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

[Tvrtko Ursulin]

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. 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.

Regards,

Tvrtko