diff mbox series

[1/2] clk: stm32: initialize syscon after clocks are registered

Message ID 20231002180854.1603452-2-ben.wolsieffer@hefring.com
State New
Headers
Series ARM: stm32: add clock for pwrcfg syscon |

Commit Message

Ben Wolsieffer Oct. 2, 2023, 6:08 p.m. UTC 
  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@hefring.com>
---
 drivers/clk/clk-stm32f4.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Comments

Stephen Boyd Dec. 17, 2023, 11:05 p.m. UTC | #1 
Quoting Ben Wolsieffer (2023-10-02 11:08:53)
> 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.

Seems like we should mark those clks as CLK_IGNORE_UNUSED and add a
comment to that fact.

> 
> 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.

There's no comment in the code that says the regmap must be set there
instead of earlier. What's to stop someone from tripping over this
problem later? At the least, please add a comment.
Ben Wolsieffer Jan. 12, 2024, 10 p.m. UTC | #2 
On Sun, Dec 17, 2023 at 03:05:01PM -0800, Stephen Boyd wrote:
> Quoting Ben Wolsieffer (2023-10-02 11:08:53)
> > 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.
> 
> Seems like we should mark those clks as CLK_IGNORE_UNUSED and add a
> comment to that fact.

That seems like a worse solution than specifying the clock dependency in
the device tree.

> 
> > 
> > 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.
> 
> There's no comment in the code that says the regmap must be set there
> instead of earlier. What's to stop someone from tripping over this
> problem later? At the least, please add a comment.

Yeah, I'll fix that. Do you have any thoughts on the race condition I
described? Should I add some kind of locking to block
enable/disable_power_domain_write_protection() until stm32f4_rcc_init()
attempts to initialize the syscon?

Thank you, Ben
diff mbox series

Patch

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);