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

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

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.

Regards,

Tvrtko
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