i965: Don't use PREAD for glGetBufferSubData().

Just map the buffer and memcpy.  This will do a CPU mmap, which should
be reasonably efficient, and doing this gives us full control over the
domains and caching instead of leaving it to the kernel.

This prevents regressions on Braswell in the next commit.  Specifically
GL45-CTS.shader_atomic_counters.basic-buffer-operations.  Because async
maps start skipping set-domain, the pread thought everything was nicely
still in the CPU domain, and returned stale data.

v2: Use _mesa_error_no_memory() if the map fails instead of crashing.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

src/mesa/drivers/dri/i965/brw_bufmgr.c

@@ -820,30 +820,6 @@ brw_bo_subdata(struct brw_bo *bo, uint64_t offset,
    return ret;
 }
 
-int
-brw_bo_get_subdata(struct brw_bo *bo, uint64_t offset,
-                   uint64_t size, void *data)
-{
-   struct brw_bufmgr *bufmgr = bo->bufmgr;
-   struct drm_i915_gem_pread pread;
-   int ret;
-
-   memclear(pread);
-   pread.handle = bo->gem_handle;
-   pread.offset = offset;
-   pread.size = size;
-   pread.data_ptr = (uint64_t) (uintptr_t) data;
-   ret = drmIoctl(bufmgr->fd, DRM_IOCTL_I915_GEM_PREAD, &pread);
-   if (ret != 0) {
-      ret = -errno;
-      DBG("%s:%d: Error reading data from buffer %d: "
-          "(%"PRIu64" %"PRIu64") %s .\n",
-          __FILE__, __LINE__, bo->gem_handle, offset, size, strerror(errno));
-   }
-
-   return ret;
-}
-
 /** Waits for all GPU rendering with the object to have completed. */
 void
 brw_bo_wait_rendering(struct brw_bo *bo)

src/mesa/drivers/dri/i965/brw_bufmgr.h

@@ -222,9 +222,6 @@ static inline int brw_bo_unmap(struct brw_bo *bo) { return 0; }
 /** Write data into an object. */
 int brw_bo_subdata(struct brw_bo *bo, uint64_t offset,
                    uint64_t size, const void *data);
-/** Read data from an object. */
-int brw_bo_get_subdata(struct brw_bo *bo, uint64_t offset,
-                       uint64_t size, void *data);
 /**
  * Waits for rendering to an object by the GPU to have completed.
  *

src/mesa/drivers/dri/i965/intel_buffer_objects.c

@@ -289,7 +289,16 @@ brw_get_buffer_subdata(struct gl_context *ctx,
    if (brw_batch_references(&brw->batch, intel_obj->buffer)) {
       intel_batchbuffer_flush(brw);
    }
-   brw_bo_get_subdata(intel_obj->buffer, offset, size, data);
+
+   void *map = brw_bo_map(brw, intel_obj->buffer, MAP_READ);
+
+   if (unlikely(!map)) {
+      _mesa_error_no_memory(__func__);
+      return;
+   }
+
+   memcpy(data, map + offset, size);
+   brw_bo_unmap(intel_obj->buffer);
 
    mark_buffer_inactive(intel_obj);
 }