[Mesa-dev] [Mesa-stable] [PATCH] egl: support EGL_LARGEST_PBUFFER in eglCreatePbufferSurface(...)

[Tapani Pälli]

On 02/18/2016 08:19 PM, Emil Velikov wrote:
> Hi guys,
>
> On 15 February 2016 at 10:54, Tapani Pälli <tapani.palli at intel.com> wrote:
>> From: Daniel Czarnowski <daniel.czarnowski at intel.com>
>>
>> Patch provides a 'sane default' for a set pbuffer surface size when
> Perhaps too cheeky of a question but "who is to determine these 'sane
> defaults'" ?

I agree this is not ideal but it's a lot better than complete failure? 
The default set in the patch is quite conservative. We could have a 
driver hook but I'm not convinced it's worth it in this particular case, 
again this is really a case for robustness testing, not for a real life 
application.

> If those are OK for one hardware/generation they won't be for another.
> I believe the whole problem boils down to the fact that we don't do
> anything in case of EGL_LARGEST_PBUFFER. Looking at GLX -
> GLX_LARGEST_PBUFFER seems to be in the same boat :-\
>
> Imho one should honour it while creating the surface. Afaics the spec
> isn't clear if height/width should be ignored when the bool is set (or
> I missed that hunk), so I'm leaning towards "yes".

The spec is "if we can't allocate the size what is requested we allocate 
max possible size":

"Use EGL_LARGEST_PBUFFER to get the largest available pbuffer when the 
allocation of the pbuffer would otherwise fail."

So we would try to allocate what is requested but we have a limit that 
says it will fail so let's not. IIRC X driver has 32k for max pixmap 
size but as they are textures nowadays in practice this limit will be 
much smaller.

>> EGL_LARGEST_PBUFFER is used by the client. MIN2 macro is moved to
>> egldefines so that it can be shared.
>>
>> Fixes following Piglit test:
>>     egl-create-largest-pbuffer-surface
>>
> With the above said, I'm not sure how much sense it makes to have
> explicit EGL_HEIGHT/WIDTH in the test, alongside EGL_LARGEST_PBUFFER
> in the mentioned test.
>
> In all fairness, fixing this properly might result in a serious
> (series of) patch(es). Which might be a bit too much for stable ?
> Better to cross that road as we reach it :-)

I did some investigation and it seems currently there is no way to query 
maximum size of a pixmap using xlib or xcb, only way would be to try 
this in a loop. Xcb uses uint16_t type so you could determine it from 
that but with most (all?) drivers it would fail with max.

// Tapani