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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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?

Regards,

Tvrtko
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