This patch prevents division by zero htotal.

In a follow-up mail Tina writes:

> > How did you manage to get here with htotal == 0? This needs backtraces (or if
> > this is just about static checkers, a mention of that).
> > -Daniel
>
> In GVT-g, we are trying to enable a virtual display w/o setting timings for a pipe
> (a.k.a htotal=0), then we met the following kernel panic:
>
> [   32.832048] divide error: 0000 [#1] SMP PTI
> [   32.833614] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.18.0-rc4-sriov+ #33
> [   32.834438] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.10.1-0-g8891697-dirty-20180511_165818-tinazhang-linux-1 04/01/2014
> [   32.835901] RIP: 0010:drm_mode_hsync+0x1e/0x40
> [   32.836004] Code: 31 c0 c3 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 8b 87 d8 00 00 00 85 c0 75 22 8b 4f 68 85 c9 78 1b 69 47 58 e8 03 00 00 99 <f7> f9 b9 d3 4d 62 10 05 f4 01 00 00 f7 e1 89 d0 c1 e8 06 f3 c3 66
> [   32.836004] RSP: 0000:ffffc900000ebb90 EFLAGS: 00010206
> [   32.836004] RAX: 0000000000000000 RBX: ffff88001c67c8a0 RCX: 0000000000000000
> [   32.836004] RDX: 0000000000000000 RSI: ffff88001c67c000 RDI: ffff88001c67c8a0
> [   32.836004] RBP: ffff88001c7d03a0 R08: ffff88001c67c8a0 R09: ffff88001c7d0330
> [   32.836004] R10: ffffffff822c3a98 R11: 0000000000000001 R12: ffff88001c67c000
> [   32.836004] R13: ffff88001c7d0370 R14: ffffffff8207eb78 R15: ffff88001c67c800
> [   32.836004] FS:  0000000000000000(0000) GS:ffff88001da00000(0000) knlGS:0000000000000000
> [   32.836004] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   32.836004] CR2: 0000000000000000 CR3: 000000000220a000 CR4: 00000000000006f0
> [   32.836004] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   32.836004] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [   32.836004] Call Trace:
> [   32.836004]  intel_mode_from_pipe_config+0x72/0x90
> [   32.836004]  intel_modeset_setup_hw_state+0x569/0xf90
> [   32.836004]  intel_modeset_init+0x905/0x1db0
> [   32.836004]  i915_driver_load+0xb8c/0x1120
> [   32.836004]  i915_pci_probe+0x4d/0xb0
> [   32.836004]  local_pci_probe+0x44/0xa0
> [   32.836004]  ? pci_assign_irq+0x27/0x130
> [   32.836004]  pci_device_probe+0x102/0x1c0
> [   32.836004]  driver_probe_device+0x2b8/0x480
> [   32.836004]  __driver_attach+0x109/0x110
> [   32.836004]  ? driver_probe_device+0x480/0x480
> [   32.836004]  bus_for_each_dev+0x67/0xc0
> [   32.836004]  ? klist_add_tail+0x3b/0x70
> [   32.836004]  bus_add_driver+0x1e8/0x260
> [   32.836004]  driver_register+0x5b/0xe0
> [   32.836004]  ? mipi_dsi_bus_init+0x11/0x11
> [   32.836004]  do_one_initcall+0x4d/0x1eb
> [   32.836004]  kernel_init_freeable+0x197/0x237
> [   32.836004]  ? rest_init+0xd0/0xd0
> [   32.836004]  kernel_init+0xa/0x110
> [   32.836004]  ret_from_fork+0x35/0x40
> [   32.836004] Modules linked in:
> [   32.859183] ---[ end trace 525608b0ed0e8665 ]---
> [   32.859722] RIP: 0010:drm_mode_hsync+0x1e/0x40
> [   32.860287] Code: 31 c0 c3 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 8b 87 d8 00 00 00 85 c0 75 22 8b 4f 68 85 c9 78 1b 69 47 58 e8 03 00 00 99 <f7> f9 b9 d3 4d 62 10 05 f4 01 00 00 f7 e1 89 d0 c1 e8 06 f3 c3 66
> [   32.862680] RSP: 0000:ffffc900000ebb90 EFLAGS: 00010206
> [   32.863309] RAX: 0000000000000000 RBX: ffff88001c67c8a0 RCX: 0000000000000000
> [   32.864182] RDX: 0000000000000000 RSI: ffff88001c67c000 RDI: ffff88001c67c8a0
> [   32.865206] RBP: ffff88001c7d03a0 R08: ffff88001c67c8a0 R09: ffff88001c7d0330
> [   32.866359] R10: ffffffff822c3a98 R11: 0000000000000001 R12: ffff88001c67c000
> [   32.867213] R13: ffff88001c7d0370 R14: ffffffff8207eb78 R15: ffff88001c67c800
> [   32.868075] FS:  0000000000000000(0000) GS:ffff88001da00000(0000) knlGS:0000000000000000
> [   32.868983] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   32.869659] CR2: 0000000000000000 CR3: 000000000220a000 CR4: 00000000000006f0
> [   32.870599] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [   32.871598] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [   32.872549] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
>
> Since drm_mode_hsync() has the logic to check mode->htotal, I just extend it to cover the case htotal==0.

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Cc: Adam Jackson <ajax@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
[danvet: Add additional explanations + cc: stable.]
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1548228539-3061-1-git-send-email-tina.zhang@intel.com