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

[Chris Wilson]

On Mon, Oct 26, 2015 at 04:00:20PM +0000, Tvrtko Ursulin wrote:
> 
> Hi,
> 
> On 23/10/15 10:50, Tvrtko Ursulin wrote:
> >On 22/10/15 17:07, Chris Wilson wrote:
> >>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?
> >
> >I've found some existing examples in uapi headers so think we are covered.
> 
> Any further thoughts on this? Would you consider re-spinning the
> patch with this addition?

I have respun it, but haven't checked it against clang nor do I know
what others think of potential portability issues with other compilers.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre