i965: Use finish_external instead of make_shareable in setTexBuffer2

The setTexBuffer2 hook from GLX is used to implement glxBindTexImageEXT
which has tighter restrictions than just "it's shared".  In particular,
it says that any rendering to the image while it is bound causes the
contents to become undefined.

The GLX_EXT_texture_from_pixmap extension provides us with an acquire
and release in the form of glXBindTexImageEXT and glXReleaseTexImageEXT.
The extension spec says,

    "Rendering to the drawable while it is bound to a texture will leave
    the contents of the texture in an undefined state.  However, no
    synchronization between rendering and texturing is done by GLX.  It
    is the application's responsibility to implement any synchronization
    required."

From the EGL 1.4 spec for eglBindTexImage:

    "After eglBindTexImage is called, the specified surface is no longer
    available for reading or writing.  Any read operation, such as
    glReadPixels or eglCopyBuffers, which reads values from any of the
    surface’s color buffers or ancillary buffers will produce
    indeterminate results.  In addition, draw operations that are done
    to the surface before its color buffer is released from the texture
    produce indeterminate results

In other words, between the bind and release calls, we effectively own
those pixels and can assume, so long as we don't crash, that no one else
is reading from/writing to the surface.  The GLX and EGL implementations
call the setTexBuffer2 and releaseTexBuffer function pointers that the
driver can hook.

In theory, this means that, between BindTexImage and ReleaseTexImage, we
own the pixels and it should be safe to track aux usage so we
can avoid redundant resolves so long as we start off with the right
assumption at the start of the bind/release pair.

In practice, however, X11 has slightly different expectations.  It's
expected that the server may be drawing to the image at the same time as
the compositor is texturing from it.  In that case, the worst expected
outcome should be tearing or partial rendering and not random corruption
like we see when rendering races with scanout with CCS.  Fortunately,
the GEM rules about texture/render dependencies save us here.  If X11
submits work to write to a pixmap after the compositor has submitted
work to texture from it, GEM inserts a dependency between the compositor
and X11.  If X11 is using a high-priority context, this will cause the
compositor to get a temporarily boosted priority while the batch from
X11 is waiting on it.  This means that we will never have an actual race
between X11 and the compositor so no corruption can happen.

Unfortunately, however, this means that X11 will likely be rendering to it
between the compositor's BindTexImage and ReleaseTexImage calls.  If we
want to avoid strange issues, we need to be a bit careful about
resolves because we can't really transition it away from the "default"
aux usage.  The only case where this would practically be a problem is
with image_load_store where we have to do a full resolve in order to use
the image via the data port.  Even there it would only be a problem if
batches were split such that X11's rendering happens between the resolve
and the use of it as a storage image.  However, the chances of this
happening are very slim so we just emit a warning and hope for the best.

This commit adds a new helper intel_miptree_finish_external which resets
all aux state to whatever ISL says is the right worst-case "default" for
the given modifier.  It feels a little awkward to call it "finish"
because it's actually an acquire from the perspective of the driver, but
it matches the semantics of the other prepare/finish functions.  This
new helper gets called in intelSetTexBuffer2 instead of make_shareable.
We also add an intelReleaseTexBuffer (we passed NULL to releaseTexBuffer
before) and call intel_miptree_prepare_external in it.  This probably
does nothing most of the time but it means that the prepare/finish calls
are properly matched.

Reviewed-by: Kenneth Graunke <[email protected]>
Reviewed-by: Chad Versace <[email protected]>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/mesa/drivers/dri/i965/intel_mipmap_tree.h b/src/mesa/drivers/dri/i965/intel_mipmap_tree.h
index 7fcf09f118a..07c85807e80 100644
--- a/src/mesa/drivers/dri/i965/intel_mipmap_tree.h
+++ b/src/mesa/drivers/dri/i965/intel_mipmap_tree.h
@@ -675,6 +675,9 @@ intel_miptree_finish_depth(struct brw_context *brw,
 void
 intel_miptree_prepare_external(struct brw_context *brw,
                                struct intel_mipmap_tree *mt);
+void
+intel_miptree_finish_external(struct brw_context *brw,
+                              struct intel_mipmap_tree *mt);
 
 void
 intel_miptree_make_shareable(struct brw_context *brw,