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Revert "glamor: Enable modifier support for xfree86 too"

Mario Kleiner requested to merge kleinerm/xserver:modesetting_disable_modifiers into master

This reverts commit 9b899941.

Turns out that defaulting glamor_egl->dmabuf_capable = TRUE breaks kms page-flipping on various Mesa+Linux/DRM-KMS+hardware combos, resulting in broken presentation timing, degraded performance and awful tearing. E.g., my testing shows that X-Server master + Mesa 21.2 + Linux 5.3 on Intel Kabylake has broken pageflipping. Similar behaviour was observed in the past on RaspberryPi 4/400 with VideoCore-6 by myself and others, and iirc by myself on some AMD gpu's, although my memories of the latter are a bit dim.

Cfe. mesa/mesa#3601 (closed) and possibly !254 (closed) for related problems.

The reason for pageflip failure on the modesetting-ddx under DRI3/Present seems to be the following sequence:

  1. Atomic modesetting for the modesetting-ddx is broken and therefore both disabled by default in the modesetting-ddx itself and also force-disabled by the Linux kernel since quite a while. If the kernel detects drmSetClientCap(fd, DRM_CLIENT_CAP_ATOMIC, 1); from the X-Server, it will reject the request, as a countermeasure to all the past and current brokeness.

  2. Without DRM_CLIENT_CAP_ATOMIC we don't get the implied universal planes support (DRM_CLIENT_CAP_UNIVERSAL_PLANES).

  3. Without DRM_CLIENT_CAP_UNIVERSAL_PLANES, drmModeGetPlaneResources() will only return overlay planes, but not primary- or cursor planes.

  4. As modesetting-ddx drmmode_crtc_create_planes() function can only operate on primary planes, but can't get any from drmModeGetPlaneResources(), the drmmode_crtc_create_planes() mostly turns into a no-op, never executes populate_format_modifiers() and therefore the Linux kernels DRM-KMS driver is not ever queried for the list of scanout/pageflip capable DRM format modifiers. Iow. the drmmode_crtc->formats[i].modifiers list stays empty with zero drmmode_crtc->formats[i].num_modifiers.

  5. The list from step 4 provides the format+modifiers for intersection which would get returned by the X-Servers DRI3 backend as response to a xcb_dri3_get_supported_modifiers_window_modifiers() request. Given an empty list was returned in step 4, this will lead to return of an empty modifiers list by xcb_dri3_get_supported_modifiers_window_modifiers().

  6. Both Mesa's DRI3/Present OpenGL backbuffer allocation logic and iirc Mesa/Vulkan/WSI/X11's swapchain image allocation logic use the list from xcb_dri3_get_supported_modifiers_window_modifiers() for format+ modifier selection for scanout/pageflip capable buffers. Cfe. Mesa's dri3_alloc_render_buffer() function.

    Due to the empty list, the Mesa code falls back to the format+modifiers reported by xcb_dri3_get_supported_modifiers_screen_modifiers() instead. This list contains all modifiers reported by GLAMOR as result of glamor_get_formats() and glamor_get_modifiers(), which in turn are query results from Mesa eglQueryDmaBufFormatsEXT() and eglQueryDmaBufModifiersEXT(). Iow. all format+modifiers which are supported for rendering are considered for the OpenGL backbuffers and Vulkan swapchain buffers.

  7. Depending on kms driver + gpu combo and Mesa version, such buffers are often not direct-scanout / pageflip capable, and so pageflipping can't be used for DRI3/Present of fullscreen windows. Whenever the system has to fallback to copies instead of pageflips, the results are broken presentation timing, degraded performance and quite horrible tearing, as the current DRI3/Present implementation does not perform any hardware synchronization of copy presents to the start of vblank or similar.

By defaulting glamor_egl->dmabuf_capable = FALSE instead, as the server 1.20 branch does, we avoid this failure:

  1. glamor_get_modifiers() turns into a no-op and returns false, not reporting any supported dmabuf modifiers to the servers DRI3 code, ie. the servers cache_formats_and_modifiers() function can't retrieve and cache any format+modifiers. Therefore the servers DRI3 code now also reports an empty format+modifiers list when Mesa does a xcb_dri3_get_supported_modifiers_screen_modifiers() query.

  2. Mesa's buffer allocation code therefore falls back to using the old DRI image extensions createImage() function to allocate buffers with use flags __DRI_IMAGE_USE_SCANOUT | __DRI_IMAGE_USE_BACKBUFFER and our OpenGL backbuffers / Vulkan swapchain images get allocated in a direct-scanout / pageflip capable format. Pageflipping works, timing and performance is good, presentation is tear-free.

Please consider merging this for branching the X-Server 1.21 branch.

Signed-off-by: Mario Kleiner mario.kleiner.de@gmail.com

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