drm/fbdev-dma: Only cleanup deferred I/O if necessary

Commit 5a498d4d ("drm/fbdev-dma: Only install deferred I/O if
necessary") initializes deferred I/O only if it is used.
drm_fbdev_dma_fb_destroy() however calls fb_deferred_io_cleanup()
unconditionally with struct fb_info.fbdefio == NULL. KASAN with the
out-of-tree Apple silicon display driver posts following warning from
__flush_work() of a random struct work_struct instead of the expected
NULL pointer derefs.

[   22.053799] ------------[ cut here ]------------
[   22.054832] WARNING: CPU: 2 PID: 1 at kernel/workqueue.c:4177 __flush_work+0x4d8/0x580
[   22.056597] Modules linked in: uhid bnep uinput nls_ascii ip6_tables ip_tables i2c_dev loop fuse dm_multipath nfnetlink zram hid_magicmouse btrfs xor xor_neon brcmfmac_wcc raid6_pq hci_bcm4377 bluetooth brcmfmac hid_apple brcmutil nvmem_spmi_mfd simple_mfd_spmi dockchannel_hid cfg80211 joydev regmap_spmi nvme_apple ecdh_generic ecc macsmc_hid rfkill dwc3 appledrm snd_soc_macaudio macsmc_power nvme_core apple_isp phy_apple_atc apple_sart apple_rtkit_helper apple_dockchannel tps6598x macsmc_hwmon snd_soc_cs42l84 videobuf2_v4l2 spmi_apple_controller nvmem_apple_efuses videobuf2_dma_sg apple_z2 videobuf2_memops spi_nor panel_summit videobuf2_common asahi videodev pwm_apple apple_dcp snd_soc_apple_mca apple_admac spi_apple clk_apple_nco i2c_pasemi_platform snd_pcm_dmaengine mc i2c_pasemi_core mux_core ofpart adpdrm drm_dma_helper apple_dart apple_soc_cpufreq leds_pwm phram
[   22.073768] CPU: 2 UID: 0 PID: 1 Comm: systemd-shutdow Not tainted 6.11.2-asahi+ #asahi-dev
[   22.075612] Hardware name: Apple MacBook Pro (13-inch, M2, 2022) (DT)
[   22.077032] pstate: 01400005 (nzcv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
[   22.078567] pc : __flush_work+0x4d8/0x580
[   22.079471] lr : __flush_work+0x54/0x580
[   22.080345] sp : ffffc000836ef820
[   22.081089] x29: ffffc000836ef880 x28: 0000000000000000 x27: ffff80002ddb7128
[   22.082678] x26: dfffc00000000000 x25: 1ffff000096f0c57 x24: ffffc00082d3e358
[   22.084263] x23: ffff80004b7862b8 x22: dfffc00000000000 x21: ffff80005aa1d470
[   22.085855] x20: ffff80004b786000 x19: ffff80004b7862a0 x18: 0000000000000000
[   22.087439] x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000005
[   22.089030] x14: 1ffff800106ddf0a x13: 0000000000000000 x12: 0000000000000000
[   22.090618] x11: ffffb800106ddf0f x10: dfffc00000000000 x9 : 1ffff800106ddf0e
[   22.092206] x8 : 0000000000000000 x7 : aaaaaaaaaaaaaaaa x6 : 0000000000000001
[   22.093790] x5 : ffffc000836ef728 x4 : 0000000000000000 x3 : 0000000000000020
[   22.095368] x2 : 0000000000000008 x1 : 00000000000000aa x0 : 0000000000000000
[   22.096955] Call trace:
[   22.097505]  __flush_work+0x4d8/0x580
[   22.098330]  flush_delayed_work+0x80/0xb8
[   22.099231]  fb_deferred_io_cleanup+0x3c/0x130
[   22.100217]  drm_fbdev_dma_fb_destroy+0x6c/0xe0 [drm_dma_helper]
[   22.101559]  unregister_framebuffer+0x210/0x2f0
[   22.102575]  drm_fb_helper_unregister_info+0x48/0x60
[   22.103683]  drm_fbdev_dma_client_unregister+0x4c/0x80 [drm_dma_helper]
[   22.105147]  drm_client_dev_unregister+0x1cc/0x230
[   22.106217]  drm_dev_unregister+0x58/0x570
[   22.107125]  apple_drm_unbind+0x50/0x98 [appledrm]
[   22.108199]  component_del+0x1f8/0x3a8
[   22.109042]  dcp_platform_shutdown+0x24/0x38 [apple_dcp]
[   22.110357]  platform_shutdown+0x70/0x90
[   22.111219]  device_shutdown+0x368/0x4d8
[   22.112095]  kernel_restart+0x6c/0x1d0
[   22.112946]  __arm64_sys_reboot+0x1c8/0x328
[   22.113868]  invoke_syscall+0x78/0x1a8
[   22.114703]  do_el0_svc+0x124/0x1a0
[   22.115498]  el0_svc+0x3c/0xe0
[   22.116181]  el0t_64_sync_handler+0x70/0xc0
[   22.117110]  el0t_64_sync+0x190/0x198
[   22.117931] ---[ end trace 0000000000000000 ]---
Signed-off-by: Janne Grunau <j@jannau.net>
Fixes: 5a498d4d ("drm/fbdev-dma: Only install deferred I/O if necessary")
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/ZwLNuZL-8Gh5UUQb@robin

drivers/gpu/drm/drm_fbdev_dma.c

@@ -50,7 +50,8 @@ static void drm_fbdev_dma_fb_destroy(struct fb_info *info)
 	if (!fb_helper->dev)
 		return;
 
-	fb_deferred_io_cleanup(info);
+	if (info->fbdefio)
+		fb_deferred_io_cleanup(info);
 	drm_fb_helper_fini(fb_helper);
 
 	drm_client_buffer_vunmap(fb_helper->buffer);