[Mesa-dev] [PATCH v2 2/3] vulkan/wsi: Report the correct min/maxImageCount

[Jason Ekstrand]

Hey, Neil!

On October 1, 2017 3:12:46 PM Neil Roberts <bpeeluk at yahoo.co.uk> wrote:

> Jason Ekstrand <jason at jlekstrand.net> writes:
>
>> +   /* For true mailbox mode, we need at least 4 images:
>> +    *  1) One to scan out from
>> +    *  2) One to have queued for scan-out
>> +    *  3) One to be currently held by the Wayland compositor
>> +    *  4) One to render to
>> +    */
>>     caps->minImageCount = 2;
>> -   caps->maxImageCount = 4;
>> +   /* There is no real maximum */
>> +   caps->maxImageCount = 0;
>
> This patch as it was applied seems to leave the minImageCount as 2 on
> X11. Is this possibly a mistake? Now it doesn’t match the comment above
> it and there’s no explanation of this in the commit message.

Probably.  Also, with current X, quad-buffering doesn't actually help but 
that's a different issue...

> It seems a little surprising that it has been changed to 4 in the
> Wayland backend. Won’t this mean that every application using Vulkan,
> even if it’s just a simple GUI app will end up with four large buffers
> just to make the few applications that want true mailbox mode behave
> correct?

Yeah... That's a bit unfortunate.  The problem is that we have no way of 
returning a different number of images depending on the mode.  In theory, 
we could start out at 2 and return SUBOPTIMAL and force the application to 
recreate the swapchain with more images until we have enough.  That would 
be a real pain though...  In not sure what the best option is.

--Jason