fb_ops.fp_pan_display() and locking

Unlike the other fb_ops, .fp_pan_display() may be called from interrupt
context.  This can happen when calling printk() from an interrupt handler,
while accelerating text console scrolling by using a virtual screen.

Hence one cannot use a mutex to provide locking in .fp_pan_display(), or
something like this happens (assumed CONFIG_DEBUG_SPINLOCK_SLEEP=y):

| BUG: sleeping function called from invalid context at linux/kernel/mutex.c:86
| in_atomic():1, irqs_disabled():1
| Call Trace:
| [c000000005f4b430] [c00000000000effc] .show_stack+0x68/0x1b0 (unreliable)
| [c000000005f4b4d0] [c000000000046ed8] .__might_sleep+0xec/0x108
| [c000000005f4b550] [c0000000003a3ce4] .mutex_lock+0x28/0x48
| [c000000005f4b5e0] [c0000000001fb1e8] .xxxfb_pan_display+0x34/0x88
| [c000000005f4b680] [c0000000001e6e10] .fb_pan_display+0x120/0x180
| [c000000005f4b710] [c0000000001f43b4] .bit_update_start+0x28/0x60
| [c000000005f4b7a0] [c0000000001f238c] .fbcon_scroll+0x6a0/0xeb8
| [c000000005f4b870] [c000000000213524] .scrup+0xa0/0x14c
| [c000000005f4b910] [c000000000213610] .lf+0x40/0x9c
| [c000000005f4b990] [c000000000218d00] .vt_console_print+0x214/0x3c8
| [c000000005f4ba40] [c00000000004f150] .__call_console_drivers+0xc8/0xf8
| [c000000005f4bad0] [c00000000004f604] .release_console_sem+0x19c/0x2bc
| [c000000005f4bb80] [c00000000004ff78] .vprintk+0x338/0x3f4
| [c000000005f4bc60] [c00000000005006c] .printk+0x38/0x48
| [c000000005f4bcf0] [c000000000053d38] .do_exit+0x3f0/0x96c
| [c000000005f4bda0] [c000000000054368] .sys_exit_group+0x0/0x8
| [c000000005f4be30] [c000000000008834] syscall_exit+0x0/0x40

Omapfb and vmlfb use a mutex in .fp_pan_display().

Probably nobody noticed this problem yet because the mere presence of a
virtual screen and panning support is not sufficient to enable panning for
accelerating text console scrolling.  One has to explicitly set
FBINFO_HWACCEL_YPAN in fb_info.flags, too, which both omapfb and vmlfb
don't do (yet).

With kind regards,
 
Geert Uytterhoeven
Software Architect

Sony Network and Software Technology Center Europe
The Corporate Village · Da Vincilaan 7-D1 · B-1935 Zaventem · Belgium
 
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