[Buildroot] [git commit branch/2021.02.x] package/sunxi-mali-mainline-driver: update help section to make it work on Linux >= 4.20

[Peter Korsgaard <peter@korsgaard.com>]

commit: https://git.buildroot.net/buildroot/commit/?id=03149de8ff93ca2483b7e79642442c17ab63479c
branch: https://git.buildroot.net/buildroot/commit/?id=refs/heads/2021.02.x

On Linux version >= 4.20 in order to have mali working we need to pass
drm_kms_helper.drm_leak_fbdev_smem=1 and at least
drm_kms_helper.drm_fbdev_overalloc=200 to have a double buffer fbdev.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 97408545ad05344cf8a4194d72ba8c6b876795ea)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
---
 package/sunxi-mali-mainline-driver/Config.in | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/package/sunxi-mali-mainline-driver/Config.in b/package/sunxi-mali-mainline-driver/Config.in
index 5db66c5bf6..90bc308bc5 100644
--- a/package/sunxi-mali-mainline-driver/Config.in
+++ b/package/sunxi-mali-mainline-driver/Config.in
@@ -26,4 +26,14 @@ config BR2_PACKAGE_SUNXI_MALI_MAINLINE_DRIVER
 	  enabled:
 	  - CONFIG_DRM_FBDEV_LEAK_PHYS_SMEM
 
+	  The following kernel command line options must be passed:
+
+	  - drm_kms_helper.drm_leak_fbdev_smem = 1
+	    since drm_kms_helper.drm_leak_fbdev_smem is set to 0 by
+	    default
+
+	  - drm_kms_helper.drm_fbdev_overalloc >= 200
+	    since the driver requires at least double buffering and
+	    fails to work with the default value(100).
+
 	  https://github.com/mripard/sunxi-mali/blob/master/README.md
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