[Intel-gfx] [RFC 1/7] drm/i915: Extract sg creation into a helper

[Tvrtko Ursulin]

On 13/10/2016 10:20, Chris Wilson wrote:
> On Thu, Oct 13, 2016 at 10:03:58AM +0100, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>>
>> In order to reuse the same logic in several places in the driver,
>> extract the logic which adds pages to the sg list and does the
>> potential coalescing, into separate functions.
>>
>> Code wanting to build the sg table needs to do the following:
>>
>> 1. Call i915_sg_create to create the state object for a given
>>     maximum number of pages.
>> 2. Iterate using i915_sg_for_each_page
>> 3. Call i915_sg_add_page to add all the pages in order
>> 4. On success call i915_sg_complete, or on failure i915_sg_abort
>>
>> In this patch only i915_gem_object_get_pages_gtt gets converted
>> to use the new functions.
> Yup, much happier. Though can we just allocate the state on the stack,
> it is the same as what we are previously using anyway. 40 bytes.

Thought about this also and had a slight preference not to use stack, 
since we got to have kmalloc and error handling in i915_sg_create anyway.

> I've also thought about doing the sg_trim, just never actually tried...
>
> Just not convinced that recreating the compact sg is better than copying
> the already compact sg (for dmabuf, partial).

Agreed already on dmabuf. Partials yes, I suppose that could be 
implemented more efficiently. You think it is worth special casing it 
rather than reuse the common API? It would have to handle a start in the 
middle of the sg entry so it wouldn't be a straightforward copy.

Regards,

Tvrtko