[PATCH v2] drm/ast: Fix start address computation

[Jocelyn Falempe]

During the driver conversion to shmem, the start address for the
scanout buffer was set to the base PCI address.
In most cases it works because only the lower 24bits are used, and
due to alignment it was almost always 0.
But on some unlucky hardware, it's not the case, and some unitilized
memory is displayed on the BMC.
With shmem, the primary plane is always at offset 0 in GPU memory.

Tested on a sr645 affected by this bug.

Fixes: f2fa5a99ca81 ("drm/ast: Convert ast to SHMEM")
Signed-off-by: Jocelyn Falempe <jfalempe at redhat.com>
---
 drivers/gpu/drm/ast/ast_mode.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
index c7443317c747..54deb29bfeb3 100644
--- a/drivers/gpu/drm/ast/ast_mode.c
+++ b/drivers/gpu/drm/ast/ast_mode.c
@@ -681,7 +681,8 @@ static void ast_primary_plane_helper_atomic_update(struct drm_plane *plane,
 	if (!old_fb || old_fb->pitches[0] != fb->pitches[0])
 		ast_set_offset_reg(ast, fb);
 	if (!old_fb) {
-		ast_set_start_address_crt1(ast, (u32)ast_plane->offset);
+		/* with shmem, the primary plane is always at offset 0 */
+		ast_set_start_address_crt1(ast, 0);
 		ast_set_index_reg_mask(ast, AST_IO_SEQ_PORT, 0x1, 0xdf, 0x00);
 	}
 }
-- 
2.39.1