Re: [REGRESSION][BISECTED] Screen goes blank with ASpeed AST2300 in 6.18-rc2

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 22.10.25 um 05:27 schrieb Peter Schneider:
> #regzbot introduced: 6f719373b943a955fee6fc2012aed207b65e2854
>
> Hi all,
>
> I have encountered a serious (for me) regression with 6.18-rc2 on my 
> 2-socket Ivy Bridge Xeon E5-2697 v2 server. After booting, my console 
> screen goes blank and stays blank. 6.18-rc1 was still fine.
>
> The machine has an Asus Z9PE-D16 server mainboard with an onboard 
> ASpeed AST2300 VGA chip with 16MB VRAM. I have attached an older HP 
> Monitor to it via old VGA jack/cable. It also has a second graphics 
> card in a PCI-E slot; an older NVidia GTX 560. It is not connected to 
> a monitor, but I have configured it via kernel command line for 
> PCI-pass-through to VMs running on this server (I use Proxmox VE, i.e. 
> QEMU/KVM virtual machines). Currently, no VMs use this yet, and also 
> no VMs are autostarting with machine boot. So when this regression 
> occurs, the server is idle. Pressing a key on the keyboard does not 
> make the screen come alive. The server is running fine though, and I 
> can access it via SSH. It just has no graphic output anymore. In case 
> this is important, the machine also has a ASMB6 BMC (can be used via 
> http).
>
> I have attached dmesg output from both 6.18-rc1 which is fine, and 
> 6.18-rc2 which exhibits this bug. I have bisected the issue, please 
> see attached git bisect.log.

Thanks for the detailed bug report.

Attached is a patch that partially reverts the broken commit. Could you 
please apply it on top of the broken kernel and report on the results?

Best regards
Thomas

>
> The bad commit is
>
> commit 6f719373b943a955fee6fc2012aed207b65e2854
> Author: Thomas Zimmermann <tzimmermann@suse.de>
> Date:   Tue Oct 14 10:46:34 2025 +0200
>
>     drm/ast: Blank with VGACR17 sync enable, always clear VGACRB6 sync 
> off
>
>     Blank the display by disabling sync pulses with VGACR17<7>. Unblank
>     by reenabling them. This VGA setting should be supported by all 
> Aspeed
>     hardware.
>
> When I revert this from 6.18-rc2, the issue goes away and my console 
> screen works again.
>
> I just saw that Greg just yesterday evening included the offending 
> patch already in stable RCs 6.12.55-rc1 and 6.17.5-rc1, so I'll test 
> these seperately and send a mail to the stable mailing list, too, if 
> affected (which I anticipate).
>
> Beste Grüße,
> Peter Schneider
>

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)


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From 5fdea2e9936a49905902e339e627a27b0ab0a884 Mon Sep 17 00:00:00 2001
From: Thomas Zimmermann <tzimmermann@suse.de>
Date: Wed, 22 Oct 2025 08:48:49 +0200
Subject: [PATCH] ast: Set/clear screen-disable bit

---
 drivers/gpu/drm/ast/ast_mode.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
index 9ce874dba69c..b73437083dbf 100644
--- a/drivers/gpu/drm/ast/ast_mode.c
+++ b/drivers/gpu/drm/ast/ast_mode.c
@@ -820,6 +820,7 @@ ast_crtc_helper_atomic_flush(struct drm_crtc *crtc,
 static void ast_crtc_helper_atomic_enable(struct drm_crtc *crtc, struct drm_atomic_state *state)
 {
 	struct ast_device *ast = to_ast_device(crtc->dev);
+	u8 vgasr1 = 0xff;
 	u8 vgacr17 = 0x00;
 	u8 vgacrb6 = 0xff;
 
@@ -828,6 +829,9 @@ static void ast_crtc_helper_atomic_enable(struct drm_crtc *crtc, struct drm_atom
 
 	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0x17, 0x7f, vgacr17);
 	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0xb6, 0xfc, vgacrb6);
+
+	vgasr1 &= ~AST_IO_VGASR1_SD;
+	ast_set_index_reg_mask(ast, AST_IO_VGASRI, 0x01, 0xdf, vgasr1);
 }
 
 static void ast_crtc_helper_atomic_disable(struct drm_crtc *crtc, struct drm_atomic_state *state)
@@ -835,6 +839,10 @@ static void ast_crtc_helper_atomic_disable(struct drm_crtc *crtc, struct drm_ato
 	struct drm_crtc_state *old_crtc_state = drm_atomic_get_old_crtc_state(state, crtc);
 	struct ast_device *ast = to_ast_device(crtc->dev);
 	u8 vgacr17 = 0xff;
+	u8 vgasr1 = 0x00;
+
+	vgasr1 |= AST_IO_VGASR1_SD;
+	ast_set_index_reg_mask(ast, AST_IO_VGASRI, 0x01, 0xdf, vgasr1);
 
 	vgacr17 &= ~AST_IO_VGACR17_SYNC_ENABLE;
 	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0x17, 0x7f, vgacr17);
-- 
2.51.0