Mesa (staging/21.1): panfrost: Fix major flaw in BO cache

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Module: Mesa
Branch: staging/21.1
Commit: b6c9f39bc0c2a27fa5c8db6503fe40e1fa4e32b1
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=b6c9f39bc0c2a27fa5c8db6503fe40e1fa4e32b1

Author: Alyssa Rosenzweig <alyssa at collabora.com>
Date:   Thu May 13 17:24:31 2021 -0400

panfrost: Fix major flaw in BO cache

BOs in the cache are chronological, so we try oldest BOs first. That
means if we find the oldest BO is busy, likely every BO is busy, and we
should bail early. This dramatically reduces the useless cycles spent in
bo_wait.

I studied the BO cache of the following drivers, all of which handle
this correctly: iris, lima, etnaviv, freedreno, vc4, v3d, v3dv.

Signed-off-by: Alyssa Rosenzweig <alyssa at collabora.com>
Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10794>
(cherry picked from commit 77d04989135233c7b276bca3912ab07a12d9f362)

---

 .pick_status.json         | 2 +-
 src/panfrost/lib/pan_bo.c | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/.pick_status.json b/.pick_status.json
index e7d76c25baf..062f79d942b 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -13,7 +13,7 @@
         "description": "panfrost: Fix major flaw in BO cache",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "master_sha": null,
         "because_sha": null
     },
diff --git a/src/panfrost/lib/pan_bo.c b/src/panfrost/lib/pan_bo.c
index dbfce02e519..8e9e257a812 100644
--- a/src/panfrost/lib/pan_bo.c
+++ b/src/panfrost/lib/pan_bo.c
@@ -201,9 +201,11 @@ panfrost_bo_cache_fetch(struct panfrost_device *dev,
                 if (entry->size < size || entry->flags != flags)
                         continue;
 
+                /* If the oldest BO in the cache is busy, likely so is
+                 * everything newer, so bail. */
                 if (!panfrost_bo_wait(entry, dontwait ? 0 : INT64_MAX,
                                       PAN_BO_ACCESS_RW))
-                        continue;
+                        break;
 
                 struct drm_panfrost_madvise madv = {
                         .handle = entry->gem_handle,