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([179.179.237.181]) by smtp.gmail.com with ESMTPSA id 956f58d0204a3-677c1b1b585sm3449969d50.5.2026.10.05.10.26.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Oct 2026 10:26:04 -0700 (PDT) From: Vinicius Pedrosa To: linux-serial@vger.kernel.org Cc: gregkh@linuxfoundation.org, jirislaby@kernel.org, robh@kernel.org, krzk+dt@kernel.org, conor+dt@kernel.org, andriy.shevchenko@linux.intel.com, ilpo.jarvinen@linux.intel.com, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-sunxi@lists.linux.dev, Enzo Adriano , stable@vger.kernel.org Subject: [PATCH 1/3] serial: 8250_dw: Keep the BUSY-safe divisor hook when DLF is present Date: Mon, 5 Oct 2026 14:25:36 -0300 Message-ID: <20261005172538.398522-2-vinicius.eduardo.pedrosa@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20261005172538.398522-1-vinicius.eduardo.pedrosa@gmail.com> References: <20261005172538.398522-1-vinicius.eduardo.pedrosa@gmail.com> Precedence: bulk X-Mailing-List: linux-sunxi@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Rspamd-Server: rspamd-worker-8404 X-Spamd-Result: default: False [-0.66 / 15.00]; BAYES_HAM(-5.50)[100.00%]; RBL_SENDERSCORE(2.00)[172.105.105.114:from]; SUSPICIOUS_RECIPS(1.50)[]; DMARC_POLICY_SOFTFAIL(1.00)[gmail.com : SPF not aligned (relaxed), No valid DKIM,none]; R_MISSING_CHARSET(0.50)[]; MAILLIST(-0.15)[generic]; BAD_REP_POLICIES(0.10)[]; MIME_GOOD(-0.10)[text/plain]; HAS_LIST_UNSUB(-0.01)[]; PRECEDENCE_BULK(0.00)[]; ARC_ALLOW(0.00)[subspace.kernel.org:s=arc-20240116:i=1]; TAGGED_RCPT(0.00)[dt]; RCPT_COUNT_TWELVE(0.00)[13]; DBL_BLOCKED_OPENRESOLVER(0.00)[tor.lore.kernel.org:rdns,tor.lore.kernel.org:helo]; FROM_HAS_DN(0.00)[]; FORGED_SENDER_MAILLIST(0.00)[]; FREEMAIL_CC(0.00)[linuxfoundation.org,kernel.org,linux.intel.com,vger.kernel.org,lists.linux.dev,gmail.com]; RCVD_VIA_SMTP_AUTH(0.00)[]; FROM_NEQ_ENVFROM(0.00)[viniciuseduardopedrosa@gmail.com,linux-sunxi@lists.linux.dev]; ASN(0.00)[asn:63949, ipnet:172.105.96.0/20, country:SG]; FREEMAIL_FROM(0.00)[gmail.com]; R_SPF_ALLOW(0.00)[+ip4:172.105.105.114]; TO_DN_SOME(0.00)[]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[100.90.174.1:received]; RCVD_COUNT_FIVE(0.00)[6]; FORGED_RECIPIENTS_MAILLIST(0.00)[]; MIME_TRACE(0.00)[0:+]; TAGGED_FROM(0.00)[bounces-26507-noreply=patchwork.local]; RECEIVED_SPAMHAUS_PBL(0.00)[179.179.237.181:received]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_LAST(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[172.105.105.114:from] X-Rspamd-Queue-Id: 2BF1B1C01E3 X-MORS-Enabled: yes X-MORS-DOMAIN: patchwork.local X-MORS-HOSTING: hosting172546 X-MORS-USER: hosting172546 X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= dw8250_probe() installs dw8250_set_divisor(), which brackets the DLAB/DLL/DLH sequence with dw8250_idle_enter()/dw8250_idle_exit() so that a BUSY UART cannot drop the divisor LCR write. dw8250_setup_port() runs afterwards and replaces that hook with the dwlib one whenever it detects a DLF register. The dwlib hook programs the divisor through serial8250_do_set_divisor() without entering idle. When termios changes while the UART is transmitting, the LCR|DLAB write is ignored and dw8250_check_lcr() retries it between dw8250_idle_enter() and dw8250_idle_exit(). dw8250_idle_exit() restores IER while DLAB is still set, so the value lands in DLH. The real IER keeps the zero written by dw8250_idle_enter(), and the port gets no further TX or RX interrupts while up->ier says they're enabled. Program DLF from dw8250_set_divisor() inside the idle section and keep that hook after dw8250_setup_port(). Observed on an Allwinner A733 (Radxa Cubie A7Z): the first interactive tcsetattr() during console output left hardware IER at 0 and up->ier at 0x07. The shell never printed its prompt or read input. With this change alone the same boot reached the prompt and echoed input, and hardware IER matched up->ier afterwards. Fixes: a7b9ce39fbe4 ("serial: 8250_dw: Ensure BUSY is deasserted") Cc: stable@vger.kernel.org Signed-off-by: Vinicius Pedrosa --- The A733 has no real DLF. Offset 0xc0 is an RS485 control register that dw8250_setup_port() mistakes for one (patch 3), which is how this path was reached. The new DLF write hasn't been tested on a UART with a real DLF register. drivers/tty/serial/8250/8250_dw.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c index 0ab558230370..ba414306c98a 100644 --- a/drivers/tty/serial/8250/8250_dw.c +++ b/drivers/tty/serial/8250/8250_dw.c @@ -189,6 +189,7 @@ static int dw8250_idle_enter(struct uart_port *p) static void dw8250_set_divisor(struct uart_port *p, unsigned int baud, unsigned int quot, unsigned int quot_frac) { + struct dw8250_data *d = to_dw8250_data(p->private_data); struct uart_8250_port *up = up_to_u8250p(p); int ret; @@ -196,6 +197,9 @@ static void dw8250_set_divisor(struct uart_port *p, unsigned int baud, if (ret < 0) return; + if (d->data.dlf_size) + dw8250_writel_ext(p, DW_UART_DLF, quot_frac); + serial_port_out(p, UART_LCR, up->lcr | UART_LCR_DLAB); if (!(serial_port_in(p, UART_LCR) & UART_LCR_DLAB)) goto idle_failed; @@ -650,7 +654,6 @@ static int dw8250_probe(struct platform_device *pdev) 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) @@ -767,6 +770,13 @@ static int dw8250_probe(struct platform_device *pdev) if (!data->skip_autocfg) dw8250_setup_port(p); + /* + * dw8250_setup_port() installs the library divisor hook when it finds + * DLF. Keep the BUSY-safe hook instead: it also programs DLF, and it + * must own DLAB so that a BUSY UART never drops the divisor LCR write. + */ + p->set_divisor = dw8250_set_divisor; + /* If we have a valid fifosize, try hooking up DMA */ if (p->fifosize) { data->data.dma.rxconf.src_maxburst = p->fifosize / 4;