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Message-ID: <aXic3pyl0xfTSYB-@tom-desktop>
Date: Tue, 27 Jan 2026 12:09:18 +0100
From: Tommaso Merciai <tommaso.merciai.xr@...renesas.com>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: Xin Zhao <jackzxcui1989@....com>, gregkh@...uxfoundation.org,
jirislaby@...nel.org, tj@...nel.org, hch@...radead.org,
linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v8] tty: tty_port: add workqueue to flip TTY buffer
Hi Marek, All,
Thanks for your comment.
On Tue, Jan 27, 2026 at 11:34:32AM +0100, Marek Szyprowski wrote:
> On 23.12.2025 04:48, Xin Zhao wrote:
> > 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_CUSTOM_WORKQUEUE flag.
> >
> > Introduce %TTY_DRIVER_CUSTOM_WORKQUEUE flag meaning not to create the
> > default single tty_driver workqueue. Two reasons why need to introduce the
> > %TTY_DRIVER_CUSTOM_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.
> >
> > Signed-off-by: Xin Zhao <jackzxcui1989@....com>
>
> This patch landed in linux-next as commit d000422a46aa ("tty: tty_port:
> add workqueue to flip TTY buffer"). In my tests I found that it causes
> some regressions, see the comments in the code below.
Same here, testing on RZ/G3E looks like s2idle is broken:
root@...rc-rzg3e:~# echo s2idle > /sys/power/mem_sleep
eroot@...rc-rzg3e:~# echo mem > /sys/power/state
[ 182.263395] PM: suspend entry (s2idle)
[ 182.267619] Filesystems sync: 0.000 seconds
[ 182.275175] Freezing user space processes
[ 182.282334] Freezing user space processes completed (elapsed 0.002 seconds)
[ 182.289447] OOM killer disabled.
[ 182.292771] Freezing remaining freezable tasks
[ 182.299126] Freezing remaining freezable tasks completed (elapsed 0.001 seconds)
[ 182.369910] renesas-gbeth 15c30000.ethernet end0: Link is Down
[ 182.379217] PM: suspend devices took 0.076 seconds
[ 185.039929] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000100
[ 185.048725] Mem abort info:
[ 185.051530] ESR = 0x0000000096000004
[ 185.055288] EC = 0x25: DABT (current EL), IL = 32 bits
[ 185.060608] SET = 0, FnV = 0
[ 185.063674] EA = 0, S1PTW = 0
[ 185.066825] FSC = 0x04: level 0 translation fault
[ 185.071708] Data abort info:
[ 185.074593] ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[ 185.080079] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[ 185.085136] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[ 185.090455] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000102465000
[ 185.096900] [0000000000000100] pgd=0000000000000000, p4d=0000000000000000
[ 185.103723] Internal error: Oops: 0000000096000004 [#1] SMP
[ 185.109389] Modules linked in: sha256 cfg80211 panfrost drm_shmem_helper bluetooth gpu_sched spi_rpc_if rcar_canfd drm_kms_helper can_dev rtc_isl1208 phy_rzg3e_usb3 ecdh_generic renesas_r
pc_if ecc rfkill fuse drm backlight ipv6
[ 185.129854] CPU: 0 UID: 0 PID: 384 Comm: sh Not tainted 6.19.0-rc7-next-20260126-00006-gdc83ac04b66e #306 PREEMPT
[ 185.140207] Hardware name: Renesas SMARC EVK version 2 based on r9a09g047e57 (DT)
[ 185.147683] pstate: 604000c5 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 185.154656] pc : __queue_work+0x20/0x474
[ 185.158621] lr : queue_work_on+0x8c/0xa8
[ 185.162571] sp : ffff80008314bbd0
[ 185.165896] x29: ffff80008314bbd0 x28: ffff800082f74268 x27: 0000000000000000
