[Mesa-dev] [PATCH 09/13] r300g, r600g, radeonsi: add support for ARB_buffer_storage

[Christian König]

Am 31.01.2014 21:36, schrieb Alex Deucher:
> On Fri, Jan 31, 2014 at 7:05 AM, Marek Olšák <maraeo at gmail.com> wrote:
>> I think we always flush the HDP cache after (before?) command submission.
>>
> The kernel flushes the HDP cache in the fence command sequence.

But we do this at the top of the pipe instead of the bottom and then 
silently assume that between the top and the bottom the CPU isn't 
accessing this VRAM buffer.

At least theoretically this can lead to a problem if the CPU wants to 
read a buffer value that's in the same cache line as a value the GPU 
writes. For example if a stupid application tries something like 
"while(!value_written_by_GPU_in_VRAM);" this won't work correctly.

Christian.

>
> Alex
>
>> This patch adds nothing new to the drivers - we have always had
>> persistent buffer mappings for all buffers and it has always worked.
>> The only thing this does is that persistent mappings are now also
>> supported by Gallium and OpenGL.
>>
>> If there is a missing cache flush somewhere, the new
>> ARB_buffer_storage piglit test will show it.
>>
>> Marek
>>
>> On Fri, Jan 31, 2014 at 2:04 AM, Michel Dänzer <michel at daenzer.net> wrote:
>>> On Don, 2014-01-30 at 23:46 +0100, Fredrik Höglund wrote:
>>>> On Thursday 30 January 2014, Marek Olšák wrote:
>>>>> From: Marek Olšák <marek.olsak at amd.com>
>>>>>
>>>>> All GTT memory mappings are coherent and therefore can be persistent.
>>>> As we discussed on IRC, I think there should be a comment somewhere
>>>> explaining that VRAM mappings are uncached, so the memory_barrier
>>>> implementations don't need to do anything for those.
>>> VRAM is mapped uncacheable by the CPU, but there is an HDP cache which
>>> must be flushed to ensure coherency between the CPU and GPU. So I
>>> suspect memory_barrier actually needs to flush the HDP cache for VRAM.
>>>
>>> I'm wondering about GTT mappings on AGP as well. I think we're using CPU
>>> write-combining for those, so we probably need to flush the
>>> write-combining buffers?
>>>
>>>
>>> --
>>> Earthling Michel Dänzer            |                  http://www.amd.com
>>> Libre software enthusiast          |                Mesa and X developer
>>>
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