] clk: stm32: initialize syscon after clocks are registered

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Date:   Mon,  2 Oct 2023 14:08:53 -0400
From:   Ben Wolsieffer <ben.wolsieffer@...ring.com>
To:     linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-clk@...r.kernel.org
Cc:     Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Ben Wolsieffer <ben.wolsieffer@...ring.com>
Subject: [PATCH 1/2] clk: stm32: initialize syscon after clocks are registered

The stm32-power-config syscon (PWR peripheral) is used in this driver
and the STM32 RTC driver to enable write access to backup domain
registers. The syscon's clock has a gate controlled by this clock
driver, but this clock is currently not registered in the device tree.
This only happens to work currently because all relevant clock setup and
RTC initialization happens before clk_disabled_unused(). After this
point, all syscon register writes are ignored.

If we simply add the syscon clock in the device tree, we end up with a
circular dependency because the clock has not been registered at the
point this driver requests the syscon.

This patch avoids this circular dependency by moving the syscon lookup
after the clocks are registered. This does appear to create a possible
race condition where someone could attempt to perform an operation on a
backup domain clock before the syscon has been initialized. This would
result in the operation having no effect because backup domain writes
could not be enabled. I'm not sure if this is a problem or if there is
a way to avoid it.

Signed-off-by: Ben Wolsieffer <ben.wolsieffer@...ring.com>
---
 drivers/clk/clk-stm32f4.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/clk/clk-stm32f4.c b/drivers/clk/clk-stm32f4.c
index 07c13ebe327d..a88e762d2b5e 100644
--- a/drivers/clk/clk-stm32f4.c
+++ b/drivers/clk/clk-stm32f4.c
@@ -1697,12 +1697,6 @@ static void __init stm32f4_rcc_init(struct device_node *np)
 		return;
 	}

-	pdrm = syscon_regmap_lookup_by_phandle(np, "st,syscfg");
-	if (IS_ERR(pdrm)) {
-		pdrm = NULL;
-		pr_warn("%s: Unable to get syscfg\n", __func__);
-	}
-
 	match = of_match_node(stm32f4_of_match, np);
 	if (WARN_ON(!match))
 		return;
@@ -1894,6 +1888,12 @@ static void __init stm32f4_rcc_init(struct device_node *np)

 	of_clk_add_hw_provider(np, stm32f4_rcc_lookup_clk, NULL);

+	pdrm = syscon_regmap_lookup_by_phandle(np, "st,syscfg");
+	if (IS_ERR(pdrm)) {
+		pdrm = NULL;
+		pr_warn("%s: Unable to get syscfg\n", __func__);
+	}
+
 	return;
 fail:
 	kfree(clks);
-- 
2.42.0