lib/igt_kms: Fix igt_display_drop_events() to work as intended

Short reads don't work, you either read the whole event or nothing
at all, so follow the recommendations in the documentation and
allocate a buffer of 4096 bytes for the event, if we capture
multiple events read them all.

Cc: Jari Tahvanainen <jari.tahvanainen@intel.com>
Reported-by: Jari Tahvanainen <jari.tahvanainen@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104538
[mlankhorst: Assign ev once in the loop, instead of 2 places. (ickle)]
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

1 files changed, 14 insertions, 8 deletions

diff --git a/lib/igt_kms.c b/lib/igt_kms.c
index ecc7a799..cc4a1f72 100644
--- a/lib/igt_kms.c
+++ b/lib/igt_kms.c
@@ -3152,14 +3152,20 @@ int igt_display_drop_events(igt_display_t *display)
 	};
 
 	while (poll(&pfd, 1, 0) > 0) {
-		struct drm_event ev;
-		char buf[128];
-
-		read(display->drm_fd, &ev, sizeof(ev));
-		igt_info("Dropping event type %u length %u\n", ev.type, ev.length);
-		igt_assert(ev.length <= sizeof(buf));
-		read(display->drm_fd, buf, ev.length);
-		ret++;
+		struct drm_event *ev;
+		char buf[4096];
+		ssize_t retval;
+
+		retval = read(display->drm_fd, &buf, sizeof(buf));
+		igt_assert_lt(0, retval);
+
+		for (int i = 0; i < retval; i += ev->length) {
+			ev = (struct drm_event *)&buf[i];
+
+			igt_info("Dropping event type %u length %u\n", ev->type, ev->length);
+			igt_assert(ev->length + i <= sizeof(buf));
+			ret++;
+		}
 	}
 
 	return ret;