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Subject[PATCH 4.9 83/93] drm/vc4: Fix race between page flip completion event and clean-up
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Derek Foreman <derekf@osg.samsung.com>

commit 26fc78f6fef39b9d7a15def5e7e9826ff68303f4 upstream.

There was a small window where a userspace program could submit
a pageflip after receiving a pageflip completion event yet still
receive EBUSY.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Cc: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/gpu/drm/vc4/vc4_crtc.c | 8 ++++++++
drivers/gpu/drm/vc4/vc4_drv.h | 1 +
drivers/gpu/drm/vc4/vc4_kms.c | 33 +++++++++++++++++++++++++--------
3 files changed, 34 insertions(+), 8 deletions(-)

--- a/drivers/gpu/drm/vc4/vc4_crtc.c
+++ b/drivers/gpu/drm/vc4/vc4_crtc.c
@@ -669,6 +669,14 @@ void vc4_disable_vblank(struct drm_devic
CRTC_WRITE(PV_INTEN, 0);
}

+/* Must be called with the event lock held */
+bool vc4_event_pending(struct drm_crtc *crtc)
+{
+ struct vc4_crtc *vc4_crtc = to_vc4_crtc(crtc);
+
+ return !!vc4_crtc->event;
+}
+
static void vc4_crtc_handle_page_flip(struct vc4_crtc *vc4_crtc)
{
struct drm_crtc *crtc = &vc4_crtc->base;
--- a/drivers/gpu/drm/vc4/vc4_drv.h
+++ b/drivers/gpu/drm/vc4/vc4_drv.h
@@ -440,6 +440,7 @@ int vc4_bo_stats_debugfs(struct seq_file
extern struct platform_driver vc4_crtc_driver;
int vc4_enable_vblank(struct drm_device *dev, unsigned int crtc_id);
void vc4_disable_vblank(struct drm_device *dev, unsigned int crtc_id);
+bool vc4_event_pending(struct drm_crtc *crtc);
int vc4_crtc_debugfs_regs(struct seq_file *m, void *arg);
int vc4_crtc_get_scanoutpos(struct drm_device *dev, unsigned int crtc_id,
unsigned int flags, int *vpos, int *hpos,
--- a/drivers/gpu/drm/vc4/vc4_kms.c
+++ b/drivers/gpu/drm/vc4/vc4_kms.c
@@ -119,17 +119,34 @@ static int vc4_atomic_commit(struct drm_

/* Make sure that any outstanding modesets have finished. */
if (nonblock) {
- ret = down_trylock(&vc4->async_modeset);
- if (ret) {
+ struct drm_crtc *crtc;
+ struct drm_crtc_state *crtc_state;
+ unsigned long flags;
+ bool busy = false;
+
+ /*
+ * If there's an undispatched event to send then we're
+ * obviously still busy. If there isn't, then we can
+ * unconditionally wait for the semaphore because it
+ * shouldn't be contended (for long).
+ *
+ * This is to prevent a race where queuing a new flip
+ * from userspace immediately on receipt of an event
+ * beats our clean-up and returns EBUSY.
+ */
+ spin_lock_irqsave(&dev->event_lock, flags);
+ for_each_crtc_in_state(state, crtc, crtc_state, i)
+ busy |= vc4_event_pending(crtc);
+ spin_unlock_irqrestore(&dev->event_lock, flags);
+ if (busy) {
kfree(c);
return -EBUSY;
}
- } else {
- ret = down_interruptible(&vc4->async_modeset);
- if (ret) {
- kfree(c);
- return ret;
- }
+ }
+ ret = down_interruptible(&vc4->async_modeset);
+ if (ret) {
+ kfree(c);
+ return ret;
}

ret = drm_atomic_helper_prepare_planes(dev, state);

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