Re: [PATCH 5/6] drm/vc4: Fix overflow mem unreferencing when the binner runs dry.

[Rob Clark]

On Tue, Jul 26, 2016 at 7:11 PM, Eric Anholt <eric@xxxxxxxxxx> wrote:
> Rob Clark <robdclark@xxxxxxxxx> writes:
>
>> On Tue, Jul 26, 2016 at 4:47 PM, Eric Anholt <eric@xxxxxxxxxx> wrote:
>>> Overflow memory handling is tricky: While it's still referenced by the
>>> BPO registers, we want to keep it from being freed.  When we are
>>> putting a new set of overflow memory in the registers, we need to
>>> assign the old one to the last rendering job using it.
>>>
>>> We were looking at "what's currently running in the binner", but since
>>> the bin/render submission split, we may end up with the binner
>>> completing and having no new job while the renderer is still
>>> processing.  So, if we don't find a bin job at all, look at the
>>> highest-seqno (last) render job to attach our overflow to.
>>
>> so, drive-by comment.. but can you allocate gem bo's without backing
>> them immediately with pages?  If so, just always allocate the bo
>> up-front and attach it as a dependency of the batch, and only pin it
>> to actual pages when you have to overflow?
>
> The amount of overflow for a given CL is arbitrary, depending on the
> geometry submitted, and the overflow pool just gets streamed into by the
> hardware as you submit bin jobs.  You'll end up allocating [0,n] new
> overflows per bin job.  I don't see where "allocate gem BOs without
> backing them immediately with pages" idea would fit into this.

well, even not knowing the size up front shouldn't really be a
show-stopper, unless you had to mmap it to userspace, perhaps..
normally backing pages aren't allocated until drm_gem_get_pages() so
allocating the gem bo as placeholder to track dependencies of the
batch/submit shouldn't be an issue.  But I noticed you don't use
drm_gem_get_pages().. maybe w/ cma helpers it is harder to decouple
allocation of the drm_gem_object from the backing store.

BR,
-R