[ 185.173083] x26: 0000000000000000 x25: ffff0000cbdf8400 x24: 0000000000000000
[ 185.180264] x23: 0000000000000200 x22: 00000000000000c0 x21: 0000000000000000
[ 185.187446] x20: 0000000000000000 x19: ffff0000cbdf8410 x18: 0000000000000001
[ 185.194629] x17: ffff8000797a8000 x16: ffff800083148000 x15: 00000dad54b1bae4
[ 185.201812] x14: 0000000000000000 x13: 0000000000000010 x12: 0000000000000010
[ 185.208992] x11: 0000000000000040 x10: 0000000000000031 x9 : ffff0000c0030be8
[ 185.216173] x8 : 0000000000000031 x7 : 72ecc8f68fa27bc6 x6 : 0000000000000031
[ 185.223353] x5 : ffff0000cbdfb421 x4 : ffff0000cbdf8410 x3 : 0000000000000000
[ 185.230534] x2 : ffff0000cbdf8410 x1 : 0000000000000000 x0 : 0000000000000200
[ 185.237717] Call trace:
[ 185.240176] __queue_work+0x20/0x474 (P)
[ 185.244141] queue_work_on+0x8c/0xa8
[ 185.247753] tty_flip_buffer_push+0x2c/0x38
[ 185.251987] put_queue+0x64/0xc0
[ 185.255261] k_unicode.part.0+0x84/0xc4
[ 185.259141] k_self+0x30/0x40
[ 185.262154] kbd_event+0x304/0x59c
[ 185.265601] input_handle_events_default+0x4c/0x70
[ 185.270437] input_pass_values+0x130/0x150
[ 185.274577] input_event_dispose+0x134/0x138
[ 185.278889] input_handle_event+0x58/0x84
[ 185.282929] input_event+0x64/0xa4
[ 185.286362] gpio_keys_irq_isr+0x78/0x130
[ 185.290414] __handle_irq_event_percpu+0x44/0x1d8
[ 185.295160] handle_irq_event+0x4c/0xac
[ 185.299035] handle_fasteoi_irq+0x104/0x194
[ 185.303246] handle_irq_desc+0x40/0x58
[ 185.307033] generic_handle_domain_irq+0x18/0x24
[ 185.311686] gic_handle_irq+0x4c/0x10c
[ 185.315465] call_on_irq_stack+0x30/0x48
[ 185.319423] do_interrupt_handler+0x80/0x84
[ 185.323645] el1_interrupt+0x3c/0x60
[ 185.327265] el1h_64_irq_handler+0x18/0x24
[ 185.331404] el1h_64_irq+0x6c/0x70
[ 185.334838] _raw_spin_unlock_irqrestore+0x8/0x44 (P)
[ 185.339931] resume_device_irqs+0x14/0x20
[ 185.343983] dpm_resume_noirq+0x14/0x24
[ 185.347860] suspend_devices_and_enter+0x674/0x738
[ 185.352684] pm_suspend+0x1f0/0x2b4
[ 185.356207] state_store+0x80/0xec
[ 185.359641] kobj_attr_store+0x18/0x2c
[ 185.363423] sysfs_kf_write+0x7c/0x94
[ 185.367122] kernfs_fop_write_iter+0x130/0x1dc
[ 185.371596] vfs_write+0x240/0x370
[ 185.375038] ksys_write+0x70/0x108
[ 185.378479] __arm64_sys_write+0x1c/0x28
[ 185.382438] invoke_syscall+0x48/0x10c
[ 185.386237] el0_svc_common.constprop.0+0xc0/0xe0
[ 185.390986] do_el0_svc+0x1c/0x28
[ 185.394350] el0_svc+0x34/0x108
[ 185.397535] el0t_64_sync_handler+0xa0/0xe4
[ 185.401761] el0t_64_sync+0x198/0x19c
[ 185.405475] Code: aa0103f4 aa0203f3 a90363f7 2a0003f7 (b9410020)
[ 185.411571] ---[ end trace 0000000000000000 ]---
[ 185.416200] Kernel panic - not syncing: Oops: Fatal exception in interrupt
[ 185.423072] SMP: stopping secondary CPUs
[ 185.427031] Kernel Offset: disabled
[ 185.430525] CPU features: 0x1000000,00080005,00230501,0400721b
[ 185.436365] Memory Limit: none
[ 185.439435] ---[ end Kernel panic - not syncing: Oops: Fatal exception in interrupt ]---
Reverting this I'm able to see s2idle work back.
Hope this help.
Thank you!
Kind Regards,
Tommaso
>
> > ---
> >
> > Change in v8:
> > - Rebase code, use system_dfl_wq instead of system_unbound_wq.
> >
> > Change in v7:
> > - Pty simply link to system_unbound_wq instead of allocating a custom one,
> > as suggested by Jiri Slaby.
> > - Modify some inappropriate expressions in the code comments,
> > as suggested by Jiri Slaby.
