linux.git/include/linux/tty_driver.h, branch v7.1-rc3

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

tty: tty_driver: introduce TTY driver sub/types enums

2025-03-20T15:00:51+00:00

Convert TTY_DRIVER_TYPE_*, and subtype macros to two enums:
tty_driver_type and tty_driver_subtype. This allows for easier
kernel-doc (later), grouping of these nicely, and proper checking.

The tty_driver's ::type and ::subtype now use these enums instead of
bare "short".

Signed-off-by: Jiri Slaby (SUSE) 
Link: https://lore.kernel.org/r/20250317070046.24386-16-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman

tty: tty_driver: document both {,__}tty_alloc_driver() properly

2025-03-20T15:00:51+00:00

__tty_alloc_driver()'s kernel-doc needed some care: describe the return
value using the standard "Returns:", and use the new enum tty_driver_flag
for @flags.

Then, the tty_alloc_driver() macro was undocumented, but referenced many
times in the docs. Copy the docs from the above (except the @owner
parameter, obviously).

Signed-off-by: Jiri Slaby (SUSE) 
Link: https://lore.kernel.org/r/20250317070046.24386-15-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman

tty: tty_driver: convert "TTY Driver Flags" to an enum

2025-03-20T15:00:51+00:00

Convert TTY_DRIVER_* macros (flags) to an enum. This allows for easier
kernel-doc (the comment needed fine tuning), grouping of these nicely,
and proper checking.

Given these are flags, define them using modern BIT() instead of hex
constants.

It turns out (thanks, kernel-doc checker) that internal
TTY_DRIVER_INSTALLED was undocumented. Fix that too.

Signed-off-by: Jiri Slaby (SUSE) 
Link: https://lore.kernel.org/r/20250317070046.24386-14-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman

tty: tty_driver: move TTY macros to the top

2025-03-20T15:00:51+00:00

So that they can be referenced in structs once converted to enums (in
the next patches).

Signed-off-by: Jiri Slaby (SUSE) 
Link: https://lore.kernel.org/r/20250317070046.24386-13-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman

Documentation: move dev-tools debugging files to process/debugging/

2024-12-17T20:46:53+00:00

Move gdb and kgdb debugging documentation to the dedicated
debugging directory (Documentation/process/debugging/).
Adjust the index.rst files to follow the file movement.
Adjust files that refer to these moved files to follow the file movement.
Update location of kgdb.rst in MAINTAINERS file.
Add a link from dev-tools/index to process/debugging/index.

Note: translations are not updated.

Signed-off-by: Randy Dunlap 
Cc: Sebastian Fricke 
Cc: Jonathan Corbet 
Cc: workflows@vger.kernel.org
Cc: Jason Wessel 
Cc: Daniel Thompson 
Cc: Douglas Anderson 
Cc: linux-debuggers@vger.kernel.org
Cc: kgdb-bugreport@lists.sourceforge.net
Cc: Doug Anderson 
Cc: Alex Shi 
Cc: Hu Haowen <2023002089@link.tyut.edu.cn>
Cc: Andrew Morton 
Cc: Greg Kroah-Hartman 
Cc: linux-serial@vger.kernel.org
Acked-by: Greg Kroah-Hartman 
Acked-by: Daniel Thompson 
Reviewed-by: Douglas Anderson 
Signed-off-by: Jonathan Corbet 
Link: https://lore.kernel.org/r/20241210000041.305477-1-rdunlap@infradead.org

tty: add the option to have a tty reject a new ldisc

2024-05-04T16:45:11+00:00

... and use it to limit the virtual terminals to just N_TTY.  They are
kind of special, and in particular, the "con_write()" routine violates
the "writes cannot sleep" rule that some ldiscs rely on.

This avoids the

   BUG: sleeping function called from invalid context at kernel/printk/printk.c:2659

when N_GSM has been attached to a virtual console, and gsmld_write()
calls con_write() while holding a spinlock, and con_write() then tries
to get the console lock.

Tested-by: Tetsuo Handa 
Cc: Jiri Slaby 
Cc: Andrew Morton 
Cc: Daniel Starke 
Reported-by: syzbot 
Closes: https://syzkaller.appspot.com/bug?extid=dbac96d8e73b61aa559c
Signed-off-by: Linus Torvalds 
Link: https://lore.kernel.org/r/20240423163339.59780-1-torvalds@linux-foundation.org
Signed-off-by: Greg Kroah-Hartman

tty: make tty_operations::send_xchar accept u8 char

2023-12-08T11:02:37+00:00

tty_operations::send_xchar is one of the last users of 'char' type for
characters in the tty layer. Convert it to u8 now.

Signed-off-by: "Jiri Slaby (SUSE)" 
Cc: Karsten Keil 
Cc: Ulf Hansson 
Cc: Marcel Holtmann 
Cc: Johan Hedberg 
Cc: Luiz Augusto von Dentz 
Cc: netdev@vger.kernel.org
Cc: linux-mmc@vger.kernel.org
Cc: linux-bluetooth@vger.kernel.org
Reviewed-by: Geert Uytterhoeven 
Link: https://lore.kernel.org/r/20231206073712.17776-5-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman

tty: fix tty_operations types in documentation

2023-11-25T07:23:17+00:00

Commits 95713967ba52 ("tty: make tty_operations::write()'s count
size_t") and dcaafbe6ee3b ("tty: propagate u8 data to
tty_operations::put_char()") changed types of characters to u8, but
omitted to fix the documentation.

Fix the latter now.

Signed-off-by: Jiri Slaby (SUSE) 
Link: https://lore.kernel.org/r/20231121092258.9334-4-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman

tty: make tty_operations::write()'s count size_t

2023-08-11T19:12:46+00:00

Unify with the rest of the code. Use size_t for counts and ssize_t for
retval.

Signed-off-by: "Jiri Slaby (SUSE)" 
Link: https://lore.kernel.org/r/20230810091510.13006-30-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman