From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 58E7727A448; Thu, 17 Jul 2025 14:14:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752761674; cv=none; b=IU7Cxvqu4VtexmKE743roTi6HVUcaHbf3SlEP5hdUF1X7IHagdQOz1BUiakIP/9MTZ+r1lziGh98QtSdzB9JFqcQ5iX9a1DJsZyQES/hke29rf+kGaUwQgJlkEf8bKO3A1nLiEmW+nwVioEV6xztq2iDPv8a4oeM/HwgGCI3AxQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752761674; c=relaxed/simple; bh=h2ovWkYi0w+2CUPEop0nRx81HZrnRolcW8rIZF7cDAc=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=IcNR0CkfFs4h+U+ba0oq7ahrT6I344rI9DkFClAbEmYa+PcF8xQge6FGzdt5+jnR7zbgpz1wsI2yg1a7bPxG8Ph8vJPdFnX4pJDqAY7q2UnLohxwqofBgyvvRYYPGJMYXXJdUvd+sVBHNA+x7YtACYHDXiwP6jdAxC4UkZMBGf8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=zzyPBc/m; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=KN43jpMp; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="zzyPBc/m"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="KN43jpMp" From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1752761670; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=f6HPPKpd3naPxlyUGoW7Ppyv/8Hu5ztE7m1CGPnk5wg=; b=zzyPBc/miPJU/u7WKM24c2CZTcIjKrDs0cVK5zkudVgEUOyqQJs43mLHdaE8Hs0t5bYuRA +0IqbnsoVET0JduaEVBts2WloZDphRnnu0POXKn+H4j9qrxN/gA2Vd764JXtLpG/tylNWp LcqvSefr4ApByOTGEcI+YEIcDKUAyjqE8PpJZcn4gA/rsCX3oubzy6DH7AiFz//tIzpyYy HqcpCCbVcM1H75HjUrFF2LteLKlbJwrjZ7v47lxtk2tcBUYMccAWS+xlkpuyS4ylSWQqU9 lBq7+mFlSgCqaJ834E3N63FsHjiHhDjKv8rU1T1kANypxAUtp4hBkXcHueMAKQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1752761670; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=f6HPPKpd3naPxlyUGoW7Ppyv/8Hu5ztE7m1CGPnk5wg=; b=KN43jpMpHBUFYieRN4GY8Lzu/uNUCcCuhlLUjFaoPW4EoF8sfgvOU33kr2IFTswtfPpXgr T86rfnsZLCapEaCA== To: yunhui cui , Douglas Anderson Cc: arnd@arndb.de, andriy.shevchenko@linux.intel.com, benjamin.larsson@genexis.eu, gregkh@linuxfoundation.org, heikki.krogerus@linux.intel.com, ilpo.jarvinen@linux.intel.com, jirislaby@kernel.org, jkeeping@inmusicbrands.com, linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, markus.mayer@linaro.org, matt.porter@linaro.org, namcao@linutronix.de, paulmck@kernel.org, pmladek@suse.com, schnelle@linux.ibm.com, sunilvl@ventanamicro.com, tim.kryger@linaro.org, stable@vger.kernel.org Subject: Re: [External] Re: [PATCH v9 2/4] serial: 8250_dw: fix PSLVERR on RX_TIMEOUT In-Reply-To: References: <20250610092135.28738-1-cuiyunhui@bytedance.com> <20250610092135.28738-3-cuiyunhui@bytedance.com> <84cyauq2nc.fsf@jogness.linutronix.de> Date: Thu, 17 Jul 2025 16:20:29 +0206 Message-ID: <84ikjqaoqi.fsf@jogness.linutronix.de> Precedence: bulk X-Mailing-List: linux-serial@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Added Douglas Anderson, author of commit 424d79183af0 ("serial: 8250_dw: Avoid "too much work" from bogus rx timeout interrupt"). On 2025-07-11, yunhui cui wrote: >> On 2025-06-23, yunhui cui wrote: >> >> The DW UART may trigger the RX_TIMEOUT interrupt without data >> >> present and remain stuck in this state indefinitely. The >> >> dw8250_handle_irq() function detects this condition by checking >> >> if the UART_LSR_DR bit is not set when RX_TIMEOUT occurs. When >> >> detected, it performs a "dummy read" to recover the DW UART from >> >> this state. >> >> >> >> When the PSLVERR_RESP_EN parameter is set to 1, reading the