[Intel-gfx] [PATCH 1/2] drm/i915: Fallback to single PAGE_SIZE segments for DMA remapping

[Tvrtko Ursulin]

On 20/12/2016 13:56, Chris Wilson wrote:
> On Tue, Dec 20, 2016 at 01:38:16PM +0000, Tvrtko Ursulin wrote:
>>
>> On 20/12/2016 12:36, Chris Wilson wrote:
>>> On Tue, Dec 20, 2016 at 11:33:27AM +0000, Chris Wilson wrote:
>>>> On Tue, Dec 20, 2016 at 11:13:43AM +0000, Tvrtko Ursulin wrote:
>>>>> How much is the cost of freeing and re-acquiring pages in the fall
>>>>> back case? It could be avoidable by using the table and adding
>>>>> something like sgt = i915_sg_copy(sgt, table_max_segment). But it
>>>>> depends on how likely is this path to be hit on swiotlb platforms. I
>>>>> have no idea. Our datasets are much bigger than the swiotlb space -
>>>>> if that is true on such platforms?
>>>>
>>>> It's below my level of care (atm). Platforms hitting this are using
>>>> swiotlb *bounce* buffers. They will not be able to support a full gfx
>>>> workload and be going through a copy. We could avoid the additional
>>>> work, the sg_table is large enough for a 1:1 copy if we do it before the
>>>> trim, but more importantly we need a simple fix for 4.10.
>>>
>>> Pushed this pair as I think this is the safe course of action. Creating
>>> i915_sg_expand() is a job for a rainy day.
>>
>> It would have been very simple and much more elegant in my opinion.
>
> I'm ready to be impressed, in my head to do an inplace rewrite was tricky.
> :)

Maybe I've missed something then. We'll see. :)

>> But I understand Tested-by tag was precious to keep. I'll send a
>> patch shortly but it won't be very tested due to time constraints.
>>
>> Also I don't know why you changed page_count and i to unsigned long
>> when the sg API can only handle unsigned int for that.
>
> Primary concern was moving them out of the way and worrying about our
> own 64bit object size issues. Hmm, can we reuse
>
> if (overflows_type(pgcount, unsigned int))
> 	return -E2BIG;
>
> to catch the mismatch?

You have already added that to i915_gem_object_create some time ago! :)

Regards,

Tvrtko