[Mesa-dev] [PATCH 06/11] loader_dri3: Honour the request to preserve back buffer content

[Michel Dänzer]

On 11/08/17 11:14 PM, Thomas Hellstrom wrote:
> EGL uses the force_copy parameter to loader_dri3_swap_buffers_msc() to indicate
> that it wants to preserve back buffer contents across a buffer swap.
> 
> While the loader then turns off server-side page-flipping there's nothing to
> guarantee that a new backbuffer isn't chosen when EGL starts to render again,
> and that buffer's content is of course undefined.
> 
> So rework the functionality:
> If the client supports local blits, allow server-side page flipping and when
> a new back is grabbed, if needed, blit the old back's content to the new back.
> If the client doesn't support local blits, disallow server-side page-flipping
> to avoid a client deadlock and then, when grabbing a new back buffer, sleep
> until the old back is idle, which may take a substantial time depending on
> swap interval.
> 
> Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>

[...]

> @@ -475,12 +476,21 @@ dri3_find_back(struct loader_dri3_drawable *draw)
>     int b;
>     xcb_generic_event_t *ev;
>     xcb_present_generic_event_t *ge;
> +   int num_to_consider = draw->num_back;
>  
>     /* Increase the likelyhood of reusing current buffer */
>     dri3_flush_present_events(draw);
>  
> +   /* Check whether we need to reuse the current back buffer as new back.
> +    * In that case, wait until it's not busy anymore.
> +    */
> +   if (!draw->have_image_blit && draw->cur_blit_source != -1) {
> +      num_to_consider = 1;
> +      draw->cur_blit_source = -1;
> +   }

Is it possible that dri3_find_back gets called with
draw->cur_blit_source != -1, entering this block (assuming
!draw->have_image_blit)...


>     for (;;) {
> -      for (b = 0; b < draw->num_back; b++) {
> +      for (b = 0; b < num_to_consider; b++) {
>           int id = LOADER_DRI3_BACK_ID((b + draw->cur_back) % draw->num_back);
>           struct loader_dri3_buffer *buffer = draw->buffers[id];

... Then later gets called again with draw->cur_blit_source == -1,
choosing a different back buffer here, because the previous one is still
busy?


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer