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[2a02:3100:ae93:f301:cdd9:d5ea:80d:9131]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-496b499cc49sm20754015e9.9.2026.07.24.14.37.19 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Fri, 24 Jul 2026 14:37:20 -0700 (PDT) From: Karl Mehltretter To: Russell King , Greg Kroah-Hartman , Jiri Slaby Cc: Karl Mehltretter , linux-arm-kernel@lists.infradead.org, linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Sebastian Andrzej Siewior , John Ogness , Petr Mladek , Steven Rostedt , Clark Williams , Toshiyuki Sato Subject: [PATCH v2 2/2] serial: amba-pl011: keep console clock enabled for atomic writes Date: Fri, 24 Jul 2026 23:33:48 +0200 Message-Id: <20260724213348.77418-3-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20260724213348.77418-1-kmehltretter@gmail.com> References: <20260724213348.77418-1-kmehltretter@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit pl011_console_write_atomic() runs from nbcon atomic context, where sleeping is not allowed. It calls clk_enable(), which takes the common-clk enable_lock. Under PREEMPT_RT that is a sleeping lock: clk_enable_lock() first tries spin_trylock_irqsave(), but on contention falls back to spin_lock_irqsave(). Therefore, an atomic-context printk on an RT kernel with a clk-backed pl011 can trip: BUG: sleeping function called from invalid context at spinlock_rt.c:48 __might_resched from rt_spin_lock rt_spin_lock from clk_enable_lock clk_enable_lock from clk_enable clk_enable from pl011_console_write_atomic ... from vprintk_emit This was found and reproduced on PREEMPT_RT. Arm32 and arm64 DT SoCs are affected; arm64 SBSA/ACPI has no clk, so clk_enable(NULL) short-circuits before the lock. In addition, write_atomic() may be invoked from NMI context and is documented to avoid locking. Removing clk_enable() from the callback also avoids a potentially unsafe NMI acquisition of the common-clock enable_lock. An nbcon atomic-capable console must be printable from any context, so the clock cannot be gated between writes. Enable the clock while the console is available for output: use clk_prepare_enable() in pl011_console_setup(), release it via clk_disable_unprepare() in the console .exit() callback, and drop the per-write clk_enable()/clk_disable() pairs from write_atomic() and write_thread(). When printk suspends consoles, drop the reference after uart_suspend_port() stops console access and restore it before uart_resume_port() -- but only if suspend actually marked the port suspended (a wake-capable tty stays running and must keep its clock), and keep it when console_suspend_enabled is false so no_console_suspend works. The active power cost of keeping the clock enabled is platform-dependent: none where the UART clock is a fixed always-on oscillator, real where it is a gateable clock branch, which then cannot be gated (nor possibly can its parent clocks) while the console is available for output. When serial core actually suspends the port, the reference is released so the clock provider can gate the clock tree. Fixes: 2eb2608618ce ("serial: amba-pl011: Implement nbcon console") Suggested-by: John Ogness Link: https://lore.kernel.org/all/8733xeaxix.fsf@jogness.linutronix.de/ Assisted-by: Claude:claude-fable-5 Signed-off-by: Karl Mehltretter --- drivers/tty/serial/amba-pl011.c | 40 +++++++++++++++++++++++---------- 1 file changed, 28 insertions(+), 12 deletions(-) diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c index 1aa43994a3cd..611d1f6a0590 100644 --- a/drivers/tty/serial/amba-pl011.c +++ b/drivers/tty/serial/amba-pl011.c @@ -2523,7 +2523,7 @@ static int pl011_console_setup(struct console *co, char *options) /* Allow pins to be muxed in and configured */ pinctrl_pm_select_default_state(uap->port.dev); - ret = clk_prepare(uap->clk); + ret = clk_prepare_enable(uap->clk); if (ret) return ret; @@ -2556,7 +2556,7 @@ static int pl011_console_exit(struct console *co) { struct uart_amba_port *uap = amba_ports[co->index]; - clk_unprepare(uap->clk); + clk_disable_unprepare(uap->clk); return 0; } @@ -2630,8 +2630,6 @@ pl011_console_write_atomic(struct console *co, struct nbcon_write_context *wctxt if (!nbcon_enter_unsafe(wctxt)) return; - clk_enable(uap->clk); - if (!uap->vendor->always_enabled) { old_cr = pl011_read(uap, REG_CR); pl011_write((old_cr & ~UART011_CR_CTSEN) | (UART01x_CR_UARTEN | UART011_CR_TXE), @@ -2648,8 +2646,6 @@ pl011_console_write_atomic(struct console *co, struct nbcon_write_context *wctxt if (!uap->vendor->always_enabled) pl011_write(old_cr, uap, REG_CR); - clk_disable(uap->clk); - nbcon_exit_unsafe(wctxt); } @@ -2662,8 +2658,6 @@ pl011_console_write_thread(struct console *co, struct nbcon_write_context *wctxt if (!nbcon_enter_unsafe(wctxt)) return; - clk_enable(uap->clk); - if (!uap->vendor->always_enabled) { old_cr = pl011_read(uap, REG_CR); pl011_write((old_cr & ~UART011_CR_CTSEN) | (UART01x_CR_UARTEN | UART011_CR_TXE), @@ -2692,8 +2686,6 @@ pl011_console_write_thread(struct console *co, struct nbcon_write_context *wctxt if (!uap->vendor->always_enabled) pl011_write(old_cr, uap, REG_CR); - clk_disable(uap->clk); - nbcon_exit_unsafe(wctxt); } @@ -3080,21 +3072,45 @@ static void pl011_remove(struct amba_device *dev) static int pl011_suspend(struct device *dev) { struct uart_amba_port *uap = dev_get_drvdata(dev); + int ret; if (!uap) return -EINVAL; - return uart_suspend_port(&amba_reg, &uap->port); + ret = uart_suspend_port(&amba_reg, &uap->port); + if (ret) + return ret; + + if (console_suspend_enabled && uap->port.suspended && + uart_console_registered(&uap->port)) + clk_disable_unprepare(uap->clk); + + return 0; } static int pl011_resume(struct device *dev) { struct uart_amba_port *uap = dev_get_drvdata(dev); + bool resume_console; + int ret; if (!uap) return -EINVAL; - return uart_resume_port(&amba_reg, &uap->port); + resume_console = console_suspend_enabled && + uap->port.suspended && + uart_console_registered(&uap->port); + if (resume_console) { + ret = clk_prepare_enable(uap->clk); + if (ret) + return ret; + } + + ret = uart_resume_port(&amba_reg, &uap->port); + if (ret && resume_console) + clk_disable_unprepare(uap->clk); + + return ret; } #endif -- 2.39.5 (Apple Git-154)