drm/i915: Handle full s64 precision for wait-ioctl

The wait-ioctl is optionally supplied a timeout with nanosecond
precision in a s64 field. We use nsecs_to_jiffies64() to convert that
into the jiffies consumed by the scheduler, but internally
nsecs_to_jiffies64() does not guard against overflow (as it's purpose is
for use by the scheduler and not drivers!). So we must guard against the
overflow ourselves, and in the process note that we may then return
much earlier than the timeout selected by the user, so don't report
ETIME unless we do hit the timeout. (Woe betold us though if the user
waits for a year (32bit) and the request is still not complete!)

v2: Refine overflow detection (to not include an overffow itself)

Reported-by: Jason Ekstrand <jason.ekstrand@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170811105731.9482-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -4176,6 +4176,11 @@ static inline unsigned long msecs_to_jiffies_timeout(const unsigned int m)
 
 static inline unsigned long nsecs_to_jiffies_timeout(const u64 n)
 {
+	/* nsecs_to_jiffies64() does not guard against overflow */
+	if (NSEC_PER_SEC % HZ &&
+	    div_u64(n, NSEC_PER_SEC) >= MAX_JIFFY_OFFSET / HZ)
+		return MAX_JIFFY_OFFSET;
+
         return min_t(u64, MAX_JIFFY_OFFSET, nsecs_to_jiffies64(n) + 1);
 }
 

drivers/gpu/drm/i915/i915_gem.c

@@ -3286,7 +3286,7 @@ static unsigned long to_wait_timeout(s64 timeout_ns)
  *  -ERESTARTSYS: signal interrupted the wait
  *  -ENONENT: object doesn't exist
  * Also possible, but rare:
- *  -EAGAIN: GPU wedged
+ *  -EAGAIN: incomplete, restart syscall
  *  -ENOMEM: damn
  *  -ENODEV: Internal IRQ fail
  *  -E?: The add request failed
@@ -3334,6 +3334,10 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 		 */
 		if (ret == -ETIME && !nsecs_to_jiffies(args->timeout_ns))
 			args->timeout_ns = 0;
+
+		/* Asked to wait beyond the jiffie/scheduler precision? */
+		if (ret == -ETIME && args->timeout_ns)
+			ret = -EAGAIN;
 	}
 
 	i915_gem_object_put(obj);