So, in the future, we'll want to have multiple jobs for different framebuffers independently in flight at the same time. As an illustrative example, we would like the app to be able to do a sequence like:
bind scanout
clear
draw something
bind FBO
clear
draw something
bind scanout
draw something else
bind FBO
draw something else
bind scanout
draw the fbo
flush
Ideally, nothing should happen until the final flush. So the order
submitted to the hardware would be:
FBO:
clear
draw something
draw something else
draw something else
Scanout:
clear
draw something
draw something else
draw the fbo
Unfortunately, right now that panfrost_flush() call makes the order
instead:
Scanout:
clear
draw something
FBO:
clear
draw something
Scanout:
clear
wallpaper scanout
draw something else
FBO:
clear
wallpaper fbo
draw something else
Scanout:
clear
wallpaper scanout
draw FBO
Clearly this is extremely suboptimal (I believe the extra wallpaper
blits are related to our poor performance on FBO-heavy apps. glmark's
-bdesktop and -bterrain are suspects here.)
The solution is to be able to have switching framebuffers be a no-op,
since all the important stuff is in the panfrost_job/batch. So
panfrost_set_framebuffer_state is just doing the copy, but no flush and
almost no logic, so we can switch back and forth freely without
incurring a flush/clear/wallpaper cycle each time.
This patch series is not the right time to focus on pipelined
performance (though I'd love if somebody gave it some love at some
point). But I do want to caution from a code smell that will make
whoever that someone is -- quite possibly you! -- quite grumpy when they
work on this in future.
On Fri, Aug 02, 2019 at 12:12:54PM +0200, Boris Brezillon wrote:
> The FB desc will be emitted/attached on the first draw targetting
> this new FB.
>
> Signed-off-by: Boris Brezillon <boris.brezil...@collabora.com>
> ---
> src/gallium/drivers/panfrost/pan_context.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/src/gallium/drivers/panfrost/pan_context.c
> b/src/gallium/drivers/panfrost/pan_context.c
> index 1091caeb1148..2b7906eea155 100644
> --- a/src/gallium/drivers/panfrost/pan_context.c
> +++ b/src/gallium/drivers/panfrost/pan_context.c
> @@ -2384,6 +2384,9 @@ panfrost_set_framebuffer_state(struct pipe_context
> *pctx,
>
> if (!is_scanout || has_draws)
> panfrost_flush(pctx, NULL, PIPE_FLUSH_END_OF_FRAME);
> + else
> +
> assert(!ctx->payloads[PIPE_SHADER_VERTEX].postfix.framebuffer &&
> +
> !ctx->payloads[PIPE_SHADER_FRAGMENT].postfix.framebuffer);
>
> util_copy_framebuffer_state(&ctx->pipe_framebuffer, fb);
>
> @@ -2391,7 +2394,8 @@ panfrost_set_framebuffer_state(struct pipe_context
> *pctx,
> struct panfrost_screen *screen = pan_screen(ctx->base.screen);
>
> panfrost_hint_afbc(screen, &ctx->pipe_framebuffer);
> - panfrost_attach_vt_framebuffer(ctx, false);
> + for (unsigned i = 0; i < PIPE_SHADER_TYPES; ++i)
> + ctx->payloads[i].postfix.framebuffer = 0;
> }
>
> static void *
> --
> 2.21.0
>
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