On my Macbook with a discrete Nvidia GPU, there is a race between selecting the integrated GPU and putting the discrete GPU into D3 [1], reliably causing a kernel oops [2].
Introducing a delay of ~1s between the calls prevents this. When the second 'OFF' write path executes, it looks like struct azx at card->private_data hasn't yet been allocated yet [3], so there is likely some locking missing.
I'm happy to perform further testing and debug of course...
Thanks, Daniel
--- [1]
echo IGD > /sys/kernel/debug/vgaswitcheroo/switch echo OFF > /sys/kernel/debug/vgaswitcheroo/switch
--- [2]
BUG: unable to handle kernel NULL pointer dereference at 0000000000000170 IP: [<ffffffffa01ba936>] azx_vs_set_state+0x26/0x178 [snd_hda_intel] PGD 259c26067 PUD 25a0fd067 PMD 0 Oops: 0000 [#1] SMP DEBUG_PAGEALLOC Modules linked in: snd_hda_codec_hdmi bnep rfcomm b43 joydev nfsd ssb nfs_acl auth_rpcgss binfmt_misc nfs lockd sunrpc uvcvideo bcm5974 videobuf2_core videobuf2_vmalloc videobuf2_memops coretemp kvm_intel snd_hda_codec_cirrus kvm applesmc input_polldev microcode bcma lpc_ich mfd_core mei snd_hda_intel(+) snd_hda_codec snd_hwdep snd_pcm snd_timer snd snd_page_alloc nls_iso8859_1 apple_gmux mac_hid apple_bl btrfs hid_apple sdhci_pci ghash_clmulni_intel tg3 sdhci i915 nouveau ttm drm_kms_helper hwmon mxm_wmi video CPU 2 Pid: 961, comm: sh Not tainted 3.6.0-rc7 #2 Apple Inc. MacBookPro10,1/Mac-C3EC7CD22292981F RIP: 0010:[<ffffffffa01ba936>] [<ffffffffa01ba936>] azx_vs_set_state+0x26/0x178 [snd_hda_intel] RSP: 0018:ffff880264271e48 EFLAGS: 00010286 RAX: 0000000000000000 RBX: ffff88025a2f5280 RCX: 0000000000000000 RDX: 0000000000000006 RSI: 0000000000000000 RDI: ffff880265479098 RBP: ffff880264271e68 R08: 2222222222222222 R09: 2222222222222222 R10: 0000000000000000 R11: 0000000000000000 R12: ffff880265479098 R13: 0000000000000000 R14: ffff880264271f50 R15: 0000000000000000 FS: 00007fa4fe183700(0000) GS:ffff88026f280000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000170 CR3: 00000002641a7000 CR4: 00000000001407e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400 Process sh (pid: 961, threadinfo ffff880264270000, task ffff880264503a00) Stack: 2222222222222222 ffff88025a2f5280 0000000000000000 ffff880264271e98 ffff880264271e88 ffffffff812e83a7 ffff8802622835c0 0000000000000004 ffff880264271ef8 ffffffff812e89ac ffff88020a46464f ffff880264503a00 Call Trace: [<ffffffff812e83a7>] set_audio_state+0x67/0x70 [<ffffffff812e89ac>] vga_switcheroo_debugfs_write+0xbc/0x380 [<ffffffff81108773>] vfs_write+0xa3/0x160 [<ffffffff81108a75>] sys_write+0x45/0xa0 [<ffffffff815231a6>] system_call_fastpath+0x1a/0x1f Code: 00 00 00 00 00 55 48 89 e5 48 83 ec 20 4c 89 65 f0 4c 8d a7 98 00 00 00 4c 89 e7 48 89 5d e8 4c 89 6d f8 41 89 f5 e8 2a 35 13 e1 <48> 8b 98 70 01 00 00 0f b6 83 55 02 00 00 a8 08 75 34 45 85 ed RIP [<ffffffffa01ba936>] azx_vs_set_state+0x26/0x178 [snd_hda_intel] RSP <ffff880264271e48> CR2: 0000000000000170
--- [3]
(gdb) list *(azx_vs_set_state+0x26) 0x2936 is in azx_vs_set_state (sound/pci/hda/hda_intel.c:2505). 2500 2501 static void azx_vs_set_state(struct pci_dev *pci, 2502 enum vga_switcheroo_state state) 2503 { 2504 struct snd_card *card = pci_get_drvdata(pci); 2505 struct azx *chip = card->private_data; 2506 bool disabled; 2507 2508 if (chip->init_failed) 2509 return;