From ada4f07bbb85b3d30ac512dd6c3fcaa3e7df5136 Mon Sep 17 00:00:00 2001
From: Bryce Harrington <bryce@osg.samsung.com>
Date: Tue, 30 Jun 2015 13:25:46 -0700
Subject: [PATCH] compositor-drm: Add comment for newly added
drm_waitvblank_pipe()
---
src/compositor-drm.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/src/compositor-drm.c b/src/compositor-drm.c
index aa6d010bd..f24fa0b48 100644
--- a/src/compositor-drm.c
+++ b/src/compositor-drm.c
@@ -597,6 +597,19 @@ drm_output_set_gamma(struct weston_output *output_base,
weston_log("set gamma failed: %m\n");
}
+/* Determine the type of vblank synchronization to use for the output.
+ *
+ * The pipe parameter indicates which CRTC is in use. Knowing this, we
+ * can determine which vblank sequence type to use for it. Traditional
+ * cards had only two CRTCs, with CRTC 0 using no special flags, and
+ * CRTC 1 using DRM_VBLANK_SECONDARY. The first bit of the pipe
+ * parameter indicates this.
+ *
+ * Bits 1-5 of the pipe parameter are 5 bit wide pipe number between
+ * 0-31. If this is non-zero it indicates we're dealing with a
+ * multi-gpu situation and we need to calculate the vblank sync
+ * using DRM_BLANK_HIGH_CRTC_MASK.
+ */
static unsigned int drm_waitvblank_pipe(struct drm_output *output)
{
if (output->pipe > 1)
--
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