kernel/linux.git/drivers/tty/serial/apbuart.c, branch linux-7.1.y

tty: tty_port: add workqueue to flip TTY buffer

2026-03-12T14:26:29+00:00

On the embedded platform, certain critical data, such as IMU data, is transmitted through UART. The tty_flip_buffer_push() interface in the TTY layer uses system_dfl_wq to handle the flipping of the TTY buffer. Although the unbound workqueue can create new threads on demand and wake up the kworker thread on an idle CPU, it may be preempted by real-time tasks or other high-prio tasks. flush_to_ldisc() needs to wake up the relevant data handle thread. When executing __wake_up_common_lock(), it calls spin_lock_irqsave(), which does not disable preemption but disables migration in RT-Linux. This prevents the kworker thread from being migrated to other cores by CPU's balancing logic, resulting in long delays. The call trace is as follows: __wake_up_common_lock __wake_up ep_poll_callback __wake_up_common __wake_up_common_lock __wake_up n_tty_receive_buf_common n_tty_receive_buf2 tty_ldisc_receive_buf tty_port_default_receive_buf flush_to_ldisc In our system, the processing interval for each frame of IMU data transmitted via UART can experience significant jitter due to this issue. Instead of the expected 10 to 15 ms frame processing interval, we see spikes up to 30 to 35 ms. Moreover, in just one or two hours, there can be 2 to 3 occurrences of such high jitter, which is quite frequent. This jitter exceeds the software's tolerable limit of 20 ms. Introduce flip_wq in tty_port which can be set by tty_port_link_wq() or as default linked to default workqueue allocated when tty_register_driver(). The default workqueue is allocated with flag WQ_SYSFS, so that cpumask and nice can be set dynamically. The execution timing of tty_port_link_wq() is not clearly restricted. The newly added function tty_port_link_driver_wq() checks whether the flip_wq of the tty_port has already been assigned when linking the default tty_driver's workqueue to the port. After the user has set a custom workqueue for a certain tty_port using tty_port_link_wq(), the system will only use this custom workqueue, even if tty_driver does not have %TTY_DRIVER_NO_WORKQUEUE flag. When tty_port register device, flip_wq link operation is done by tty_port_link_driver_wq(), but for in-memory devices the link operation cannot cover all the cases. Although tty_port_install() is dedicated for in-memory devices lik PTY to link port allocated on demand, the logic of tty_port_install() is so simple that people may not call it, vc_cons[0].d->port is one such case. We check the buf.flip_wq when flip TTY buffer, if buf.flip_wq of TTY port is NULL, use system_dfl_wq as a backup. To avoid naming conflict of the default tty_driver's workqueue, using '"%s-%s", driver->name, driver->driver_name' as the workqueue name. In cases where driver_name is not specified and therefore is NULL, the workqueue is not created. Drivers that do not define driver_name are potentially in-memory devices like vty, which generally do not require special workqueue settings. Even with the combination of name and driver_name, the workqueue names can still be duplicated, as many tty serial drivers use "ttyS" as dev_name and "serial" as driver_name. I modified the conflicting driver_name of these drivers by appending a suffix of _xx based on the corresponding .c file. If this modification is not made, it could not only lead to duplicate workqueue names but also result in duplicate entries for the /proc/tty/driver/ nodes. Introduce %TTY_DRIVER_NO_WORKQUEUE flag meaning not to create the default single tty_driver workqueue. Two reasons why need to introduce the %TTY_DRIVER_NO_WORKQUEUE flag: 1. If the WQ_SYSFS parameter is enabled, workqueue_sysfs_register() will fail when trying to create a workqueue with the same name. The pty is an example of this; if both CONFIG_LEGACY_PTYS and CONFIG_UNIX98_PTYS are enabled, the call to tty_register_driver() in unix98_pty_init() will fail. 2. Different TTY ports may be used for different tasks, which may require separate core binding control via workqueues. In this case, the workqueue created by default in the TTY driver is unnecessary. Enabling this flag prevents the creation of this redundant workqueue. After applying this patch, we can set the related UART TTY flip buffer workqueue by sysfs. We set the cpumask to CPU cores associated with the IMU tasks, and set the nice to -20. Testing has shown significant improvement in the previously described issue, with almost no stuttering occurring anymore. Tested-by: Tommaso Merciai Tested-by: Marek Szyprowski Signed-off-by: Xin Zhao Link: https://patch.msgid.link/20260213085039.3274704-1-jackzxcui1989@163.com Signed-off-by: Greg Kroah-Hartman

serial: apbuart: fix console prompt on qemu

2024-01-04T15:21:06+00:00

When using a leon kernel with qemu there where no console prompt. The root cause is the handling of the fifo size in the tx part of the apbuart driver. The qemu uart driver only have a very rudimentary status handling and do not report the number of chars queued in the tx fifo in the status register. So the driver ends up with a fifo size of 1. In the tx path the fifo size is divided by 2 - resulting in a fifo size of zero. The original implementation would always try to send one char, but after the introduction of uart_port_tx_limited() the fifo size is respected even for the first char. There seems to be no good reason to divide the fifo size with two - so remove this. It looks like something copied from the original amba driver. With qemu we now have a minimum fifo size of one char, so we show the prompt. Signed-off-by: Sam Ravnborg Fixes: d11cc8c3c4b6 ("tty: serial: use uart_port_tx_limited()") Cc: Andreas Larsson Cc: Greg Kroah-Hartman Cc: Jiri Slaby Cc: Cc: Link: https://lore.kernel.org/r/20231226121607.GA2622970@ravnborg.org Signed-off-by: Greg Kroah-Hartman

