[PATCH RFC] drm/mediatek: Remove all conflicting aperture devices during probe

[Luca Leonardo Scorcia <l.scorcia@gmail.com>]

If a device has a framebuffer available it might be already used as
display by simple-framebuffer or simpledrm when mediatek-drm is probed.
This is actually helpful when porting to a new device as
framebuffers are simple to setup in device trees and fbcon can be
used to monitor the kernel boot process.

When drm-mediatek loads a new fb device is initialized, however
fbcon remains attached to the initial framebuffer which is no longer
connected to the actual display - the early fb is never removed.

We can gracefully transition from framebuffer handling to drm-managed
display by calling aperture_remove_all_conflicting_devices as we probe
mediatek-drm. This takes care of unloading other fb devices/drivers
and disconnects fbcon which then automatically reconnects to
mediatekdrmfb as soon as it's available.

Signed-off-by: Luca Leonardo Scorcia <l.scorcia@gmail.com>
---
This patch has been tested on Xiaomi Mi Smart Clock, however
I'd like to get some feedback from more knowledgeable people,
especially about those points:
1. Is aperture support on mtk-drm desired at all?
2. mtk-drm does not know about the fb address therefore as far 
   as I can see we can't use the more specific function
   aperture_remove_conflicting_devices. This means that all generic
   aperture drivers will be unloaded. It might not be a real world issue 
   as I can't see why one might want to load mtk-drm while keeping 
   other aperture  device drivers active, but my experience is very limited.
Thanks!

 drivers/gpu/drm/mediatek/mtk_drm_drv.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
index a94c51a83261..17e45b93fe49 100644
--- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
+++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
@@ -4,6 +4,7 @@
  * Author: YT SHEN <yt.shen@mediatek.com>
  */
 
+#include <linux/aperture.h>
 #include <linux/component.h>
 #include <linux/module.h>
 #include <linux/of.h>
@@ -1116,6 +1117,11 @@ static int mtk_drm_probe(struct platform_device *pdev)
 	if (!private->all_drm_private)
 		return -ENOMEM;
 
+	/* Remove framebuffer owners, this will release fbcon if active */
+	ret = aperture_remove_all_conflicting_devices(DRIVER_NAME);
+	if (ret < 0)
+		dev_err(dev, "Failed to remove conflicting aperture devices (%d)", ret);
+
 	/* Bringup ovl_adaptor */
 	if (mtk_drm_find_mmsys_comp(private, DDP_COMPONENT_DRM_OVL_ADAPTOR)) {
 		ovl_adaptor = platform_device_register_data(dev, "mediatek-disp-ovl-adaptor",
-- 
2.43.0