[Mesa-dev] [PATCH] st/dri: Add an option to always request a front buffer from the loader

[Thomas Hellstrom]

When an application decides to read from the front buffer of a window,
typically a fake front is created and initialized with the real front window
contents. However, if there was a window manager reparenting operation between
the last swapbuffer and the read the real front window contents would be
invalid. This hurts piglit applications that read from the front.

So add an option to always request a front buffer from the dri loader. This
makes the fake front initialization typically happen before any drawing- or
swapbuffer operations take place and, as a result, piglit results from tests
that read from the frontbuffer become robust with dri3.
While there is a memory usage overhead with this option enabled, the
performance overhead with dri3 should be minimal.

With dri2 the situation is different and require more work.

Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
---
 .../auxiliary/pipe-loader/driinfo_gallium.h        |  1 +
 src/gallium/state_trackers/dri/dri2.c              | 22 +++++++++++++++++++++-
 src/gallium/state_trackers/dri/dri_screen.c        |  2 ++
 src/gallium/state_trackers/dri/dri_screen.h        |  1 +
 src/util/xmlpool/t_options.h                       |  6 ++++++
 5 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/src/gallium/auxiliary/pipe-loader/driinfo_gallium.h b/src/gallium/auxiliary/pipe-loader/driinfo_gallium.h
index 48a57c9..fe69570 100644
--- a/src/gallium/auxiliary/pipe-loader/driinfo_gallium.h
+++ b/src/gallium/auxiliary/pipe-loader/driinfo_gallium.h
@@ -32,4 +32,5 @@ DRI_CONF_SECTION_END
 DRI_CONF_SECTION_MISCELLANEOUS
    DRI_CONF_ALWAYS_HAVE_DEPTH_BUFFER("false")
    DRI_CONF_GLSL_ZERO_INIT("false")
+   DRI_CONF_ALWAYS_REQUEST_FRONT_BUFFER("false")
 DRI_CONF_SECTION_END
diff --git a/src/gallium/state_trackers/dri/dri2.c b/src/gallium/state_trackers/dri/dri2.c
index 1e8bb48..8398888 100644
--- a/src/gallium/state_trackers/dri/dri2.c
+++ b/src/gallium/state_trackers/dri/dri2.c
@@ -312,6 +312,8 @@ dri2_drawable_get_buffers(struct dri_drawable *drawable,
    int num_buffers;
    unsigned attachments[10];
    unsigned num_attachments, i;
+   boolean have_front = FALSE;
+   int back_depth = 0;
 
    assert(loader);
    with_format = dri_with_format(drawable->sPriv);
@@ -319,8 +321,10 @@ dri2_drawable_get_buffers(struct dri_drawable *drawable,
    num_attachments = 0;
 
    /* for Xserver 1.6.0 (DRI2 version 1) we always need to ask for the front */
-   if (!with_format)
+   if (!with_format) {
       attachments[num_attachments++] = __DRI_BUFFER_FRONT_LEFT;
+      have_front = TRUE;
+   }
 
    for (i = 0; i < *count; i++) {
       enum pipe_format format;
@@ -333,6 +337,7 @@ dri2_drawable_get_buffers(struct dri_drawable *drawable,
 
       switch (atts[i]) {
       case ST_ATTACHMENT_FRONT_LEFT:
+         have_front = TRUE;
          /* already added */
          if (!with_format)
             continue;
@@ -376,6 +381,17 @@ dri2_drawable_get_buffers(struct dri_drawable *drawable,
       if (with_format) {
          attachments[num_attachments++] = depth;
       }
+
+      if (att == __DRI_BUFFER_BACK_LEFT)
+         back_depth = depth;
+   }
+
+   if (dri_screen(drawable->sPriv)->always_request_front &&
+       !have_front &&
+       back_depth != 0) {
+      attachments[num_attachments++] = __DRI_BUFFER_FRONT_LEFT;
+      if (with_format)
+         attachments[num_attachments++] = back_depth;
    }
 
    if (with_format) {
@@ -449,6 +465,10 @@ dri_image_drawable_get_buffers(struct dri_drawable *drawable,
       }
    }
 
+   if (dri_screen(sPriv)->always_request_front &&
+       image_format != __DRI_IMAGE_FORMAT_NONE)
+      buffer_mask |= __DRI_IMAGE_BUFFER_FRONT;
+
    return (*sPriv->image.loader->getBuffers) (dPriv, image_format,
                                        (uint32_t *) &drawable->base.stamp,
                                        dPriv->loaderPrivate, buffer_mask,
diff --git a/src/gallium/state_trackers/dri/dri_screen.c b/src/gallium/state_trackers/dri/dri_screen.c
index 1d9f441..eeb125d 100644
--- a/src/gallium/state_trackers/dri/dri_screen.c
+++ b/src/gallium/state_trackers/dri/dri_screen.c
@@ -485,6 +485,8 @@ dri_init_options(struct dri_screen *screen)
    pipe_loader_load_options(screen->dev);
 
    dri_fill_st_options(screen);
+   screen->always_request_front = driQueryOptionb(&screen->dev->option_cache,
+                                                  "always_request_front");
 }
 
 const __DRIconfig **
diff --git a/src/gallium/state_trackers/dri/dri_screen.h b/src/gallium/state_trackers/dri/dri_screen.h
index 677e945..5b076ce 100644
--- a/src/gallium/state_trackers/dri/dri_screen.h
+++ b/src/gallium/state_trackers/dri/dri_screen.h
@@ -59,6 +59,7 @@ struct dri_screen
    __DRIscreen *sPriv;
    boolean throttling_enabled;
    int default_throttle_frames;
+   boolean always_request_front;
 
    struct st_config_options options;
 
diff --git a/src/util/xmlpool/t_options.h b/src/util/xmlpool/t_options.h
index d3f31fc..2744024 100644
--- a/src/util/xmlpool/t_options.h
+++ b/src/util/xmlpool/t_options.h
@@ -380,6 +380,12 @@ DRI_CONF_OPT_BEGIN_B(glsl_zero_init, def) \
         DRI_CONF_DESC(en,gettext("Force uninitialized variables to default to zero")) \
 DRI_CONF_OPT_END
 
+#define DRI_CONF_ALWAYS_REQUEST_FRONT_BUFFER(def) \
+DRI_CONF_OPT_BEGIN_B(always_request_front, def) \
+        DRI_CONF_DESC(en,"Always request a front buffer from the dri loader. This makes dri3 always maintain a fake front buffer for window drawables, which in turn makes frontbuffer reads in, for example, piglit more reliable.") \
+DRI_CONF_OPT_END
+
+
 /**
  * \brief Initialization configuration options
  */
-- 
2.7.4