From nobody Sat Apr 4 10:01:22 2026 Received: from smtpfb1-g21.free.fr (smtpfb1-g21.free.fr [212.27.42.9]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 8D431396D03; Tue, 24 Feb 2026 12:22:56 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=212.27.42.9 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771935781; cv=none; b=XhFoJxk6ERf4m9wezQ2/IqWEL9kjlc8J/R1tSvHtIIenr6ikwjqG/YawH3utSBp3slCLpRk4SMO+iArm7vptYrY9dN+BRgjhGXq08Z0Adjc4L3SHQr7xbA1wyCsukU41BizI93XvNxJjCGDly2U2phTUAHcIOstwSVDURplsAp0= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771935781; c=relaxed/simple; bh=TXKJIKiEPtHfcGEJx/r052JjVn3m4tuf41dI1DM7msk=; h=From:To:Cc:Subject:Date:Message-Id:MIME-Version; b=CiC/HaFcpOECUwxSUQGu82gGIQS3rE2g9VQVFuDkJoq4b0A2ZkKvWBtgGutUkuEko1+xVt4TSsJTZLeHiBCGUElQQVvmu5mFZvORW6+ZzbHhfViq5KKR9NdrepBxacwfjYGCYYTAYWY/XYm1Y2lVrwW+VwmTTDB1MHxUPg3kc5U= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=lht.dlh.de; spf=pass smtp.mailfrom=free.fr; arc=none smtp.client-ip=212.27.42.9 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=lht.dlh.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=free.fr Received: from smtp2-g21.free.fr (smtp2-g21.free.fr [212.27.42.2]) by smtpfb1-g21.free.fr (Postfix) with ESMTP id C56BCDF9503; Tue, 24 Feb 2026 13:17:18 +0100 (CET) Received: from albans-vm.. (unknown [213.61.141.186]) (Authenticated sender: albeu@free.fr) by smtp2-g21.free.fr (Postfix) with ESMTPSA id 9DB3420039E; Tue, 24 Feb 2026 13:16:58 +0100 (CET) From: Alban Bedel To: linux-serial@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Andy Shevchenko , =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= , Jiri Slaby , Greg Kroah-Hartman , Peng Zhang , stable@vger.kernel.org, Muchun Song , Alban Bedel , Maximilian Lueer Subject: [PATCH v4] serial: 8250: always disable IRQ during THRE test Date: Tue, 24 Feb 2026 13:16:39 +0100 Message-Id: <20260224121639.579404-1-alban.bedel@lht.dlh.de> X-Mailer: git-send-email 2.39.5 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: Peng Zhang commit 039d4926379b ("serial: 8250: Toggle IER bits on only after irq has been set up") moved IRQ setup before the THRE test, in combination with commit 205d300aea75 ("serial: 8250: change lock order in serial8250_do_startup()") the interrupt handler can run during the test and race with its IIR reads. This can produce wrong THRE test results and cause spurious registration of the serial8250_backup_timeout timer. Unconditionally disable the IRQ for the short duration of the test and re-enable it afterwards to avoid the race. Cc: stable@vger.kernel.org Fixes: 039d4926379b ("serial: 8250: Toggle IER bits on only after irq has b= een set up") Depends-on: 205d300aea75 ("serial: 8250: change lock order in serial8250_do= _startup()") Signed-off-by: Peng Zhang Reviewed-by: Muchun Song Signed-off-by: Alban Bedel Tested-by: Maximilian Lueer --- Changelog: v2: Replaced disable_irq_nosync() with disable_irq() to prevent interrupts that are currently being handled v3: Added changelog v4: Updated commit message to mention the addtional relation to commit 205d300aea75 --- Commit 039d4926379b ("serial: 8250: Toggle IER bits on only after irq has been set up"), in combination with 205d300aea75 ("serial: 8250: change lock order in serial8250_do_startup()"), introduced a bug which showed in some of our devices after updating them to kernel 5.15.169. These devices have an I2C touch screen which is behind an UART to I2C bridge. This bug lead to an ever growing latency on the bus, delaying the touch events by up to several seconds. Looking for other solutions than reverting commit 039d4926379b I found Peng Zhang's patch, backported it to 5.15 and could confirm that it solve the high delay issue. As this a quiet sever regression I'm taking the liberty to re-submit Peng Zhang's patch in the hope it will be considered for inclusion. I added the changelog requested by greg k-h's patch email bot after the v2 submission, as well as the mention of commit 205d300aea75 requested by Jiri Slaby after v3. Also please note that commit 039d4926379b was backported as far back as 5.10, so quiet a few stable kernels are affected. This patch doesn't apply as-is on older kernels, I can provide a patch for 5.15 if desired. Alban Bedel --- drivers/tty/serial/8250/8250_port.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/= 8250_port.c index cc94af2d578a6..a743964c9d227 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2147,8 +2147,7 @@ static void serial8250_THRE_test(struct uart_port *po= rt) if (up->port.flags & UPF_NO_THRE_TEST) return; =20 - if (port->irqflags & IRQF_SHARED) - disable_irq_nosync(port->irq); + disable_irq(port->irq); =20 /* * Test for UARTs that do not reassert THRE when the transmitter is idle = and the interrupt @@ -2170,8 +2169,7 @@ static void serial8250_THRE_test(struct uart_port *po= rt) serial_port_out(port, UART_IER, 0); } =20 - if (port->irqflags & IRQF_SHARED) - enable_irq(port->irq); + enable_irq(port->irq); =20 /* * If the interrupt is not reasserted, or we otherwise don't trust the ii= r, setup a timer to --=20 2.39.5