linux-kernel - [PATCH] drm/msm: Fix dirtyfb refcounting

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Date:   Fri,  4 Mar 2022 12:21:45 -0800
From:   Rob Clark <robdclark@...il.com>
To:     dri-devel@...ts.freedesktop.org
Cc:     freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        Rob Clark <robdclark@...omium.org>,
        Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] drm/msm: Fix dirtyfb refcounting

From: Rob Clark <robdclark@...omium.org>

refcount_t complains about 0->1 transitions, which isn't *quite* what we
wanted.  So use dirtyfb==1 to mean that the fb is not connected to any
output that requires dirtyfb flushing, so that we can keep the underflow
and overflow checking.

Fixes: 9e4dde28e9cd ("drm/msm: Avoid dirtyfb stalls on video mode displays (v2)")
Signed-off-by: Rob Clark <robdclark@...omium.org>
---
 drivers/gpu/drm/msm/msm_fb.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/msm/msm_fb.c b/drivers/gpu/drm/msm/msm_fb.c
index 96b379a08327..7137492fe78e 100644
--- a/drivers/gpu/drm/msm/msm_fb.c
+++ b/drivers/gpu/drm/msm/msm_fb.c
@@ -37,7 +37,7 @@ static int msm_framebuffer_dirtyfb(struct drm_framebuffer *fb,
 	/* If this fb is not used on any display requiring pixel data to be
 	 * flushed, then skip dirtyfb
 	 */
-	if (refcount_read(&msm_fb->dirtyfb) == 0)
+	if (refcount_read(&msm_fb->dirtyfb) == 1)
 		return 0;
 
 	return drm_atomic_helper_dirtyfb(fb, file_priv, flags, color,
@@ -221,6 +221,8 @@ static struct drm_framebuffer *msm_framebuffer_init(struct drm_device *dev,
 		goto fail;
 	}
 
+	refcount_set(&msm_fb->dirtyfb, 1);
+
 	drm_dbg_state(dev, "create: FB ID: %d (%p)", fb->base.id, fb);
 
 	return fb;
-- 
2.35.1