Created attachment 45537 [details]
simple app the exibhits the problem

When using glReadPixels(0, 0, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer) there's a speed difference between reading back from framebuffer or from an FBO.

Using gdb I found that format field of renderbuffer structure is always equal PIPE_FORMAT_NONE for the FBO which prevents using st_fast_readpixels.

I've built a simple patch which init format field in "st_renderbuffer_alloc_storage" and there is an improvement (a.k.a st_fast_readpixels is used everywhere).

What do you think of this (potential) issue ?

(I'm using git version of Mesa, and r600g)