[Mesa-dev] [PATCH] intel: Fix ReadPixels on buffers whose width >= 32kbytes

[Alex Deucher]

On Thu, Jan 24, 2013 at 2:10 PM, Paul Berry <stereotype441 at gmail.com> wrote:
> When possible, glReadPixels calls are performed using the hardware
> blitter.  However, according to the Ivy Bridge PRM, Vol1 Part4,
> section 1.2.1.2 (Graphics Data Size Limitations):
>
>     The BLT engine is capable of transferring very large quantities of
>     graphics data. Any graphics data read from and written to the
>     destination is permitted to represent a number of pixels that
>     occupies up to 65,536 scan lines and up to 32,768 bytes per scan
>     line at the destination. The maximum number of pixels that may be
>     represented per scan line’s worth of graphics data depends on the
>     color depth.
>
> With an RGBA32F color buffer (which has 16 bytes per pixel) this
> imposes a maximum width of 2048 pixels.
>
> To make matters worse, if the pitch of the buffer is 32k or greater,
> intel_miptree_map_blit's call to intelEmitCopyBlit will overflow
> intelEmitCopyBlit's src_pitch and dst_pitch parameters (which are
> 16-bit signed integers).
>
> We can conveniently avoid both problems by avoiding the readpixels
> blit path when the miptree's pitch is >= 32k.
>
> Fixes gles3conform "half_float" tests when the buffer width is greater
> than 2048.
> ---
>  src/mesa/drivers/dri/intel/intel_mipmap_tree.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/src/mesa/drivers/dri/intel/intel_mipmap_tree.c b/src/mesa/drivers/dri/intel/intel_mipmap_tree.c
> index ce03afa..f2571bd 100644
> --- a/src/mesa/drivers/dri/intel/intel_mipmap_tree.c
> +++ b/src/mesa/drivers/dri/intel/intel_mipmap_tree.c
> @@ -1568,7 +1568,8 @@ intel_miptree_map_singlesample(struct intel_context *intel,
>     } else if (intel->has_llc &&
>               !(mode & GL_MAP_WRITE_BIT) &&
>               !mt->compressed &&
> -             mt->region->tiling == I915_TILING_X) {
> +             mt->region->tiling == I915_TILING_X &&
> +              mt->region->pitch < 32768) {

You may want to put a comment here about why you have this pitch check.

Alex

>        intel_miptree_map_blit(intel, mt, map, level, slice);
>     } else {
>        intel_miptree_map_gtt(intel, mt, map, level, slice);
> --
> 1.8.1.1
>
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