linux-kernel - [PATCH v2] drm/crc-debugfs: Add notes about CRC<->commit interactions

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Message-Id: <20190806104835.26075-1-brian.starkey@arm.com>
Date:   Tue,  6 Aug 2019 11:48:35 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Liviu Dudau <Liviu.Dudau@....com>,
        "james qian wang (Arm Technology China)" <james.qian.wang@....com>,
        "Lowry Li (Arm Technology China)" <Lowry.Li@....com>, nd@....com,
        Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel@...ts.freedesktop.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH v2] drm/crc-debugfs: Add notes about CRC<->commit interactions

CRC generation can be impacted by commits coming from userspace, and
enabling CRC generation may itself trigger a commit. Add notes about
this to the kerneldoc.

Signed-off-by: Brian Starkey <brian.starkey@....com>
---
 drivers/gpu/drm/drm_debugfs_crc.c | 17 +++++++++++++----
 include/drm/drm_crtc.h            |  4 ++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_debugfs_crc.c b/drivers/gpu/drm/drm_debugfs_crc.c
index 7ca486d750e9..77159b6e77c3 100644
--- a/drivers/gpu/drm/drm_debugfs_crc.c
+++ b/drivers/gpu/drm/drm_debugfs_crc.c
@@ -65,10 +65,19 @@
  * it submits. In this general case, the maximum userspace can do is to compare
  * the reported CRCs of frames that should have the same contents.
  *
- * On the driver side the implementation effort is minimal, drivers only need to
- * implement &drm_crtc_funcs.set_crc_source. The debugfs files are automatically
- * set up if that vfunc is set. CRC samples need to be captured in the driver by
- * calling drm_crtc_add_crc_entry().
+ * On the driver side the implementation effort is minimal, drivers only need
+ * to implement &drm_crtc_funcs.set_crc_source. The debugfs files are
+ * automatically set up if that vfunc is set. CRC samples need to be captured
+ * in the driver by calling drm_crtc_add_crc_entry(). Depending on the driver
+ * and HW requirements, &drm_crtc_funcs.set_crc_source may result in a commit
+ * (even a full modeset).
+ *
+ * CRC results must be reliable across non-full-modeset atomic commits, so if a
+ * commit via DRM_IOCTL_MODE_ATOMIC would disable or otherwise interfere with
+ * CRC generation, then the driver must mark that commit as a full modeset
+ * (drm_atomic_crtc_needs_modeset() should return true). As a result, to ensure
+ * consistent results, generic userspace must re-setup CRC generation after a
+ * legacy SETCRTC or an atomic commit with DRM_MODE_ATOMIC_ALLOW_MODESET.
  */
 
 static int crc_control_show(struct seq_file *m, void *data)
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index 128d8b210621..7d14c11bdc0a 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -756,6 +756,9 @@ struct drm_crtc_funcs {
 	 * provided from the configured source. Drivers must accept an "auto"
 	 * source name that will select a default source for this CRTC.
 	 *
+	 * This may trigger an atomic modeset commit if necessary, to enable CRC
+	 * generation.
+	 *
 	 * Note that "auto" can depend upon the current modeset configuration,
 	 * e.g. it could pick an encoder or output specific CRC sampling point.
 	 *
@@ -767,6 +770,7 @@ struct drm_crtc_funcs {
 	 * 0 on success or a negative error code on failure.
 	 */
 	int (*set_crc_source)(struct drm_crtc *crtc, const char *source);
+
 	/**
 	 * @verify_crc_source:
 	 *
-- 
2.17.1