From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 B95CA3AD527; Mon, 23 Mar 2026 13:52:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774273979; cv=none; b=ms62xwalrmPStIq0Hm/2hKPtUxKjJBf5oCFxkaLsrEBuabbkICG+5sPCP37tUowt3SLPH5Rf2lALRwatO8tCMUtTQKIqLpZMg14u9/PBbaPasH+q2H4RVpuv+N+Xi4AeISUjgr0xnT0nxpL5GXWXTtXQkJdiwT7nr5HDVscIVqU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774273979; c=relaxed/simple; bh=L+4gCllCUeHmnWHvAvIIQJhNyESJVEZFpnPwZTGgJeM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=W+1U953kpY7EOPtLqzOgQqjlcKcMXkQeSjrfJxZ2tJNiWHz/fcV23Rt2vlNcLn70bXZDy95U11rd1n0RFJq9UwDcuSuJowYjYxQiaqiI0UsuOh+cFXtoLgXSLXKbYdNV4uvlX/GvXJG1bxEGm2mBWrY+xfOsLHuPPFdjCwI+nk8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=vPz9Puoe; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="vPz9Puoe" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3DA92C4CEF7; Mon, 23 Mar 2026 13:52:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1774273979; bh=L+4gCllCUeHmnWHvAvIIQJhNyESJVEZFpnPwZTGgJeM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=vPz9PuoePzg9sAtGJfjk1EAtZAAaGJ29RUEEfpnSrwP6oJaXSyeVVGp4DnHCG6gNi m9IHuzXluooedlkumflTh7kfdYMjvptVvLYRRv+IUZwjQtd3XtlMFXtUZPHazirkga ty6z6cxos0QqChYAj2VROzl3KiKro2iWZ17Hx3IE= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, stable , qianfan Zhao , Adriana Nicolae , "Bandal, Shankar" , "Murthy, Shanth" , Andy Shevchenko , =?UTF-8?q?Ilpo=20J=C3=A4rvinen?= Subject: [PATCH 6.19 068/220] serial: 8250_dw: Ensure BUSY is deasserted Date: Mon, 23 Mar 2026 14:44:05 +0100 Message-ID: <20260323134506.739180957@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260323134504.575022936@linuxfoundation.org> References: <20260323134504.575022936@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 6.19-stable review patch. If anyone has any objections, please let me know. ------------------ From: Ilpo Järvinen commit a7b9ce39fbe4ae2919fe4f7ac16c293cb6632d30 upstream. DW UART cannot write to LCR, DLL, and DLH while BUSY is asserted. Existance of BUSY depends on uart_16550_compatible, if UART HW is configured with it those registers can always be written. There currently is dw8250_force_idle() which attempts to achieve non-BUSY state by disabling FIFO, however, the solution is unreliable when Rx keeps getting more and more characters. Create a sequence of operations that ensures UART cannot keep BUSY asserted indefinitely. The new sequence relies on enabling loopback mode temporarily to prevent incoming Rx characters keeping UART BUSY. Ensure no Tx in ongoing while the UART is switches into the loopback mode (requires exporting serial8250_fifo_wait_for_lsr_thre() and adding DMA Tx pause/resume functions). According to tests performed by Adriana Nicolae , simply disabling FIFO or clearing FIFOs only once does not always ensure BUSY is deasserted but up to two tries may be needed. This could be related to ongoing Rx of a character (a guess, not known for sure). Therefore, retry FIFO clearing a few times (retry limit 4 is arbitrary number but using, e.g., p->fifosize seems overly large). Tests performed by others did not exhibit similar challenge but it does not seem harmful to leave the FIFO clearing loop in place for all DW UARTs with BUSY functionality. Use the new dw8250_idle_enter/exit() to do divisor writes and LCR writes. In case of plain LCR writes, opportunistically try to update LCR first and only invoke dw8250_idle_enter() if the write did not succeed (it has been observed that in