> > - Link to v7: https://lore.kernel.org/all/20251210125028.4174917-1-jackzxcui1989@163.com/T/#u
> >
> > Change in v6:
> > - Modify many inappropriate expressions in the commit log and code comments,
> > as suggested by Jiri Slaby.
> > - Add reasons why need to introduce the %TTY_DRIVER_CUSTOM_WORKQUEUE in
> > commit log.
> > - Modify the error handling related to the allocation failure of workqueue in
> > tty_register_driver(), as suggested by Jiri Slaby.
> > - Add description of tty_port_link_driver_wq() in the commit log,
> > as suggested by Jiri Slaby.
> > - Link to v6: https://lore.kernel.org/all/20251210031827.3771327-1-jackzxcui1989@163.com/
> >
> > Change in v5:
> > - Do not allocate workqueue twice when CONFIG_UNIX98_PTYS and
> > CONFIG_LEGACY_PTYS are all enabled.
> > - Link to v5: https://lore.kernel.org/all/20251205030829.1829987-1-jackzxcui1989@163.com/
> >
> > Change in v4:
> > - Simplify the logic for creating and releasing the workqueue,
> > as suggested by Tejun Heo.
> > - Allocate single workqueue of one tty_driver as default, link it to
> > port when tty_port register device or tty_driver.
> > - Introduce tty_port_link_wq() to link specific workqueue to port.
> > - Add driver flag %TTY_DRIVER_CUSTOM_WORKQUEUE meaning not to create the
> > default single tty_driver workqueue.
> > - Link to v4: https://lore.kernel.org/all/202512041303.7192024b-lkp@intel.com/T/#t
> >
> > Change in v3:
> > - Add tty flip workqueue for all tty ports, as suggested by Greg KH.
> > Every tty port use an individual flip workqueue, while all pty ports
> > share the same workqueue created in pty_flip_wq_init().
> > - Modify the commit log to describe the reason for latency spikes in
> > RT-Linux.
> > - Link to v3: https://lore.kernel.org/all/20251027060929.394053-1-jackzxcui1989@163.com/
> >
> > Change in v2:
> > - Do not add new module parameters
> > as suggested by Greg KH
> > - Set WQ_SYSFS to allow properties changes from userspace
> > as suggested by Tejun Heo
> > - Link to v2: https://lore.kernel.org/all/20251024155534.2302590-1-jackzxcui1989@163.com
> > ---
> > drivers/tty/pty.c | 14 ++++++++++----
> > drivers/tty/tty_buffer.c | 8 ++++----
> > drivers/tty/tty_io.c | 21 ++++++++++++++++++++-
> > drivers/tty/tty_port.c | 23 +++++++++++++++++++++++
> > include/linux/tty_buffer.h | 1 +
> > include/linux/tty_driver.h | 7 +++++++
> > include/linux/tty_port.h | 13 +++++++++++++
> > 7 files changed, 78 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> > index 6120d827a..1f17575f8 100644
> > --- a/drivers/tty/pty.c
> > +++ b/drivers/tty/pty.c
> > @@ -403,6 +403,8 @@ static int pty_common_install(struct tty_driver *driver, struct tty_struct *tty,
> > o_tty->link = tty;
> > tty_port_init(ports[0]);
> > tty_port_init(ports[1]);
> > + tty_port_link_wq(ports[0], system_dfl_wq);
> > + tty_port_link_wq(ports[1], system_dfl_wq);
> > tty_buffer_set_limit(ports[0], 8192);
> > tty_buffer_set_limit(ports[1], 8192);
> > o_tty->port = ports[0];
> > @@ -532,14 +534,16 @@ static void __init legacy_pty_init(void)
> > pty_driver = tty_alloc_driver(legacy_count,
> > TTY_DRIVER_RESET_TERMIOS |
> > TTY_DRIVER_REAL_RAW |
> > - TTY_DRIVER_DYNAMIC_ALLOC);
> > + TTY_DRIVER_DYNAMIC_ALLOC |
> > + TTY_DRIVER_CUSTOM_WORKQUEUE);
> > if (IS_ERR(pty_driver))