serial: apb: Use port lock wrappers

2023-09-18T09:18:10+00:00

When a serial port is used for kernel console output, then all modifications to the UART registers which are done from other contexts, e.g. getty, termios, are interference points for the kernel console. So far this has been ignored and the printk output is based on the principle of hope. The rework of the console infrastructure which aims to support threaded and atomic consoles, requires to mark sections which modify the UART registers as unsafe. This allows the atomic write function to make informed decisions and eventually to restore operational state. It also allows to prevent the regular UART code from modifying UART registers while printk output is in progress. All modifications of UART registers are guarded by the UART port lock, which provides an obvious synchronization point with the console infrastructure. To avoid adding this functionality to all UART drivers, wrap the spin_[un]lock*() invocations for uart_port::lock into helper functions which just contain the spin_[un]lock*() invocations for now. In a subsequent step these helpers will gain the console synchronization mechanisms. Converted with coccinelle. No functional change. Signed-off-by: Thomas Gleixner Signed-off-by: John Ogness Link: https://lore.kernel.org/r/20230914183831.587273-19-john.ogness@linutronix.de Signed-off-by: Greg Kroah-Hartman

tty: Explicitly include correct DT includes

2023-07-25T18:19:05+00:00

The DT of_device.h and of_platform.h date back to the separate of_platform_bus_type before it as merged into the regular platform bus. As part of that merge prepping Arm DT support 13 years ago, they "temporarily" include each other. They also include platform_device.h and of.h. As a result, there's a pretty much random mix of those include files used throughout the tree. In order to detangle these headers and replace the implicit includes with struct declarations, users need to explicitly include the correct includes. Signed-off-by: Rob Herring Acked-by: Uwe Kleine-König # for imx Link: https://lore.kernel.org/r/20230724205440.767071-1-robh@kernel.org Signed-off-by: Greg Kroah-Hartman

serial: drivers: switch ch and flag to u8

2023-07-25T17:21:04+00:00

Now that the serial layer explicitly expects 'u8' for flags and characters, propagate this type to drivers' (RX) routines. Note that amba-pl011's, clps711x's and st-asc's 'ch' are left unchanged because 'ch' contains not only a character, but whole status. Signed-off-by: Jiri Slaby (SUSE) Cc: Tobias Klauser Cc: Russell King Cc: Vineet Gupta Cc: Richard Genoud Cc: Nicolas Ferre Cc: Alexandre Belloni Cc: Claudiu Beznea Cc: Alexander Shiyan Cc: Baruch Siach Cc: "Maciej W. Rozycki" Cc: Taichi Sugaya Cc: Takao Orito Cc: Shawn Guo Cc: Sascha Hauer Cc: Pengutronix Kernel Team Cc: Fabio Estevam Cc: NXP Linux Team Cc: Kevin Cernekee Cc: Krzysztof Kozlowski Cc: Alim Akhtar Cc: Laxman Dewangan Cc: Thierry Reding Cc: Jonathan Hunter Cc: Palmer Dabbelt Cc: Paul Walmsley Cc: Orson Zhai Cc: Baolin Wang Cc: Chunyan Zhang Cc: Patrice Chotard Cc: Maxime Coquelin Cc: Alexandre Torgue Cc: Hammer Hsieh Acked-by: Richard GENOUD Acked-by: Tobias Klauser Acked-by: Thierry Reding Acked-by: Maciej W. Rozycki Link: https://lore.kernel.org/r/20230712081811.29004-11-jirislaby@kernel.org Signed-off-by: Greg Kroah-Hartman

tty: serial: use uart_port_tx_limited()

2022-11-03T02:32:40+00:00

uart_port_tx_limited() is a new helper to send characters to the device. Use it in these drivers. mux.c also needs to define tx_done(). But I'm not sure if the driver really wants to wait for all the characters to dismiss from the HW fifo at this code point. Hence I marked this as FIXME. Cc: Russell King Cc: Florian Fainelli Cc: bcm-kernel-feedback-list@broadcom.com Cc: "Pali Rohár" Cc: Kevin Cernekee Cc: Palmer Dabbelt Cc: Paul Walmsley Cc: Orson Zhai Cc: Baolin Wang Cc: Chunyan Zhang Cc: Patrice Chotard Cc: linux-riscv@lists.infradead.org Reviewed-by: Ilpo Järvinen Signed-off-by: Jiri Slaby (SUSE) Link: https://lore.kernel.org/r/20221004104927.14361-4-jirislaby@kernel.org Signed-off-by: Greg Kroah-Hartman