practice most LCR writes do succeed without complications). This issue was first reported by qianfan Zhao who put lots of debugging effort into understanding the solution space. Fixes: c49436b657d0 ("serial: 8250_dw: Improve unwritable LCR workaround") Fixes: 7d4008ebb1c9 ("tty: add a DesignWare 8250 driver") Cc: stable Reported-by: qianfan Zhao Link: https://lore.kernel.org/linux-serial/289bb78a-7509-1c5c-2923-a04ed3b6487d@163.com/ Reported-by: Adriana Nicolae Link: https://lore.kernel.org/linux-serial/20250819182322.3451959-1-adriana@arista.com/ Reported-by: Bandal, Shankar Tested-by: Bandal, Shankar Tested-by: Murthy, Shanth Reviewed-by: Andy Shevchenko Signed-off-by: Ilpo Järvinen Link: https://patch.msgid.link/20260203171049.4353-8-ilpo.jarvinen@linux.intel.com Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250.h | 25 +++++ drivers/tty/serial/8250/8250_dw.c | 163 ++++++++++++++++++++++++++---------- drivers/tty/serial/8250/8250_port.c | 28 +++--- 3 files changed, 161 insertions(+), 55 deletions(-) --- a/drivers/tty/serial/8250/8250.h +++ b/drivers/tty/serial/8250/8250.h @@ -175,7 +175,9 @@ static unsigned int __maybe_unused seria return value; } +void serial8250_clear_fifos(struct uart_8250_port *p); void serial8250_clear_and_reinit_fifos(struct uart_8250_port *p); +void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, unsigned int count); void serial8250_rpm_get(struct uart_8250_port *p); void serial8250_rpm_put(struct uart_8250_port *p); @@ -400,6 +402,26 @@ static inline bool serial8250_tx_dma_run return dma && dma->tx_running; } + +static inline void serial8250_tx_dma_pause(struct uart_8250_port *p) +{ + struct uart_8250_dma *dma = p->dma; + + if (!dma->tx_running) + return; + + dmaengine_pause(dma->txchan); +} + +static inline void serial8250_tx_dma_resume(struct uart_8250_port *p) +{ + struct uart_8250_dma *dma = p->dma; + + if (!dma->tx_running) + return; + + dmaengine_resume(dma->txchan); +} #else static inline int serial8250_tx_dma(struct uart_8250_port *p) { @@ -421,6 +443,9 @@ static inline bool serial8250_tx_dma_run { return false; } + +static inline void serial8250_tx_dma_pause(struct uart_8250_port *p) { } +static inline void serial8250_tx_dma_resume(struct uart_8250_port *p) { } #endif static inline int ns16550a_goto_highspeed(struct uart_8250_port *up) --- a/drivers/tty/serial/8250/8250_dw.c +++ b/drivers/tty/serial/8250/8250_dw.c @@ -16,6 +16,7 @@ #include #include #include +#include #include #include #include @@ -47,6 +48,8 @@ #define DW_UART_MCR_SIRE BIT(6) +#define DW_UART_USR_BUSY BIT(0) + /* Renesas specific register fields */ #define RZN1_UART_xDMACR_DMA_EN BIT(0) #define RZN1_UART_xDMACR_1_WORD_BURST (0 << 1) @@ -89,6 +92,7 @@ struct dw8250_data { unsigned int skip_autocfg:1; unsigned int uart_16550_compatible:1; + unsigned int in_idle:1; u8 no_int_count; }; @@ -121,78 +125,151 @@ static inline u32 dw8250_modify_msr(stru return value; } +static void dw8250_idle_exit(struct uart_port *p) +{ + struct dw8250_data *d = to_dw8250_data(p->private_data); + struct uart_8250_port *up = up_to_u8250p(p); + + if (d->uart_16550_compatible) + return; + + if (up->capabilities & UART_CAP_FIFO) + serial_port_out(p, UART_FCR, up->fcr); + serial_port_out(p, UART_MCR, up->mcr); + serial_port_out(p, UART_IER, up->ier); + + /* DMA Rx is restarted by IRQ handler as needed. */ + if (up->dma) + serial8250_tx_dma_resume(up); + + d->in_idle = 0; +} + /* - * This function is being called as part of the