[Mesa-dev] [PATCH 5/5] dri3: Use SwapBuffer flips for back- and fake front

[Thomas Hellstrom]

On 06/28/2017 08:48 PM, Axel Davy wrote:
> On 28/06/2017 20:40, Thomas Hellstrom wrote:
>> On 06/28/2017 07:36 PM, Axel Davy wrote:
>>> Hi,
>>>
>>> To my knowledge, this is invalid to switch the front fake buffer 
>>> with the back buffer.
>>>
>>> The front buffer is supposed to take into account what the app draws 
>>> with the xserver commands, etc.
>>
>> SwapBuffers should bring the contents of the back buffer to the front 
>> (and fake front if applicable) buffer, and the backbuffer contents 
>> become undefined, or tagged with a certain age. None of this is 
>> violated here. The old back buffer will become the new fake front and 
>> appropriately synced with X rendering as needed.
>>
>> Is there a specific scenario you are concerned with?
>
> When I investigated the use of front buffer when writing the patch to 
> add secondary gpu support, I understood almost all apps don't use 
> front buffer at all, but when they do, it's to interact with the real 
> content displayed to the user, which can see some modifications.

So this behaviour doesn't change with the patch. As soon as the old back 
buffer is swapped in place as the new fake front, it starts 
synchronizing with the X rendered content when needed. Specifically that 
happens during glXWaitGL and glXWaitX.

/Thomas