[PATCH v3] video: fbdev: pm2fb: avoid stall on fb_sync

[Tong Zhang]

pm2fb_sync is called when doing /dev/fb read or write.
The original pm2fb_sync wait indefinitely on hardware flags which can
possibly stall kernel and make everything unresponsive.
Instead of waiting indefinitely, we can timeout to give user a chance to
get back control.

[   39.503356] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-48-gd9c812dda5194
[   39.503947] Call Trace:
[   39.504081]  <IRQ>
[   39.504193]  dump_stack+0x7d/0xa3
[   39.504377]  nmi_cpu_backtrace.cold+0x32/0x7e
[   39.504613]  ? lapic_can_unplug_cpu+0x70/0x70
[   39.504850]  nmi_trigger_cpumask_backtrace+0xdf/0x100
[   39.505121]  rcu_dump_cpu_stacks+0xed/0x130
[   39.505349]  rcu_sched_clock_irq.cold+0x3b1/0x61d
[   39.505602]  ? hrtimer_run_queues+0x2c/0x1b0
[   39.505833]  ? __acct_update_integrals+0x136/0x160
[   39.506091]  update_process_times+0xb9/0xf0
[   39.506317]  tick_sched_handle.isra.0+0x5c/0x80
[   39.506562]  tick_sched_timer+0x70/0x90
[   39.506770]  __hrtimer_run_queues+0x1c6/0x3e0
[   39.517095]  ? tick_sched_handle.isra.0+0x80/0x80
[   39.517349]  ? enqueue_hrtimer+0xd0/0xd0
[   39.517561]  ? _raw_write_lock_irqsave+0xd0/0xd0
[   39.517812]  ? ktime_get+0x45/0xb0
[   39.517997]  ? ktime_get_update_offsets_now+0x96/0x150
[   39.518273]  hrtimer_interrupt+0x1a0/0x340
[   39.518496]  __sysvec_apic_timer_interrupt+0x7f/0x160
[   39.518768]  asm_call_irq_on_stack+0xf/0x20
[   39.518997]  </IRQ>
[   39.519114]  sysvec_apic_timer_interrupt+0x6f/0x80
[   39.519372]  asm_sysvec_apic_timer_interrupt+0x12/0x20
[   39.519647] RIP: 0010:pm2fb_sync+0x47/0x70 [pm2fb]
[   39.519907] Code: 89 ef e8 0c 87 2c c1 48 8b 53 08 31 c0 89 82 40 8c 00 00 0f ae f0 48 8b 53 08 1
[   39.520885] RSP: 0018:ffff88810a1f7df8 EFLAGS: 00000202
[   39.521165] RAX: 0000000072d5d49f RBX: ffff88810a034418 RCX: ffffc900000b0020
[   39.521542] RDX: ffffc900000b0000 RSI: 0000000000000246 RDI: ffff88810a034420
[   39.521920] RBP: ffff88810a034420 R08: 0000000000000000 R09: ffffed102143ef64
[   39.522297] R10: 0000000000000003 R11: ffffed102143ef63 R12: ffff88810a1f7ed0
[   39.522673] R13: ffff88810a034000 R14: ffffc90002800000 R15: ffff888109e5e000
[   39.523053]  ? pm2fb_sync+0x24/0x70 [pm2fb]
[   39.523280]  fb_write+0x1c2/0x2d0
[   39.523461]  vfs_write+0x108/0x380
[   39.523647]  ksys_write+0xb4/0x150
[   39.523832]  ? __ia32_sys_read+0x40/0x40
[   39.524043]  ? fpregs_assert_state_consistent+0x4d/0x60
[   39.524322]  do_syscall_64+0x33/0x40
[   39.524517]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   39.524788] RIP: 0033:0x7f5e50097970
[   39.524981] Code: 73 01 c3 48 8b 0d 28 d5 2b 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 4
[   39.525952] RSP: 002b:00007ffec6895b38 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[   39.526352] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f5e50097970
[   39.526726] RDX: 0000000000000200 RSI: 000056491a6ad000 RDI: 0000000000000001
[   39.537261] RBP: 0000000000000200 R08: 0000000000000000 R09: 000056491a6ad030
[   39.537633] R10: 0000000000000871 R11: 0000000000000246 R12: 0000000000000800
[   39.538008] R13: 000056491a6ad000 R14: 0000000000000200 R15: 0000000000000000

Signed-off-by: Tong Zhang <ztong0001 at gmail.com>
---
 v3: initialize timeout_fifo to 0, and added comment for WAIT_FIFO_TIMEOUT
 v2: fix typo and add console log according to Randy's <rdunlap at infradead.org> comment
 drivers/video/fbdev/pm2fb.c | 30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbdev/pm2fb.c b/drivers/video/fbdev/pm2fb.c
index 27893fa139b0..37db7e740221 100644
--- a/drivers/video/fbdev/pm2fb.c
+++ b/drivers/video/fbdev/pm2fb.c
@@ -183,12 +183,24 @@ static inline void pm2v_RDAC_WR(struct pm2fb_par *p, s32 idx, u32 v)
 
 #ifdef CONFIG_FB_PM2_FIFO_DISCONNECT
 #define WAIT_FIFO(p, a)
+#define WAIT_FIFO_TIMEOUT(p, a) (0)
 #else
 static inline void WAIT_FIFO(struct pm2fb_par *p, u32 a)
 {
 	while (pm2_RD(p, PM2R_IN_FIFO_SPACE) < a)
 		cpu_relax();
 }
+/* return 1 for timeout, otherwise 0 */
+static inline int WAIT_FIFO_TIMEOUT(struct pm2fb_par *p, u32 a)
+{
+	int timeout = 10000;
+	while (pm2_RD(p, PM2R_IN_FIFO_SPACE) < a) {
+		cpu_relax();
+		if (--timeout == 0)
+			return 1;
+	}
+	return 0;
+}
 #endif
 
 /*
@@ -1031,15 +1043,27 @@ static int pm2fb_blank(int blank_mode, struct fb_info *info)
 static int pm2fb_sync(struct fb_info *info)
 {
 	struct pm2fb_par *par = info->par;
+	int timeout_sync = 10000;
+	int timeout_fifo = 0;
 
-	WAIT_FIFO(par, 1);
+	if (WAIT_FIFO_TIMEOUT(par, 1))
+		goto end;
 	pm2_WR(par, PM2R_SYNC, 0);
 	mb();
 	do {
-		while (pm2_RD(par, PM2R_OUT_FIFO_WORDS) == 0)
+		timeout_fifo = 10000;
+		while (pm2_RD(par, PM2R_OUT_FIFO_WORDS) == 0) {
 			cpu_relax();
-	} while (pm2_RD(par, PM2R_OUT_FIFO) != PM2TAG(PM2R_SYNC));
+			if (--timeout_fifo == 0)
+				goto end;
+		}
+		if (pm2_RD(par, PM2R_OUT_FIFO) == PM2TAG(PM2R_SYNC))
+			break;
+	} while (--timeout_sync > 0);
 
+end:
+	if ((!timeout_sync) || (!timeout_fifo))
+		printk_ratelimited(KERN_WARNING "pm2fb: sync timeout!\n");
 	return 0;
 }
 
-- 
2.25.1