[PATCH 6.8 126/172] drm/ast: Fix soft lockup

[Greg Kroah-Hartman]

6.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jammy Huang <jammy_huang at aspeedtech.com>

commit bc004f5038220b1891ef4107134ccae44be55109 upstream.

There is a while-loop in ast_dp_set_on_off() that could lead to
infinite-loop. This is because the register, VGACRI-Dx, checked in
this API is a scratch register actually controlled by a MCU, named
DPMCU, in BMC.

These scratch registers are protected by scu-lock. If suc-lock is not
off, DPMCU can not update these registers and then host will have soft
lockup due to never updated status.

DPMCU is used to control DP and relative registers to handshake with
host's VGA driver. Even the most time-consuming task, DP's link
training, is less than 100ms. 200ms should be enough.

Signed-off-by: Jammy Huang <jammy_huang at aspeedtech.com>
Fixes: 594e9c04b586 ("drm/ast: Create the driver for ASPEED proprietory Display-Port")
Reviewed-by: Jocelyn Falempe <jfalempe at redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann at suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
Cc: KuoHsiang Chou <kuohsiang_chou at aspeedtech.com>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Jocelyn Falempe <jfalempe at redhat.com>
Cc: dri-devel at lists.freedesktop.org
Cc: <stable at vger.kernel.org> # v5.19+
Link: https://patchwork.freedesktop.org/patch/msgid/20240403090246.1495487-1-jammy_huang@aspeedtech.com
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
---
 drivers/gpu/drm/ast/ast_dp.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/drivers/gpu/drm/ast/ast_dp.c
+++ b/drivers/gpu/drm/ast/ast_dp.c
@@ -180,6 +180,7 @@ void ast_dp_set_on_off(struct drm_device
 {
 	struct ast_device *ast = to_ast_device(dev);
 	u8 video_on_off = on;
+	u32 i = 0;
 
 	// Video On/Off
 	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0xE3, (u8) ~AST_DP_VIDEO_ENABLE, on);
@@ -192,6 +193,8 @@ void ast_dp_set_on_off(struct drm_device
 						ASTDP_MIRROR_VIDEO_ENABLE) != video_on_off) {
 			// wait 1 ms
 			mdelay(1);
+			if (++i > 200)
+				break;
 		}
 	}
 }