UART_RX >> >> while the FIFO is enabled and UART_LSR_DR is not set will generate a >> >> PSLVERR error, which may lead to a system panic. There are two methods >> >> to prevent PSLVERR: one is to check if UART_LSR_DR is set before reading >> >> UART_RX when the FIFO is enabled, and the other is to read UART_RX when >> >> the FIFO is disabled. >> >> >> >> Given these two scenarios, the FIFO must be disabled before the >> >> "dummy read" operation and re-enabled afterward to maintain normal >> >> UART functionality. >> >> >> >> Fixes: 424d79183af0 ("serial: 8250_dw: Avoid "too much work" from bogus rx timeout interrupt") >> >> Signed-off-by: Yunhui Cui >> >> Cc: stable@vger.kernel.org >> >> --- >> >> drivers/tty/serial/8250/8250_dw.c | 10 +++++++++- >> >> 1 file changed, 9 insertions(+), 1 deletion(-) >> >> >> >> diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c >> >> index 1902f29444a1c..082b7fcf251db 100644 >> >> --- a/drivers/tty/serial/8250/8250_dw.c >> >> +++ b/drivers/tty/serial/8250/8250_dw.c >> >> @@ -297,9 +297,17 @@ static int dw8250_handle_irq(struct uart_port *p) >> >> uart_port_lock_irqsave(p, &flags); >> >> status = serial_lsr_in(up); >> >> >> >> - if (!(status & (UART_LSR_DR | UART_LSR_BI))) >> >> + if (!(status & (UART_LSR_DR | UART_LSR_BI))) { >> >> + /* To avoid PSLVERR, disable the FIFO first. */ >> >> + if (up->fcr & UART_FCR_ENABLE_FIFO) >> >> + serial_out(up, UART_FCR, 0); >> >> + >> >> serial_port_in(p, UART_RX); >> >> >> >> + if (up->fcr & UART_FCR_ENABLE_FIFO) >> >> + serial_out(up, UART_FCR, up->fcr); >> >> + } >> >> + >> >> uart_port_unlock_irqrestore(p, flags); >> >> } >> >> I do not know enough about the hardware. Is a dummy read really the only >> way to exit the RX_TIMEOUT state? >> >> What if there are bytes in the TX-FIFO. Are they in danger of being >> cleared? >> >> From [0] I see: >> >> "Writing a "0" to bit 0 will disable the FIFOs, in essence turning the >> UART into 8250 compatibility mode. In effect this also renders the rest >> of the settings in this register to become useless. If you write a "0" >> here it will also stop the FIFOs from sending or receiving data, so any >> data that is sent through the serial data port may be scrambled after >> this setting has been changed. It would be recommended to disable FIFOs >> only if you are trying to reset the serial communication protocol and >> clearing any working buffers you may have in your application >> software. Some documentation suggests that setting this bit to "0" also >> clears the FIFO buffers, but I would recommend explicit buffer clearing >> instead using bits 1 and 2." >> >> Have you performed tests where you fill the TX-FIFO and then >> disable/enable the FIFO to see if the TX-bytes survive? > > Sorry, I haven't conducted relevant tests. The reason I made this > modification is that it clearly contradicts the logic of avoiding > PSLVERR. Disabling the FIFO can at least prevent the Panic() caused by > PSVERR. I am just wondering if there is some other way to avoid this. But since we are talking about a hardware quirk and it is only related to suspend/resume, maybe it is acceptable to risk data corruption in this case. (?) I am hoping Douglas can chime in. John Ogness >> [0] https://en.wikibooks.org/wiki/Serial_Programming/8250_UART_Programming