[PATCH] serial/pmac_zilog: Remove flawed mitigation for rx irq flood

From: Finn Thain
Date: Thu Mar 28 2024 - 00:13:46 EST

The mitigation was intended to stop the irq completely. That might have
been better than a hard lock-up but it turns out that you get a crash
anyway if you're using pmac_zilog as a serial console.

That's because the pr_err() call in pmz_receive_chars() results in
pmz_console_write() attempting to lock a spinlock already locked in
pmz_interrupt(). With CONFIG_DEBUG_SPINLOCK=y, this produces a fatal
BUG splat like the one below. (The spinlock at 0x62e140 is the one in
struct uart_port.)

Even when it's not fatal, the serial port rx function ceases to work.
Also, the iteration limit doesn't play nicely with QEMU. Please see
bug report linked below.

A web search for reports of the error message "pmz: rx irq flood" didn't
produce anything. So I don't think this code is needed any more. Remove it.

[ 14.560000] ttyPZ0: pmz: rx irq flood !
[ 14.560000] BUG: spinlock recursion on CPU#0, swapper/0
[ 14.560000] lock: 0x62e140, .magic: dead4ead, .owner: swapper/0, .owner_cpu: 0
[ 14.560000] CPU: 0 PID: 0 Comm: swapper Not tainted 6.8.0-mac-dbg-preempt-00004-g4143b7b9144a #1
[ 14.560000] Stack from 0059bcc4:
[ 14.560000] 0059bcc4 0056316f 0056316f 00002700 004b6444 0059bce4 004ad8c6 0056316f
[ 14.560000] 0059bd10 004a6546 00556759 0062e140 dead4ead 0059f892 00000000 00000000
[ 14.560000] 0062e140 0059bde8 005c03d0 0059bd24 0004daf6 0062e140 005567bf 0062e140
[ 14.560000] 0059bd34 004b64c2 0062e140 00000001 0059bd50 002e15ea 0062e140 00000001
[ 14.560000] 0059bde7 0059bde8 005c03d0 0059bdac 0005124e 005c03d0 005cdc00 0000002b
[ 14.560000] 005a3caa 005a3caa 00000000 0059bde8 0004ff00 0059be8b 00038200 000529ba
[ 14.560000] Call Trace: [<00002700>] ret_from_kernel_thread+0xc/0x14
[ 14.560000] [<004b6444>] _raw_spin_lock+0x0/0x28
[ 14.560000] [<004ad8c6>] dump_stack+0x10/0x16
[ 14.560000] [<004a6546>] spin_dump+0x6e/0x7c
[ 14.560000] [<0004daf6>] do_raw_spin_lock+0x9c/0xa6
[ 14.560000] [<004b64c2>] _raw_spin_lock_irqsave+0x2a/0x34
[ 14.560000] [<002e15ea>] pmz_console_write+0x32/0x9a
[ 14.560000] [<0005124e>] console_flush_all+0x112/0x3a2
[ 14.560000] [<0004ff00>] console_trylock+0x0/0x7a
[ 14.560000] [<00038200>] parameq+0x48/0x6e
[ 14.560000] [<000529ba>] __printk_safe_enter+0x0/0x36
[ 14.560000] [<0005113c>] console_flush_all+0x0/0x3a2
[ 14.560000] [<000542c4>] prb_read_valid+0x0/0x1a
[ 14.560000] [<004b65a4>] _raw_spin_unlock+0x0/0x38
[ 14.560000] [<0005151e>] console_unlock+0x40/0xb8
[ 14.560000] [<00038200>] parameq+0x48/0x6e
[ 14.560000] [<002c778c>] __tty_insert_flip_string_flags+0x0/0x14e
[ 14.560000] [<00051798>] vprintk_emit+0x156/0x238
[ 14.560000] [<00051894>] vprintk_default+0x1a/0x1e
[ 14.560000] [<000529a8>] vprintk+0x74/0x86
[ 14.560000] [<004a6596>] _printk+0x12/0x16
[ 14.560000] [<002e23be>] pmz_receive_chars+0x1cc/0x394
[ 14.560000] [<004b6444>] _raw_spin_lock+0x0/0x28
[ 14.560000] [<00038226>] parse_args+0x0/0x3a6
[ 14.560000] [<004b6466>] _raw_spin_lock+0x22/0x28
[ 14.560000] [<002e26b4>] pmz_interrupt+0x12e/0x1e0
[ 14.560000] [<00048680>] arch_cpu_idle_enter+0x0/0x8
[ 14.560000] [<00054ebc>] __handle_irq_event_percpu+0x24/0x106
[ 14.560000] [<004ae576>] default_idle_call+0x0/0x46
[ 14.560000] [<00055020>] handle_irq_event+0x30/0x90
[ 14.560000] [<00058320>] handle_simple_irq+0x5e/0xc0
[ 14.560000] [<00048688>] arch_cpu_idle_exit+0x0/0x8
[ 14.560000] [<00054800>] generic_handle_irq+0x3c/0x4a
[ 14.560000] [<00002978>] do_IRQ+0x24/0x3a
[ 14.560000] [<004ae508>] cpu_idle_poll.isra.0+0x0/0x6e
[ 14.560000] [<00002874>] auto_irqhandler_fixup+0x4/0xc
[ 14.560000] [<004ae508>] cpu_idle_poll.isra.0+0x0/0x6e
[ 14.560000] [<004ae576>] default_idle_call+0x0/0x46
[ 14.560000] [<004ae598>] default_idle_call+0x22/0x46
[ 14.560000] [<00048710>] do_idle+0x6a/0xf0
[ 14.560000] [<000486a6>] do_idle+0x0/0xf0
[ 14.560000] [<000367d2>] find_task_by_pid_ns+0x0/0x2a
[ 14.560000] [<0005d064>] __rcu_read_lock+0x0/0x12
[ 14.560000] [<00048a5a>] cpu_startup_entry+0x18/0x1c
[ 14.560000] [<00063a06>] __rcu_read_unlock+0x0/0x26
[ 14.560000] [<004ae65a>] kernel_init+0x0/0xfa
[ 14.560000] [<0049c5a8>] strcpy+0x0/0x1e
[ 14.560000] [<004a6584>] _printk+0x0/0x16
[ 14.560000] [<0049c72a>] strlen+0x0/0x22
[ 14.560000] [<006452d4>] memblock_alloc_try_nid+0x0/0x82
[ 14.560000] [<0063939a>] arch_post_acpi_subsys_init+0x0/0x8
[ 14.560000] [<0063991e>] console_on_rootfs+0x0/0x60
[ 14.560000] [<00638410>] _sinittext+0x410/0xadc
[ 14.560000]

Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxx>
Cc: "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxx>
Cc: linux-m68k@xxxxxxxxxxxxxxxxxxxx
Link: https://github.com/vivier/qemu-m68k/issues/44
Link: https://lore.kernel.org/all/1078874617.9746.36.camel@gaston/
Signed-off-by: Finn Thain <fthain@xxxxxxxxxxxxxx>
---
drivers/tty/serial/pmac_zilog.c | 14 --------------
1 file changed, 14 deletions(-)

diff --git a/drivers/tty/serial/pmac_zilog.c b/drivers/tty/serial/pmac_zilog.c
index c8bf08c19c64..77691fbbf779 100644
--- a/drivers/tty/serial/pmac_zilog.c
+++ b/drivers/tty/serial/pmac_zilog.c
@@ -210,7 +210,6 @@ static bool pmz_receive_chars(struct uart_pmac_port *uap)
{
struct tty_port *port;
unsigned char ch, r1, drop, flag;
- int loops = 0;

/* Sanity check, make sure the old bug is no longer happening */
if (uap->port.state == NULL) {
@@ -291,24 +290,11 @@ static bool pmz_receive_chars(struct uart_pmac_port *uap)
if (r1 & Rx_OVR)
tty_insert_flip_char(port, 0, TTY_OVERRUN);
next_char:
- /* We can get stuck in an infinite loop getting char 0 when the
- * line is in a wrong HW state, we break that here.
- * When that happens, I disable the receive side of the driver.
- * Note that what I've been experiencing is a real irq loop where
- * I'm getting flooded regardless of the actual port speed.
- * Something strange is going on with the HW
- */
- if ((++loops) > 1000)
- goto flood;
ch = read_zsreg(uap, R0);
if (!(ch & Rx_CH_AV))
break;
}

- return true;
- flood:
- pmz_interrupt_control(uap, 0);
- pmz_error("pmz: rx irq flood !\n");
return true;
}

--
2.39.3