[Mesa-dev] Mesa (master): 29 new commits

[Alex Deucher]

On Thu, Apr 14, 2016 at 9:21 AM, Alex Deucher <alexdeucher at gmail.com> wrote:
> On Thu, Apr 14, 2016 at 3:29 AM, Michel Dänzer <michel at daenzer.net> wrote:
>> On 14.04.2016 11:37, Michel Dänzer wrote:
>>> On 12.04.2016 21:33, Marek =?UNKNOWN?B?T2zFocOhaw==?= wrote:
>>>>
>>>> URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=5a4b74d1ba2c156766a7a5dbfef099c7db5d6694
>>>> Author: Marek Olšák <marek.olsak at amd.com>
>>>> Date:   Mon Apr 11 19:56:07 2016 +0200
>>>>
>>>>     gallium/radeon: relax requirements on VRAM placements on APUs
>>>
>>> This change caused a bunch of ARB_shader_load_image_store piglit tests
>>> to fail on my Kaveri, see some examples below. The incorrect values
>>> seem consistent.
>>>
>>> I suppose some buffers end up in GTT instead of VRAM with this
>>> change, but I'm not sure how that could cause problems. Any ideas?
>>
>> Also, with the code modified to use GTT only for everything but
>> (potential) scanout buffers, the performance of Unigine Valley and the
>> Unreal Engine 4 Elemental demo is reduced by about 30%. So the premise
>> that GTT is about as fast as VRAM doesn't seem to hold true in practice
>> (at least with Kaveri and presumably other (pre-)CIK APUs; maybe it's
>> better with Carrizo and newer), which means that this change may cause
>> performance of long-running processes to drop significantly over time.
>
> We need to make sure the buffers end up in WC GTT.  The cache snoops
> for coherent GTT reduce performance.
>

On APUs (sumo and up), there are two different interfaces to memory.
"vram" and WC GTT use the faster interface and coherent GTT (snoop bit
set) uses the slower interface.  On pre-APU IGPs, I'm not sure how the
"vram" vs. GTT interfaces differ.

Alex

> Alex
>
>>
>> Given all these issues, I'm afraid it may be better to revert this
>> change for now, until we have a better plan for dealing with this.
>>
>>
>> --
>> Earthling Michel Dänzer               |               http://www.amd.com
>> Libre software enthusiast             |             Mesa and X developer
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