> > panic("Couldn't allocate pty driver");
> >
> > pty_slave_driver = tty_alloc_driver(legacy_count,
> > TTY_DRIVER_RESET_TERMIOS |
> > TTY_DRIVER_REAL_RAW |
> > - TTY_DRIVER_DYNAMIC_ALLOC);
> > + TTY_DRIVER_DYNAMIC_ALLOC |
> > + TTY_DRIVER_CUSTOM_WORKQUEUE);
> > if (IS_ERR(pty_slave_driver))
> > panic("Couldn't allocate pty slave driver");
> >
> > @@ -849,7 +853,8 @@ static void __init unix98_pty_init(void)
> > TTY_DRIVER_REAL_RAW |
> > TTY_DRIVER_DYNAMIC_DEV |
> > TTY_DRIVER_DEVPTS_MEM |
> > - TTY_DRIVER_DYNAMIC_ALLOC);
> > + TTY_DRIVER_DYNAMIC_ALLOC |
> > + TTY_DRIVER_CUSTOM_WORKQUEUE);
> > if (IS_ERR(ptm_driver))
> > panic("Couldn't allocate Unix98 ptm driver");
> > pts_driver = tty_alloc_driver(NR_UNIX98_PTY_MAX,
> > @@ -857,7 +862,8 @@ static void __init unix98_pty_init(void)
> > TTY_DRIVER_REAL_RAW |
> > TTY_DRIVER_DYNAMIC_DEV |
> > TTY_DRIVER_DEVPTS_MEM |
> > - TTY_DRIVER_DYNAMIC_ALLOC);
> > + TTY_DRIVER_DYNAMIC_ALLOC |
> > + TTY_DRIVER_CUSTOM_WORKQUEUE);
> > if (IS_ERR(pts_driver))
> > panic("Couldn't allocate Unix98 pts driver");
> >
> > diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> > index 1a5673acd..86e1e7178 100644
> > --- a/drivers/tty/tty_buffer.c
> > +++ b/drivers/tty/tty_buffer.c
> > @@ -76,7 +76,7 @@ void tty_buffer_unlock_exclusive(struct tty_port *port)
> > mutex_unlock(&buf->lock);
> >
> > if (restart)
> > - queue_work(system_dfl_wq, &buf->work);
> > + queue_work(buf->flip_wq, &buf->work);
> > }
> > EXPORT_SYMBOL_GPL(tty_buffer_unlock_exclusive);
> >
> > @@ -530,7 +530,7 @@ void tty_flip_buffer_push(struct tty_port *port)
> > struct tty_bufhead *buf = &port->buf;
> >
> > tty_flip_buffer_commit(buf->tail);
> > - queue_work(system_dfl_wq, &buf->work);
> > + queue_work(buf->flip_wq, &buf->work);
> > }
> > EXPORT_SYMBOL(tty_flip_buffer_push);
> >
> > @@ -560,7 +560,7 @@ int tty_insert_flip_string_and_push_buffer(struct tty_port *port,
> > tty_flip_buffer_commit(buf->tail);
> > spin_unlock_irqrestore(&port->lock, flags);
> >
> > - queue_work(system_dfl_wq, &buf->work);
> > + queue_work(buf->flip_wq, &buf->work);
> >
> > return size;
> > }
> > @@ -613,7 +613,7 @@ void tty_buffer_set_lock_subclass(struct tty_port *port)
> >
> > bool tty_buffer_restart_work(struct tty_port *port)
> > {
> > - return queue_work(system_dfl_wq, &port->buf.work);
> > + return queue_work(port->buf.flip_wq, &port->buf.work);
> > }
> >
> > bool tty_buffer_cancel_work(struct tty_port *port)
> > diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> > index e2d92cf70..d64fb08ba 100644
> > --- a/drivers/tty/tty_io.c
> > +++ b/drivers/tty/tty_io.c
> > @@ -3446,10 +3446,23 @@ int tty_register_driver(struct tty_driver *driver)
> > if (error < 0)
> > goto err;
> >
> > + if (!(driver->flags & TTY_DRIVER_CUSTOM_WORKQUEUE)) {
> > + driver->flip_wq = alloc_workqueue("%s-flip-wq", WQ_UNBOUND | WQ_SYSFS,
> > + 0, driver->name);
>
> It looks that driver->name is not unique on some systems, see:
>
> $ git grep ttyMSM drivers/tty/
> drivers/tty/serial/msm_serial.c: .name = "ttyMSM",
> drivers/tty/serial/msm_serial.c: .dev_name = "ttyMSM",
> drivers/tty/serial/qcom_geni_serial.c: .name = "ttyMSM",
> drivers/tty/serial/qcom_geni_serial.c: .dev_name = "ttyMSM",
>
> This fails on Qualcomm RB5 boards, breaking the boot process (booting
