[Mesa-dev] [PATCH 2/2] RFC i965: Share the GTT between non-robust contexts

[Chris Wilson]

Every client (everyone instance that opens /dev/dri/card0 or the render
nodes), receives a unique per-process GTT (where supported by the
hardware, unfortunately that means only Broadwell and later). Every
context created by each client, in turns receives its own unique ppGTT.
This is overkill in terms of allocations and tracking, both in the
kernel and in the hardware, as we could be sharing the per-client GTT
amongst all of its contexts. The downside is that context segregation is
reduced, a stray write from one context may affect another, and so we
must honour any client requests that require robust segregation (e.g.
ARB_robustness).

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 src/mesa/drivers/dri/i965/brw_bufmgr.c  | 23 +++++++++++++++++------
 src/mesa/drivers/dri/i965/brw_bufmgr.h  |  2 +-
 src/mesa/drivers/dri/i965/brw_context.c |  2 +-
 3 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_bufmgr.c b/src/mesa/drivers/dri/i965/brw_bufmgr.c
index 52b5bf97a1..d8a9635f5d 100644
--- a/src/mesa/drivers/dri/i965/brw_bufmgr.c
+++ b/src/mesa/drivers/dri/i965/brw_bufmgr.c
@@ -1297,13 +1297,24 @@ init_cache_buckets(struct brw_bufmgr *bufmgr)
 }
 
 uint32_t
-brw_create_hw_context(struct brw_bufmgr *bufmgr)
+brw_create_hw_context(struct brw_bufmgr *bufmgr, unsigned int flags)
 {
-   struct drm_i915_gem_context_create create = { };
-   int ret = drmIoctl(bufmgr->fd, DRM_IOCTL_I915_GEM_CONTEXT_CREATE, &create);
-   if (ret != 0) {
-      DBG("DRM_IOCTL_I915_GEM_CONTEXT_CREATE failed: %s\n", strerror(errno));
-      return 0;
+   struct local_i915_gem_context_create_v2 {
+      uint32_t ctx_id; /* out */
+      uint32_t flags;
+#define I915_GEM_CONTEXT_SHARE_GTT 0x1
+      uint32_t share_ctx;
+      uint32_t pad;
+   } create =  { };
+#define LOCAL_IOCTL_I915_GEM_CONTEXT_CREATE      DRM_IOWR (DRM_COMMAND_BASE + DRM_I915_GEM_CONTEXT_CREATE, struct local_i915_gem_context_create_v2)
+
+   if (!(flags & __DRI_CTX_FLAG_ROBUST_BUFFER_ACCESS))
+      create.flags |= I915_GEM_CONTEXT_SHARE_GTT;
+
+   if (drmIoctl(bufmgr->fd, LOCAL_IOCTL_I915_GEM_CONTEXT_CREATE, &create)) {
+      create.flags = 0;
+      if (drmIoctl(bufmgr->fd, LOCAL_IOCTL_I915_GEM_CONTEXT_CREATE, &create))
+         return 0;
    }
 
    return create.ctx_id;
diff --git a/src/mesa/drivers/dri/i965/brw_bufmgr.h b/src/mesa/drivers/dri/i965/brw_bufmgr.h
index 0ae541cda0..f5191aff76 100644
--- a/src/mesa/drivers/dri/i965/brw_bufmgr.h
+++ b/src/mesa/drivers/dri/i965/brw_bufmgr.h
@@ -321,7 +321,7 @@ void brw_bufmgr_enable_reuse(struct brw_bufmgr *bufmgr);
 
 int brw_bo_wait(struct brw_bo *bo, int64_t timeout_ns);
 
-uint32_t brw_create_hw_context(struct brw_bufmgr *bufmgr);
+uint32_t brw_create_hw_context(struct brw_bufmgr *bufmgr, unsigned int flags);
 
 #define BRW_CONTEXT_LOW_PRIORITY ((I915_CONTEXT_MIN_USER_PRIORITY-1)/2)
 #define BRW_CONTEXT_MEDIUM_PRIORITY (I915_CONTEXT_DEFAULT_PRIORITY)
diff --git a/src/mesa/drivers/dri/i965/brw_context.c b/src/mesa/drivers/dri/i965/brw_context.c
index 9e0f875b27..e374236b6b 100644
--- a/src/mesa/drivers/dri/i965/brw_context.c
+++ b/src/mesa/drivers/dri/i965/brw_context.c
@@ -986,7 +986,7 @@ brwCreateContext(gl_api api,
     * This is required for transform feedback buffer offsets, query objects,
     * and also allows us to reduce how much state we have to emit.
     */
-   brw->hw_ctx = brw_create_hw_context(brw->bufmgr);
+   brw->hw_ctx = brw_create_hw_context(brw->bufmgr, ctx_config->flags);
    if (!brw->hw_ctx && devinfo->gen >= 6) {
       fprintf(stderr, "Failed to create hardware context.\n");
       intelDestroyContext(driContextPriv);
-- 
2.15.1