[Mesa-dev] [Intel-gfx] [PATCH 00/68] Broadwell 48b addressing and prelocations (no relocs)

[Daniel Vetter]

On Fri, Aug 22, 2014 at 9:03 AM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
>> > > If a GPU
>> > > client uses only prelocations, the relocation process can be entirely
>> > > skipped. This sounds like a big win initially,
>> >
>> > Close to zero if the client uses existing interfaces.
>> > -Chris
>>
>> Chris,
>>
>> I don't know if you've seen Ben's libdrm and Mesa patches, but with a few patches to libdrm and virtually zero Mesa changes, he's apparently eliminated our need to do any relocations for the 3D driver.  It wasn't invasive at all---I was surprised.
>
> Indeed, you could do everything inside libdrm with the code I posted 2
> years ago.

I915_EXEC_NO_RELOC can be used to tell the kernel that it doesn't need
to walk all the reloc tables (if nothing moved) because userspace
didn't go insane and reuse reloc trees. So you'd need to implement a
flag + a libdrm function to set that (iirc mesa has been non-stupid
since years). And yeah I kinda expect any new reloc-less thing to get
benchmarked against an implementation using that, since the 48bit
specific thing proposed looks like a fairly short-lived stop-gap, and
since the current no-reloc we already have would work everywhere. And
yeah I've been poking people to look at this for years. too.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch