[Intel-gfx] [PATCH 12/41] drm/i915: Introduce an internal allocator for disposable private objects

[Tvrtko Ursulin]

On 20/10/2016 21:36, Chris Wilson wrote:
> On Thu, Oct 20, 2016 at 05:22:23PM +0100, Tvrtko Ursulin wrote:
>>
>> On 20/10/2016 16:03, Chris Wilson wrote:
>>> Quite a few of our objects used for internal hardware programming do not
>>> benefit from being swappable or from being zero initialised. As such
>>> they do not benefit from using a shmemfs backing storage and since they
>>> are internal and never directly exposed to the user, we do not need to
>>> worry about providing a filp. For these we can use an
>>> drm_i915_gem_object wrapper around a sg_table of plain struct page. They
>>> are not swap backed and not automatically pinned. If they are reaped
>>> by the shrinker, the pages are released and the contents discarded. For
>>> the internal use case, this is fine as for example, ringbuffers are
>>> pinned from being written by a request to be read by the hardware. Once
>>> they are idle, they can be discarded entirely. As such they are a good
>>> match for execlist ringbuffers and a small variety of other internal
>>> objects.
>>>
>>> In the first iteration, this is limited to the scratch batch buffers we
>>> use (for command parsing and state initialisation).
>>
>> And the status page.
>
> Yeah, I was just thinking of the runtime allocated blocks where the
> change can be measured.
>
>>> +	max_order = MAX_ORDER;
>>> +#ifdef CONFIG_SWIOTLB
>>> +	if (swiotlb_nr_tbl())
>>> +		max_order = min(max_order, ilog2(IO_TLB_SEGSIZE));
>>> +#endif
>>
>> I couldn't figure out what IO_TLB_SEGSIZE actually is in some
>> minutes of cross-referencing. Did not seem to be in units of bytes
>> according to swiotlb.h.
>
> Pages.
>
>> In either case my question is - why use different parameters than
>> swiotlb_max_size you recently added to i915_gem.c?
>
> I was trying to exploit the compile time constants, and I did not care
> to grow the search for even higher orders.

Sorry pages, but it is not AFAICS, but it is units of IO_TLB_SHIFT size 
which is 2k. Happens to be fine since it is smaller (5) than MAX_ORDER 
(11), however to me it still looks like a unrelated number.

ilog2(IO_TLB_SEGSIZE) + 1 would be the same units. It would still look 
arbitrary but I suppose it would be passable.

Regards,

Tvrtko