[PATCH 8/8] drm/ast: Avoid reprogramming primary-plane scanout address

[Thomas Zimmermann]

Some AST-based BMCs stop display output for up to 5 seconds after
reprogramming the scanout address. As the address is fixed, avoid
re-setting the address' value.

Reported-by: Jocelyn Falempe <jfalempe at redhat.com>
Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
---
 drivers/gpu/drm/ast/ast_mode.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
index 1b991658290b..54a9643d86ce 100644
--- a/drivers/gpu/drm/ast/ast_mode.c
+++ b/drivers/gpu/drm/ast/ast_mode.c
@@ -672,9 +672,17 @@ static void ast_primary_plane_helper_atomic_update(struct drm_plane *plane,
 	}
 
 	ast_set_offset_reg(ast, fb);
-	ast_set_start_address_crt1(ast, (u32)ast_plane->offset);
 
-	ast_set_index_reg_mask(ast, AST_IO_SEQ_PORT, 0x1, 0xdf, 0x00);
+	/*
+	 * Some BMCs stop scanning out the video signal after the driver
+	 * reprogrammed the scanout address. This stalls display output
+	 * for several seconds and makes the display unusable. Therefore
+	 * only reprogram the address after enabling the plane.
+	 */
+	if (!old_fb && fb) {
+		ast_set_start_address_crt1(ast, (u32)ast_plane->offset);
+		ast_set_index_reg_mask(ast, AST_IO_SEQ_PORT, 0x1, 0xdf, 0x00);
+	}
 }
 
 static void ast_primary_plane_helper_atomic_disable(struct drm_plane *plane,
-- 
2.37.3