> hangs, because drivers try to use the unregistered wq):
>
> sysfs: cannot create duplicate filename
> '/devices/virtual/workqueue/ttyMSM-flip-wq'
> CPU: 6 UID: 0 PID: 1 Comm: swapper/0 Not tainted
> 6.19.0-rc7-next-20260126+ #16440 PREEMPT
> Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
> Call trace:
> show_stack+0x18/0x24 (C)
> dump_stack_lvl+0xc0/0xd0
> dump_stack+0x18/0x24
> sysfs_warn_dup+0x64/0x80
> sysfs_create_dir_ns+0xe8/0x108
> kobject_add_internal+0x98/0x270
> kobject_add+0x94/0x10c
> device_add+0xec/0x720
> device_register+0x20/0x30
> workqueue_sysfs_register+0x8c/0x170
> __alloc_workqueue+0x554/0x668
> alloc_workqueue_noprof+0x5c/0xfc
> tty_register_driver+0x120/0x2d0
> uart_register_driver+0x120/0x1b0
> qcom_geni_serial_init+0x20/0xc8
> do_one_initcall+0x64/0x308
> kernel_init_freeable+0x284/0x508
> kernel_init+0x24/0x1dc
> ret_from_fork+0x10/0x20
> kobject: kobject_add_internal failed for ttyMSM-flip-wq with -EEXIST,
> don't try to register things with the same name in the same directory.
>
> Changing the above alloc_workqueue() to use 'driver->driver_name' fixes
> the boot issue. If keeping the driver->name is desired, then maybe the
> '"%s-%s-flip-wq", ..., driver->name, driver->driver_name' format is a
> better one.
>
> The other issue with this patch I've observed on ARM Juno R1 board. With
> one of the above fixes for the workqueue name, the boot process is still
> broken because of the NULL pointer dereference:
>
> input: gpio-keys as /de ** replaying previous printk message ** input:
> gpio-keys as /devices/platform/gpio-keys/input/input3 Unable to handle
> kernel NULL pointer dereference at virtual address 00000000000001c0 Mem
> abort info: ... [00000000000001c0] user address but active_mm is swapper
> Internal error: Oops: 0000000096000004 [#1] SMP Modules linked in: CPU:
> 1 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.19.0-rc7-next-20260126+
> #16443 PREEMPT Hardware name: ARM Juno development board (r1) (DT)
> pstate: 400000c5 (nZcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc :
> __queue_work+0x30/0x7c4 lr : queue_work_on+0xac/0xdc ... Call trace:
> __queue_work+0x30/0x7c4 (P) queue_work_on+0xac/0xdc
> tty_flip_buffer_push+0x2c/0x38 k_fn.part.0+0x7c/0xc8 k_fn+0x20/0x2c
> kbd_event+0x2d8/0x504 input_handle_events_default+0x50/0x74
> input_pass_values+0x148/0x2b4 input_handle_event+0xcc/0x5e0
> input_event+0x64/0xa4 gpio_keys_open+0x9c/0xc4
> input_open_device+0x8c/0x128 kbd_connect+0x84/0xa0
> input_attach_handler+0x9c/0xf4 input_register_device+0x308/0x48c
> gpio_keys_probe+0x40c/0xafc platform_probe+0x5c/0xac
> really_probe+0xbc/0x298 __driver_probe_device+0x78/0x12c
> driver_probe_device+0xdc/0x164 __driver_attach+0xe4/0x224
> bus_for_each_dev+0x74/0xd0 driver_attach+0x24/0x30
> bus_add_driver+0xe4/0x208 driver_register+0x60/0x128
> __platform_driver_register+0x24/0x30 gpio_keys_init+0x1c/0x28
> do_one_initcall+0x64/0x308 kernel_init_freeable+0x284/0x508
> kernel_init+0x24/0x1dc ret_from_fork+0x10/0x20 Code: a9025bf5 a90573fb
> aa0203fb 35001843 (b941c260) ---[ end trace 0000000000000000 ]--- note:
> swapper/0[1] exited with irqs disabled note: swapper/0[1] exited with
> preempt_count 3 Kernel panic - not syncing: Attempted to kill init!
