rhi: Do not rely on unspecified relation between readbacks and FramesInFlight

Add a new queriable resource limit value MaxAsyncReadbackFrames. Change
the autotest to rely on this instead of relying on the unspecified,
works-by-accident relation between readbacks and FramesInFlight. This
way even if the behavior diverges in some backend in the future, clients
(well written ones, that is), will continue to function correctly.

Also clarify the docs for FramesInFlight, and change d3d and gl to return
the correct value (which is 1 from QRhi perspective; the expanded docs now
explain a bit what this really means and what it does not).

Change-Id: I0486715570a9e6fc5d3dc431694d1712875dfe01
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

1 files changed, 5 insertions, 5 deletions

diff --git a/tests/auto/gui/rhi/qrhi/tst_qrhi.cpp b/tests/auto/gui/rhi/qrhi/tst_qrhi.cpp
index ee6ac950c9..ab435caa40 100644
--- a/tests/auto/gui/rhi/qrhi/tst_qrhi.cpp
+++ b/tests/auto/gui/rhi/qrhi/tst_qrhi.cpp
@@ -1672,9 +1672,9 @@ void tst_QRhi::renderToWindowSimple()
 
     QVERIFY(pipeline->build());
 
-    const int framesInFlight = rhi->resourceLimit(QRhi::FramesInFlight);
-    QVERIFY(framesInFlight >= 1);
-    const int FRAME_COUNT = framesInFlight + 1;
+    const int asyncReadbackFrames = rhi->resourceLimit(QRhi::MaxAsyncReadbackFrames);
+    // one frame issues the readback, then we do MaxAsyncReadbackFrames more to ensure the readback completes
+    const int FRAME_COUNT = asyncReadbackFrames + 1;
     bool readCompleted = false;
     QRhiReadbackResult readResult;
     QImage result;
@@ -1721,8 +1721,8 @@ void tst_QRhi::renderToWindowSimple()
     }
 
     // The readback is asynchronous here. However it is guaranteed that it
-    // finished at latest after rendering QRhi::FramesInFlight frames after the
-    // one that enqueues the readback.
+    // finished at latest after rendering QRhi::MaxAsyncReadbackFrames frames
+    // after the one that enqueues the readback.
     QVERIFY(readCompleted);
     QVERIFY(readbackWidth > 0);