uart_port::serial_out() - * routine. Hence, it must not call serial_port_out() or serial_out() - * against the modified registers here, i.e. LCR. + * Ensure BUSY is not asserted. If DW UART is configured with + * !uart_16550_compatible, the writes to LCR, DLL, and DLH fail while + * BUSY is asserted. + * + * Context: port's lock must be held */ -static void dw8250_force_idle(struct uart_port *p) +static int dw8250_idle_enter(struct uart_port *p) { + struct dw8250_data *d = to_dw8250_data(p->private_data); + unsigned int usr_reg = d->pdata ? d->pdata->usr_reg : DW_UART_USR; struct uart_8250_port *up = up_to_u8250p(p); - unsigned int lsr; + int retries; + u32 lsr; - /* - * The following call currently performs serial_out() - * against the FCR register. Because it differs to LCR - * there will be no infinite loop, but if it ever gets - * modified, we might need a new custom version of it - * that avoids infinite recursion. - */ - serial8250_clear_and_reinit_fifos(up); + lockdep_assert_held_once(&p->lock); + + if (d->uart_16550_compatible) + return 0; + + d->in_idle = 1; + + /* Prevent triggering interrupt from RBR filling */ + serial_port_out(p, UART_IER, 0); + + if (up->dma) { + serial8250_rx_dma_flush(up); + if (serial8250_tx_dma_running(up)) + serial8250_tx_dma_pause(up); + } /* - * With PSLVERR_RESP_EN parameter set to 1, the device generates an - * error response when an attempt to read an empty RBR with FIFO - * enabled. + * Wait until Tx becomes empty + one extra frame time to ensure all bits + * have been sent on the wire. + * + * FIXME: frame_time delay is too long with very low baudrates. */ - if (up->fcr & UART_FCR_ENABLE_FIFO) { - lsr = serial_port_in(p, UART_LSR); - if (!(lsr & UART_LSR_DR)) - return; + serial8250_fifo_wait_for_lsr_thre(up, p->fifosize); + ndelay(p->frame_time); + + serial_port_out(p, UART_MCR, up->mcr | UART_MCR_LOOP); + + retries = 4; /* Arbitrary limit, 2 was always enough in tests */ + do { + serial8250_clear_fifos(up); + if (!(serial_port_in(p, usr_reg) & DW_UART_USR_BUSY)) + break; + /* FIXME: frame_time delay is too long with very low baudrates. */ + ndelay(p->frame_time); + } while (--retries); + + lsr = serial_lsr_in(up); + if (lsr & UART_LSR_DR) { + serial_port_in(p, UART_RX); + up->lsr_saved_flags = 0; } - serial_port_in(p, UART_RX); + /* Now guaranteed to have BUSY deasserted? Just sanity check */ + if (serial_port_in(p, usr_reg) & DW_UART_USR_BUSY) { + dw8250_idle_exit(p); + return -EBUSY; + } + + return 0; +} + +static void dw8250_set_divisor(struct uart_port *p, unsigned int baud, + unsigned int quot, unsigned int quot_frac) +{ + struct uart_8250_port *up = up_to_u8250p(p); + int ret; + + ret = dw8250_idle_enter(p); + if (ret < 0) + return; + + serial_port_out(p, UART_LCR, up->lcr | UART_LCR_DLAB); + if (!(serial_port_in(p, UART_LCR) & UART_LCR_DLAB)) + goto idle_failed; + + serial_dl_write(up, quot); + serial_port_out(p, UART_LCR, up->lcr); + +idle_failed: + dw8250_idle_exit(p); } /* * This function is being called as part of the uart_port::serial_out() - * routine. Hence, it must not call serial_port_out() or serial_out() - * against the modified registers here, i.e. LCR. + * routine. Hence, special care must be taken when serial_port_out() or + * serial_out() against the modified registers here, i.e. LCR (d->in_idle is + * used to break recursion loop). */ static void dw8250_check_lcr(struct uart_port *p, unsigned int offset, u32 value) { struct dw8250_data *d = to_dw8250_data(p->private_data); - void __iomem *addr = p->membase + (offset << p->regshift); - int tries = 1000; + u32 lcr; + int ret; if (offset != UART_LCR || d->uart_16550_compatible) return; + lcr = serial_port_in(p, UART_LCR); + /* Make sure LCR write wasn't ignored */ - while (tries--) { - u32 lcr = serial_port_in(p, offset); + if ((value & ~UART_LCR_SPAR) == (lcr & ~UART_LCR_SPAR)) + return; - if ((value & ~UART_LCR_SPAR) == (lcr & ~UART_LCR_SPAR)) - return; + if (d->in_idle) + goto write_err; - dw8250_force_idle(p); + ret = dw8250_idle_enter(p); + if (ret < 0) + goto write_err; + + serial_port_out(p, UART_LCR, value); + dw8250_idle_exit(p); + return; -#ifdef CONFIG_64BIT - if (p->type == PORT_OCTEON) - __raw_writeq(value & 0xff, addr); - else -#endif - if (p->iotype == UPIO_MEM32) - writel(value, addr); - else if (p->iotype == UPIO_MEM32BE) - iowrite32be(value, addr); - else - writeb(value, addr); - } +write_err: /* * FIXME: this deadlocks if port->lock is already held * dev_err(p->dev, "Couldn't set LCR to %d\n", value); */ + return; /* Silences "label at the end of compound statement" */ } /* @@ -632,8 +709,10 @@ static int dw8250_probe(struct platform_ p->type = PORT_8250; p->flags = UPF_FIXED_PORT; p->dev = dev; + p->set_ldisc = dw8250_set_ldisc; p->set_termios = dw8250_set_termios; + p->set_divisor = dw8250_set_divisor; data = devm_kzalloc(dev, sizeof(*data), GFP_KERNEL); if (!data) --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -489,7 +489,7 @@ serial_port_out_sync(struct uart_port *p /* * FIFO support. */ -static void serial8250_clear_fifos(struct uart_8250_port *p) +void serial8250_clear_fifos(struct uart_8250_port *p) { if (p->capabilities & UART_CAP_FIFO) { serial_out(p, UART_FCR, UART_FCR_ENABLE_FIFO); @@ -498,6 +498,7 @@ static void serial8250_clear_fifos(struc serial_out(p, UART_FCR, 0); } } +EXPORT_SYMBOL_NS_GPL(serial8250_clear_fifos, "SERIAL_8250"); static enum hrtimer_restart serial8250_em485_handle_start_tx(struct hrtimer *t); static enum hrtimer_restart serial8250_em485_handle_stop_tx(struct hrtimer *t); @@ -3198,6 +3199,17 @@ void serial8250_set_defaults(struct uart } EXPORT_SYMBOL_GPL(serial8250_set_defaults); +void serial8250_fifo_wait_for_lsr_thre(struct uart_8250_port *up, unsigned int count) +{ + unsigned int i; + + for (i = 0; i < count; i++) { + if (wait_for_lsr(up, UART_LSR_THRE)) + return; + } +} +EXPORT_SYMBOL_NS_GPL(serial8250_fifo_wait_for_lsr_thre, "SERIAL_8250"); + #ifdef CONFIG_SERIAL_8250_CONSOLE static void serial8250_console_putchar(struct uart_port *port, unsigned char ch) @@ -3239,16 +3251,6 @@ static void serial8250_console_restore(s serial8250_out_MCR(up, up->mcr | UART_MCR_DTR | UART_MCR_RTS); } -static void fifo_wait_for_lsr(struct uart_8250_port *up, unsigned int count) -{ - unsigned int i; - - for (i = 0; i < count; i++) { - if (wait_for_lsr(up, UART_LSR_THRE)) - return; - } -} - /* * Print a string to the serial port using the device FIFO * @@ -3267,7 +3269,7 @@ static void serial8250_console_fifo_writ while (s != end) { /* Allow timeout for each byte of a possibly full FIFO */ - fifo_wait_for_lsr(up, fifosize); + serial8250_fifo_wait_for_lsr_thre(up, fifosize); for (i = 0; i < fifosize && s != end; ++i) { if (*s == '\n' && !cr_sent) { @@ -3285,7 +3287,7 @@ static void serial8250_console_fifo_writ * Allow timeout for each byte written since the caller will only wait * for UART_LSR_BOTH_EMPTY using the timeout of a single character */ - fifo_wait_for_lsr(up, tx_count); + serial8250_fifo_wait_for_lsr_thre(up, tx_count); } /*