> exitcode=0x0000000b SMP: stopping secondary CPUs Kernel Offset: disabled
> CPU features: 0x1040000,41858004,00020000,0400421b Memory Limit: none
> ---[ end Kernel panic - not syncing: Attempted to kill init!
> exitcode=0x0000000b ]---
>
> Reverting $subject on top of current linux-next fixes this issue.
>
> > + if (!driver->flip_wq) {
> > + error = -ENOMEM;
> > + goto err_unreg_char;
> > + }
> > + for (i = 0; i < driver->num; i++) {
> > + if (driver->ports[i])
> > + tty_port_link_driver_wq(driver->ports[i], driver);
> > + }
> > + }
> > +
> > if (driver->flags & TTY_DRIVER_DYNAMIC_ALLOC) {
> > error = tty_cdev_add(driver, dev, 0, driver->num);
> > if (error)
> > - goto err_unreg_char;
> > + goto err_destroy_wq;
> > }
> >
> > scoped_guard(mutex, &tty_mutex)
> > @@ -3475,6 +3488,10 @@ int tty_register_driver(struct tty_driver *driver)
> > scoped_guard(mutex, &tty_mutex)
> > list_del(&driver->tty_drivers);
> >
> > +err_destroy_wq:
> > + if (!(driver->flags & TTY_DRIVER_CUSTOM_WORKQUEUE))
> > + destroy_workqueue(driver->flip_wq);
> > +
> > err_unreg_char:
> > unregister_chrdev_region(dev, driver->num);
> > err:
> > @@ -3494,6 +3511,8 @@ void tty_unregister_driver(struct tty_driver *driver)
> > driver->num);
> > scoped_guard(mutex, &tty_mutex)
> > list_del(&driver->tty_drivers);
> > + if (!(driver->flags & TTY_DRIVER_CUSTOM_WORKQUEUE))
> > + destroy_workqueue(driver->flip_wq);
> > }
> > EXPORT_SYMBOL(tty_unregister_driver);
> >
> > diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
> > index fe67c5cb0..611f87814 100644
> > --- a/drivers/tty/tty_port.c
> > +++ b/drivers/tty/tty_port.c
> > @@ -99,6 +99,26 @@ void tty_port_init(struct tty_port *port)
> > }
> > EXPORT_SYMBOL(tty_port_init);
> >
> > +/**
> > + * tty_port_link_wq - link tty_port and flip workqueue
> > + * @port: tty_port of the device
> > + * @flip_wq: workqueue to queue flip buffer work on
> > + *
> > + * When %TTY_DRIVER_CUSTOM_WORKQUEUE is used, every tty_port shall be linked to
> > + * a workqueue manually by this function, otherwise tty_flip_buffer_push() will
> > + * see %NULL flip_wq pointer on queue_work.
> > + * When %TTY_DRIVER_CUSTOM_WORKQUEUE is NOT used, the function can be used to
> > + * link a certain port to a specific workqueue, instead of using the workqueue
> > + * allocated in tty_register_driver().
> > + *
> > + * Note that TTY port API will NOT destroy the workqueue.
