Before this patch, X server detects/sets the primary device by:
- The "PrimaryGPU" option in extra X configuration
- pci_device_is_boot_vga() for PCI devices
- Set the first (0 index) device as the primary device, if it is not found yet.
However, the other display controllers like Amlogic's meson cannot be detected as the primary device by pci_device_is_boot_vga(). Thus, it has to set the extra X configuration for the "PrimaryGPU" option. Otherwise, X server will set the first (0 index) device as the primary device. But it may not be the correct one, because it has no output. For example, Amlogic puts the GPU and display controller as different devices:
(II) xfree86: Adding drm device (/dev/dri/card0) (II) Platform probe for /sys/devices/platform/soc/d0000000.apb/d00c0000.gpu/drm/card0 (II) xfree86: Adding drm device (/dev/dri/card1) (II) Platform probe for /sys/devices/platform/soc/d0100000.vpu/drm/card1
This patch adds a new way to set the primary device by detecting the output of devices. It refers to the probe_hw() and related functions in modesetting driver.