serial: Make ->set_termios() old ktermios const

2022-08-30T12:22:35+00:00

There should be no reason to adjust old ktermios which is going to get discarded anyway. Reviewed-by: Andy Shevchenko Signed-off-by: Ilpo Järvinen Link: https://lore.kernel.org/r/20220816115739.10928-7-ilpo.jarvinen@linux.intel.com Signed-off-by: Greg Kroah-Hartman

serial: make uart_console_write->putchar()'s character an unsigned char

2022-03-03T14:06:31+00:00

Currently, uart_console_write->putchar's second parameter (the character) is of type int. It makes little sense, provided uart_console_write() accepts the input string as "const char *s" and passes its content -- the characters -- to putchar(). So switch the character's type to unsigned char. We don't use char as that is signed on some platforms. That would cause troubles for drivers which (implicitly) cast the char to u16 when writing to the device. Sign extension would happen in that case and the value written would be completely different to the provided char. DZ is an example of such a driver -- on MIPS, it uses u16 for dz_out in dz_console_putchar(). Note we do the char -> uchar conversion implicitly in uart_console_write(). Provided we do not change size of the data type, sign extension does not happen there, so the problem is void. This makes the types consistent and unified with the rest of the uart layer, which uses unsigned char in most places already. One exception is xmit_buf, but that is going to be converted later. Cc: Paul Cercueil Cc: Tobias Klauser Cc: Russell King Cc: Vineet Gupta Cc: Nicolas Ferre Cc: Alexandre Belloni Cc: Ludovic Desroches Cc: Florian Fainelli Cc: bcm-kernel-feedback-list@broadcom.com Cc: Alexander Shiyan Cc: Baruch Siach Cc: "Maciej W. Rozycki" Cc: Paul Walmsley Cc: Palmer Dabbelt Cc: Albert Ou Cc: Shawn Guo Cc: Sascha Hauer Cc: Pengutronix Kernel Team Cc: Fabio Estevam Cc: NXP Linux Team Cc: Karol Gugala Cc: Mateusz Holenko Cc: Vladimir Zapolskiy Cc: Neil Armstrong Cc: Kevin Hilman Cc: Jerome Brunet Cc: Martin Blumenstingl Cc: Taichi Sugaya Cc: Takao Orito Cc: Liviu Dudau Cc: Sudeep Holla Cc: Lorenzo Pieralisi Cc: "Andreas Färber" Cc: Manivannan Sadhasivam Cc: Michael Ellerman Cc: Benjamin Herrenschmidt Cc: Paul Mackerras Cc: Andy Gross Cc: Bjorn Andersson Cc: Krzysztof Kozlowski Cc: Orson Zhai Cc: Baolin Wang Cc: Chunyan Zhang Cc: Patrice Chotard Cc: Maxime Coquelin Cc: Alexandre Torgue Cc: "David S. Miller" Cc: Peter Korsgaard Cc: Michal Simek Acked-by: Richard Genoud [atmel_serial] Acked-by: Uwe Kleine-König Acked-by: Paul Cercueil Acked-by: Neil Armstrong # meson_serial Signed-off-by: Jiri Slaby Link: https://lore.kernel.org/r/20220303080831.21783-1-jslaby@suse.cz Signed-off-by: Greg Kroah-Hartman

serial: apbuart: drop low-latency workaround

2021-04-22T10:09:25+00:00

Commit 78d34d75c84d ("tty: serial: apbuart: drop uart_port->lock before calling tty_flip_buffer_push()") claimed to address a locking issue but only provided a dubious lockdep splat from an unrelated driver, which in the end turned out to be due a broken local change carried by the author. Unfortunately these patches were merged before the issue had been analysed properly so the commit messages makes no sense whatsoever. The real issue was first seen on RT which at the time effectively always set the low_latency flag for all serial drivers by patching tty_flip_buffer_push(). This in turn revealed that many drivers did not handle the infamous low_latency behaviour which meant that data was pushed immediately to the line discipline instead of being deferred to a work queue. Since commit a9c3f68f3cd8 ("tty: Fix low_latency BUG"), tty_flip_buffer_push() always schedules a work item to push data to the line discipline and there's no need to keep any low_latency hacks around. Link: https://lore.kernel.org/linux-serial/cover.1376923198.git.viresh.kumar@linaro.org/ Signed-off-by: Johan Hovold Link: https://lore.kernel.org/r/20210421095509.3024-7-johan@kernel.org Signed-off-by: Greg Kroah-Hartman

tty/serial: Migrate apbuart to use has_sysrq

2019-12-18T14:04:44+00:00

The SUPPORT_SYSRQ ifdeffery is not nice as: - May create misunderstanding about sizeof(struct uart_port) between different objects - Prevents moving functions from serial_core.h - Reduces readability (well, it's ifdeffery - it's hard to follow) In order to remove SUPPORT_SYSRQ, has_sysrq variable has been added. Initialise it in driver's probe and remove ifdeffery. Signed-off-by: Dmitry Safonov Link: https://lore.kernel.org/r/20191213000657.931618-11-dima@arista.com Signed-off-by: Greg Kroah-Hartman