[Intel-gfx] [PATCH] drm/915: Pad GTT views of exec objects up to user specified size

[Chris Wilson]

On Thu, Oct 22, 2015 at 03:15:55PM +0100, Tvrtko Ursulin wrote:
> 
> Hi,
> 
> On 21/10/15 16:24, Chris Wilson wrote:
> >Our GPUs impose certain requirements upon buffers that depend upon how
> >exactly they are used. Typically this is expressed as that they require
> >a larger surface than would be naively computed by pitch * height.
> >Normally such requirements are hidden away in the userspace driver, but
> >when we accept pointers from strangers and later impose extra conditions
> >on them, the original client allocator has no idea about the
> >monstrosities in the GPU and we require the userspace driver to inform
> >the kernel how many padding pages are required beyond the client
> >allocation.
> >
> >v2: Long time, no see
> 
> [snip]
> 
> >diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> >index 08e047cba76a..678f7d5320ae 100644
> >--- a/include/uapi/drm/i915_drm.h
> >+++ b/include/uapi/drm/i915_drm.h
> >@@ -691,10 +691,11 @@ struct drm_i915_gem_exec_object2 {
> >  #define EXEC_OBJECT_NEEDS_GTT	(1<<1)
> >  #define EXEC_OBJECT_WRITE	(1<<2)
> >  #define EXEC_OBJECT_SUPPORTS_48B_ADDRESS (1<<3)
> >-#define __EXEC_OBJECT_UNKNOWN_FLAGS -(EXEC_OBJECT_SUPPORTS_48B_ADDRESS<<1)
> >+#define EXEC_OBJECT_PAD_TO_SIZE	(1<<4)
> >+#define __EXEC_OBJECT_UNKNOWN_FLAGS -(EXEC_OBJECT_PAD_TO_SIZE<<1)
> >  	__u64 flags;
> >
> >-	__u64 rsvd1;
> >+	__u64 rsvd1; /* pad_to_size */
> >  	__u64 rsvd2;
> >  };
> 
> What do you think about:
> 
> union {
> 	__u64 pad_to_size;
> 	__u64 rsvd1;
> } ?
> 
> Kind of like a migration path for userspace?

Hmm, I think that just might work. Clang? Does clang support anonymous
unions yet? Do we care if it doesn't?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre