drm/i915: prevent division by zero when asking for chipset power

commit 4ed0b577457eb6aeb7cdc7e7316576e63d15abb2 upstream.

This prevents an in-kernel division by zero which happens when we are
asking for i915_chipset_val too quickly, or within a race condition
between the power monitoring thread and userspace accesses via debugfs.

The issue can be reproduced easily via the following command:
while ``; do cat /sys/kernel/debug/dri/0/i915_emon_status; done

This is particularly dangerous because it can be triggered by
a non-privileged user by just reading the debugfs entry.

This issue was also found independently by Konstantin Belousov
<kostikbel@gmail.com>, who proposed a similar patch.

Reported-by: Konstantin Belousov <kostikbel@gmail.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Change-Id: Ic86534499300b66168ad29ac08ab1e1f987de909
Reviewed-on: http://git-master/r/74160
Reviewed-by: Varun Wadekar <vwadekar@nvidia.com>
Tested-by: Varun Wadekar <vwadekar@nvidia.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 7916bd97d5c1..1a2a2d1790b6 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -707,6 +707,7 @@ typedef struct drm_i915_private {
 
 	u64 last_count1;
 	unsigned long last_time1;
+	unsigned long chipset_power;
 	u64 last_count2;
 	struct timespec last_time2;
 	unsigned long gfx_power;