[Mesa-stable] [PATCH v4] i965: fixed clamping in set_scissor_bits when the y is flipped

[Dylan Baker]

Quoting Eleni Maria Stea (2019-02-22 13:02:30)
> Calculating the scissor rectangle fields with the y flipped (0 on top)
> can generate negative values that will cause assertion failure later on
> as the scissor fields are all unsigned. We must clamp the bbox values
> again to make sure they don't exceed the fb_height. Also fixed a
> calculation error.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108999
>           https://bugs.freedesktop.org/show_bug.cgi?id=109594
> 
> v2:
>    - I initially clamped the values inside the if (Y is flipped) case
>    and I made a mistake in the calculation: the clamp of the bbox[2] should
>    be a check if (bbox[2] >= fbheight) bbox[2] = fbheight - 1 instead and I
>    shouldn't have changed the ScissorRectangleYMax calculation. As the
>    fixed code is equivalent with using CLAMP instead of MAX2 at the top of
>    the function when bbox[2] and bbox[3] are calculated, and the 2nd is more
>    clear, I replaced it. (Nanley Chery)
> 
> v3:
>    - Reversed the CLAMP change in bbox[3] as the API guarantees that the
>    viewport height is positive. (Nanley Chery)
> 
> v4:
>   - Added nomination for the mesa-stable branch and the link to the second
>   bugzilla bug (Nanley Chery)
> 
> CC: <mesa-stable at lists.freedesktop.org>
> Tested-by: Paul Chelombitko <qamonstergl at gmail.com>
> Reviewed-by: Nanley Chery <nanley.g.chery at intel.com>
> ---
>  src/mesa/drivers/dri/i965/genX_state_upload.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/genX_state_upload.c b/src/mesa/drivers/dri/i965/genX_state_upload.c
> index 027dad1e089..73c983ce742 100644
> --- a/src/mesa/drivers/dri/i965/genX_state_upload.c
> +++ b/src/mesa/drivers/dri/i965/genX_state_upload.c
> @@ -2446,7 +2446,7 @@ set_scissor_bits(const struct gl_context *ctx, int i,
>  
>     bbox[0] = MAX2(ctx->ViewportArray[i].X, 0);
>     bbox[1] = MIN2(bbox[0] + ctx->ViewportArray[i].Width, fb_width);
> -   bbox[2] = MAX2(ctx->ViewportArray[i].Y, 0);
> +   bbox[2] = CLAMP(ctx->ViewportArray[i].Y, 0, fb_height);
>     bbox[3] = MIN2(bbox[2] + ctx->ViewportArray[i].Height, fb_height);
>     _mesa_intersect_scissor_bounding_box(ctx, i, bbox);
>  
> -- 
> 2.20.1
> 

Do you have push access? I'd like to get this merged so we can close said bugs,
and Nanley or I can push this for you if you don't have access.

Dylan
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