[Mesa-dev] [PATCH v4] virgl: Add support for glGetMultisample

[Erik Faye-Lund]

On Fri, Jun 29, 2018 at 12:39 PM Gert Wollny <gert.wollny at collabora.com> wrote:
>
> Use caps to obtain the multisample sample positions for up to 16
> positions and implement the according Gallium interface.
>
> This implemenation (plus its counterpart in virglrenderer) assume that
> the fixed sample position are always the same for a given number of samples
> over the whole live time of a qemu session. It also assumes that sample
> series are only given for 2, 4, 8, and 16 samples, and for intermediate
> numbers N of samples the next higher supported set from above list is picked
> and the sample positions for the first N samples are returned accordingly.
>
> Fixes (when run on GL host):
>     dEQP-GLES31.functional.texture.multisample.samples_1.sample_position
>     dEQP-GLES31.functional.texture.multisample.samples_2.sample_position
>     dEQP-GLES31.functional.texture.multisample.samples_3.sample_position
>     dEQP-GLES31.functional.texture.multisample.samples_4.sample_position
>     dEQP-GLES31.functional.texture.multisample.samples_8.sample_position
>     dEQP-GLES31.functional.texture.multisample.samples_10.sample_position
>     dEQP-GLES31.functional.texture.multisample.samples_12.sample_position
>     dEQP-GLES31.functional.texture.multisample.samples_13.sample_position
>     dEQP-GLES31.functional.texture.multisample.samples_16.sample_position
>
> v2: remove unrelated chunk (thanks Ilia Mirkin)
> v3: - also return positions for intermediate sample counts
>     - fix unused varible warning
>     - update description
> v4: explain better what this patch assumes and how it handles sample numbers
>     that are not directly advertised (thanks go to Erik Faye-Lund for making
>     me aware that this should be documented)
>
> Signed-off-by: Gert Wollny <gert.wollny at collabora.com>
> ---
> I left the debug_printf in there, because this patch (together with the
> related virglrenderer patch) is not sufficient to fix above tests on a GLES
> host.
>
>  src/gallium/drivers/virgl/virgl_context.c | 38 +++++++++++++++++++++++++++++++
>  src/gallium/drivers/virgl/virgl_hw.h      |  1 +
>  2 files changed, 39 insertions(+)
>
> diff --git a/src/gallium/drivers/virgl/virgl_context.c b/src/gallium/drivers/virgl/virgl_context.c
> index e6f8dc8525..43c141e42d 100644
> --- a/src/gallium/drivers/virgl/virgl_context.c
> +++ b/src/gallium/drivers/virgl/virgl_context.c
> @@ -920,6 +920,42 @@ virgl_context_destroy( struct pipe_context *ctx )
>     FREE(vctx);
>  }
>
> +static void virgl_get_sample_position(struct pipe_context *ctx,
> +                                      unsigned sample_count,
> +                                      unsigned index,
> +                                      float *out_value)
> +{
> +   struct virgl_context *vctx = virgl_context(ctx);
> +   struct virgl_screen *vs = virgl_screen(vctx->base.screen);
> +
> +   if (sample_count > vs->caps.caps.v1.max_samples) {
> +      debug_printf("VIRGL: requested %d MSAA samples, but only %d supported\n",
> +                   sample_count, vs->caps.caps.v1.max_samples);
> +      return;
> +   }
> +
> +   /* The following is basically copied from dri/i965gen6_get_sample_position
> +    * The only addition is that we hold the msaa positions for all sample
> +    * counts in a flat array. */
> +   uint32_t bits = 0;
> +   if (sample_count == 1) {
> +      out_value[0] = out_value[1] = 0.5f;
> +      return;
> +   } else if (sample_count == 2) {
> +      bits = vs->caps.caps.v2.msaa_sample_positions[0] >> (8 * index);
> +   } else if (sample_count < 4) {
> +      bits = vs->caps.caps.v2.msaa_sample_positions[1] >> (8 * index);
> +   } else if (sample_count < 8) {
> +      bits = vs->caps.caps.v2.msaa_sample_positions[2 + (index >> 2)] >> (8 * (index & 3));
> +   } else if (sample_count < 8) {
> +      bits = vs->caps.caps.v2.msaa_sample_positions[4 + (index >> 2)] >> (8 * (index & 3));
> +   }
> +   out_value[0] = ((bits >> 4) & 0xf) / 16.0f;
> +   out_value[1] = (bits & 0xf) / 16.0f;
> +   debug_printf("VIRGL: sample postion [%2d/%2d] = (%f, %f)\n",
> +                index, sample_count, out_value[0], out_value[1]);

Are you sure that an unconditional debug_printf in the success case
here is a good idea?

AFAIK, most debug_printfs are either in error paths, or guarded by a
compile-time flag, a run-time flag (typically environment variables)
or something like that.

The most kosher way of doing this would be with a driver-specific
debug-flag. Sadly, it seems there's no such flag in virgl yet for
this, and maybe this is a too small detail to bother with this? If so,
quite a lot of places just throw an #if 0 around it so it's easy to
enable for developers in the future...

Either way:
Reviewed-by: Erik Faye-Lund <erik.faye-lund at collabora.com>