linux-kernel - [PATCH v3] drm/crc-debugfs: Add notes about CRC<->commit interactions

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Message-Id: <20190806124622.28399-1-brian.starkey@arm.com>
Date:   Tue,  6 Aug 2019 13:46:22 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     dri-devel@...ts.freedesktop.org, nd@....com
Cc:     Liviu Dudau <Liviu.Dudau@....com>,
        "james qian wang (Arm Technology China)" <james.qian.wang@....com>,
        LKML <linux-kernel@...r.kernel.org>, ayan.halder@....com,
        Daniel Vetter <daniel@...ll.ch>
Subject: [PATCH v3] 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.

Changes since v1:
 - Clarified that anything that would disable CRCs counts as a full
   modeset, and so userspace needs to reconfigure after full modesets

Changes since v2:
 - Add these notes
 - Rebase onto drm-misc-next (trivial conflict in comment)

Signed-off-by: Brian Starkey <brian.starkey@....com>
Reviewed-by: Daniel Vetter <daniel.vetter@...ll.ch>
---
 drivers/gpu/drm/drm_debugfs_crc.c | 9 +++++++++
 include/drm/drm_crtc.h            | 4 ++++
 2 files changed, 13 insertions(+)

diff --git a/drivers/gpu/drm/drm_debugfs_crc.c b/drivers/gpu/drm/drm_debugfs_crc.c
index 6604ed223160..be1b7ba92ffe 100644
--- a/drivers/gpu/drm/drm_debugfs_crc.c
+++ b/drivers/gpu/drm/drm_debugfs_crc.c
@@ -69,6 +69,15 @@
  * implement &drm_crtc_funcs.set_crc_source and &drm_crtc_funcs.verify_crc_source.
  * The debugfs files are automatically set up if those vfuncs are 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