pan/decode: Check for MFBD preload chicken bit

If this bit is clear, MFBD preload will be enabled, and you.. don't want
that. (At least, when the bit is clear, the old contents of the
framebuffer will be preserved. I'm assuming this is what "MFBD preload"
refers to in kbase.)

Validate that this bit is always set.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>

diff --git a/src/panfrost/pandecode/decode.c b/src/panfrost/pandecode/decode.c
index 9c29d87033236e3e31d6e607f8a8707ab1d56954..29bf2c2dc3a0782a7d3888cfa02e8c0dad885461 100644 (file)
--- a/src/panfrost/pandecode/decode.c
+++ b/src/panfrost/pandecode/decode.c
@@ -779,7 +779,16 @@ pandecode_rt_format(struct mali_rt_format format)
 
         pandecode_swizzle(format.swizzle);
 
-        pandecode_prop("no_preload = 0x%" PRIx32, format.no_preload);
+        /* In theory, the no_preload bit can be cleared to enable MFBD preload,
+         * which is a faster hardware-based alternative to the wallpaper method
+         * to preserve framebuffer contents across frames. In practice, MFBD
+         * preload is buggy on Midgard, and so this is a chicken bit. If this
+         * bit isn't set, most likely something broke unrelated to preload */
+
+        if (!format.no_preload) {
+                pandecode_msg("XXX: buggy MFBD preload enabled - chicken bit should be clear\n");
+                pandecode_prop("no_preload = 0x%" PRIx32, format.no_preload);
+        }
 
         if (format.zero)
                 pandecode_prop("zero = 0x%" PRIx32, format.zero);