> > + */
> > +void tty_port_link_wq(struct tty_port *port, struct workqueue_struct *flip_wq)
> > +{
> > + port->buf.flip_wq = flip_wq;
> > +}
> > +EXPORT_SYMBOL_GPL(tty_port_link_wq);
> > +
> > /**
> > * tty_port_link_device - link tty and tty_port
> > * @port: tty_port of the device
> > @@ -157,6 +177,7 @@ struct device *tty_port_register_device_attr(struct tty_port *port,
> > const struct attribute_group **attr_grp)
> > {
> > tty_port_link_device(port, driver, index);
> > + tty_port_link_driver_wq(port, driver);
> > return tty_register_device_attr(driver, index, device, drvdata,
> > attr_grp);
> > }
> > @@ -183,6 +204,7 @@ struct device *tty_port_register_device_attr_serdev(struct tty_port *port,
> > struct device *dev;
> >
> > tty_port_link_device(port, driver, index);
> > + tty_port_link_driver_wq(port, driver);
> >
> > dev = serdev_tty_port_register(port, host, parent, driver, index);
> > if (PTR_ERR(dev) != -ENODEV) {
> > @@ -703,6 +725,7 @@ int tty_port_install(struct tty_port *port, struct tty_driver *driver,
> > struct tty_struct *tty)
> > {
> > tty->port = port;
> > + tty_port_link_driver_wq(port, driver);
> > return tty_standard_install(driver, tty);
> > }
> > EXPORT_SYMBOL_GPL(tty_port_install);
> > diff --git a/include/linux/tty_buffer.h b/include/linux/tty_buffer.h
> > index 31125e3be..48adcb0e8 100644
> > --- a/include/linux/tty_buffer.h
> > +++ b/include/linux/tty_buffer.h
> > @@ -34,6 +34,7 @@ static inline u8 *flag_buf_ptr(struct tty_buffer *b, unsigned int ofs)
> >
> > struct tty_bufhead {
> > struct tty_buffer *head; /* Queue head */
> > + struct workqueue_struct *flip_wq;
> > struct work_struct work;
> > struct mutex lock;
> > atomic_t priority;
> > diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
> > index 188ee9b76..9c65854f7 100644
> > --- a/include/linux/tty_driver.h
> > +++ b/include/linux/tty_driver.h
> > @@ -69,6 +69,10 @@ struct serial_struct;
> > * Do not create numbered ``/dev`` nodes. For example, create
> > * ``/dev/ttyprintk`` and not ``/dev/ttyprintk0``. Applicable only when a
> > * driver for a single tty device is being allocated.
> > + *
> > + * @TTY_DRIVER_CUSTOM_WORKQUEUE:
> > + * Do not create workqueue when tty_register_driver(). When set, flip
> > + * buffer workqueue shall be set by tty_port_link_wq() for every port.
> > */
> > enum tty_driver_flag {
> > TTY_DRIVER_INSTALLED = BIT(0),
> > @@ -79,6 +83,7 @@ enum tty_driver_flag {
> > TTY_DRIVER_HARDWARE_BREAK = BIT(5),
> > TTY_DRIVER_DYNAMIC_ALLOC = BIT(6),
> > TTY_DRIVER_UNNUMBERED_NODE = BIT(7),
> > + TTY_DRIVER_CUSTOM_WORKQUEUE = BIT(8),
> > };
> >
> > enum tty_driver_type {
> > @@ -506,6 +511,7 @@ struct tty_operations {
> > * @flags: tty driver flags (%TTY_DRIVER_)
> > * @proc_entry: proc fs entry, used internally
> > * @other: driver of the linked tty; only used for the PTY driver
> > + * @flip_wq: workqueue to queue flip buffer work on
> > * @ttys: array of active &struct tty_struct, set by tty_standard_install()
> > * @ports: array of &struct tty_port; can be set during initialization by
> > * tty_port_link_device() and similar
> > @@ -539,6 +545,7 @@ struct tty_driver {
> > unsigned long flags;
> > struct proc_dir_entry *proc_entry;
> > struct tty_driver *other;
> > + struct workqueue_struct *flip_wq;
> >
> > /*
> > * Pointer to the tty data structures
> > diff --git a/include/linux/tty_port.h b/include/linux/tty_port.h
> > index 660c254f1..c1b87f3c5 100644
> > --- a/include/linux/tty_port.h
> > +++ b/include/linux/tty_port.h
> > @@ -138,6 +138,7 @@ struct tty_port {
> > kernel */
> >
> > void tty_port_init(struct tty_port *port);
> > +void tty_port_link_wq(struct tty_port *port, struct workqueue_struct *flip_wq);
> > void tty_port_link_device(struct tty_port *port, struct tty_driver *driver,
> > unsigned index);
> > struct device *tty_port_register_device(struct tty_port *port,
> > @@ -165,6 +166,18 @@ static inline struct tty_port *tty_port_get(struct tty_port *port)
> > return NULL;
> > }
> >
> > +/*
> > + * Never overwrite the workqueue set by tty_port_link_wq().
> > + * No effect when %TTY_DRIVER_CUSTOM_WORKQUEUE is set, as driver->flip_wq is
> > + * %NULL.
> > + */
> > +static inline void tty_port_link_driver_wq(struct tty_port *port,
> > + struct tty_driver *driver)
> > +{
> > + if (!port->buf.flip_wq)
> > + port->buf.flip_wq = driver->flip_wq;
> > +}
> > +
> > /* If the cts flow control is enabled, return true. */
> > static inline bool tty_port_cts_enabled(const struct tty_port *port)
> > {
>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>
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