I have laptop with AMD A8-5557M APU (Radeon HD 8550G) and discrete videocard Radeon HD 8750M.
When I start steam on discrete videocard, system often hangs completely. I can only reboot it using magic SysRq key.

Steps to Reproduce:
1. Get list of videocards using this command:
$ xrandr --listproviders
2. Use xrandr to tell discrete card to output to integrated card:
$ xrandr --setprovideroffloadsink 0x41 0x78
3. Then I launch some program on discrete card:
$ DRI_PRIME=1 steam
4. Clicking on steam window menu often produces system hang.

This is from system log:

May 19 17:03:43 linux.local kernel: BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
May 19 17:03:43 linux.local kernel: IP: [<ffffffffc0376ebe>] ttm_bo_wait+0x7e/0x190 [ttm]
May 19 17:03:44 linux.local kernel: PGD 1a6a47067 PUD 1a6a46067 PMD 0 
May 19 17:03:44 linux.local kernel: Oops: 0000 [#1] SMP 
May 19 17:03:44 linux.local kernel: Modules linked in: nf_log_ipv6 nf_log_common xt_LOG xt_recent ip6table_nat nf_nat_ipv6 nf_nat xt_comment ip6t_REJ
May 19 17:03:44 linux.local kernel:  videobuf2_core btusb v4l2_common videodev bluetooth media acer_wmi wl(PO) sparse_keymap kvm_amd snd_hda_codec_re
May 19 17:03:44 linux.local kernel:  dm_region_hash dm_log dm_mod [last unloaded: fglrx]
May 19 17:03:44 linux.local kernel: CPU: 1 PID: 3703 Comm: X Tainted: P           O   3.19.8-desktop-1.mga5 #1
May 19 17:03:44 linux.local kernel: Hardware name: Acer Aspire V5-552G/Dazzle_RL , BIOS V2.07 12/23/2013
May 19 17:03:44 linux.local kernel: task: ffff8801a69d0000 ti: ffff8801a5778000 task.ti: ffff8801a5778000
May 19 17:03:44 linux.local kernel: RIP: 0010:[<ffffffffc0376ebe>]  [<ffffffffc0376ebe>] ttm_bo_wait+0x7e/0x190 [ttm]
May 19 17:03:44 linux.local kernel: RSP: 0018:ffff8801a577b9c8  EFLAGS: 00010246
May 19 17:03:44 linux.local kernel: RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
May 19 17:03:44 linux.local kernel: RDX: 0000000000003a98 RSI: 0000000000000000 RDI: ffff88018c85f068
May 19 17:03:44 linux.local kernel: RBP: ffff8801a577ba18 R08: ffff88015c7d5178 R09: 0000000000000001
May 19 17:03:44 linux.local kernel: R10: 00000000000002d0 R11: 00000000000002d0 R12: 0000000000000000
May 19 17:03:44 linux.local kernel: R13: ffffea000272c680 R14: 0000000000000001 R15: ffff8801a572d200
May 19 17:03:44 linux.local kernel: FS:  00007f8ca45f7700(0000) GS:ffff8801aec80000(0000) knlGS:00000000f445fb40
May 19 17:03:44 linux.local kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
May 19 17:03:44 linux.local kernel: CR2: 0000000000000018 CR3: 00000001a5c50000 CR4: 00000000000407e0
May 19 17:03:44 linux.local kernel: Stack:
May 19 17:03:44 linux.local kernel:  0000000000820000 ffff88018c85f068 ffff88008dc869f0 0000000080347e00
May 19 17:03:44 linux.local kernel:  ffff8801a577ba18 ffff88018c85f098 ffff88003672c7b8 ffff88018c85f068
May 19 17:03:44 linux.local kernel:  ffff88003672c710 ffff880036fee0c0 ffff8801a577ba68 ffffffffc03777b1
May 19 17:03:44 linux.local kernel: Call Trace:
May 19 17:03:44 linux.local kernel:  [<ffffffffc03777b1>] ttm_bo_release+0x141/0x2d0 [ttm]
May 19 17:03:44 linux.local kernel:  [<ffffffffc0377969>] ttm_bo_unref+0x29/0x30 [ttm]
May 19 17:03:44 linux.local kernel:  [<ffffffffc03ae72a>] radeon_bo_unref+0x2a/0x50 [radeon]
May 19 17:03:44 linux.local kernel:  [<ffffffffc0479fb5>] radeon_vm_clear_freed+0x75/0xc0 [radeon]
May 19 17:03:44 linux.local kernel:  [<ffffffffc03c7c8d>] radeon_cs_ioctl+0x58d/0x7a0 [radeon]
May 19 17:03:44 linux.local kernel:  [<ffffffffc02969f7>] drm_ioctl+0x197/0x670 [drm]
May 19 17:03:44 linux.local kernel:  [<ffffffff8136ca74>] ? tomoyo_path_number_perm+0x64/0x1d0
May 19 17:03:44 linux.local kernel:  [<ffffffffc038e04c>] radeon_drm_ioctl+0x4c/0x80 [radeon]
May 19 17:03:44 linux.local kernel:  [<ffffffff811ccc00>] do_vfs_ioctl+0x2e0/0x4e0
May 19 17:03:44 linux.local kernel:  [<ffffffff811cce81>] SyS_ioctl+0x81/0xa0
May 19 17:03:44 linux.local kernel:  [<ffffffff816c864d>] system_call_fastpath+0x16/0x1b
May 19 17:03:44 linux.local kernel: Code: 85 c0 74 6d 31 db eb 0e 0f 1f 44 00 00 83 c3 01 41 3b 5f 10 73 5b 48 63 cb 4d 8b 6c cf 18 49 8b 75 40 83 e6
May 19 17:03:44 linux.local kernel: RIP  [<ffffffffc0376ebe>] ttm_bo_wait+0x7e/0x190 [ttm]
May 19 17:03:44 linux.local kernel:  RSP <ffff8801a577b9c8>
May 19 17:03:44 linux.local kernel: CR2: 0000000000000018
May 19 17:03:44 linux.local kernel: ---[ end trace b06ae4af91282299 ]---