linux-kernel - [REGRESSION][BISECTED] Screen goes blank with ASpeed AST2300 in 6.18-rc2 (was: Re: [PATCH v2] drm/ast: Blank with VGACR17 sync enable, always clear VGACRB6 sync off)

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Message-ID: <a40caf8e-58ad-4f9c-af7f-54f6f69c29bb@googlemail.com>
Date: Wed, 22 Oct 2025 05:27:09 +0200
From: Peter Schneider <pschneider1968@...glemail.com>
To: regressions@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Cc: dri-devel@...ts.freedesktop.org, stable@...r.kernel.org,
 Thomas Zimmermann <tzimmermann@...e.de>, jfalempe@...hat.com,
 airlied@...hat.com, dianders@...omium.org, nbowler@...conx.ca,
 stable@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [REGRESSION][BISECTED] Screen goes blank with ASpeed AST2300 in
 6.18-rc2 (was: Re: [PATCH v2] drm/ast: Blank with VGACR17 sync enable, always
 clear VGACRB6 sync off)

#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.

The bad commit is

commit 6f719373b943a955fee6fc2012aed207b65e2854
Author: Thomas Zimmermann <tzimmermann@...e.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

-- 
Climb the mountain not to plant your flag, but to embrace the challenge,
enjoy the air and behold the view. Climb it so you can see the world,
not so the world can see you.                    -- David McCullough Jr.

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Am 14.10.2025 um 10:46 schrieb Thomas Zimmermann:
> Blank the display by disabling sync pulses with VGACR17<7>. Unblank
> by reenabling them. This VGA setting should be supported by all Aspeed
> hardware.
> 
> Ast currently blanks via sync-off bits in VGACRB6. Not all BMCs handle
> VGACRB6 correctly. After disabling sync during a reboot, some BMCs do
> not reenable it after the soft reset. The display output remains dark.
> When the display is off during boot, some BMCs set the sync-off bits in
> VGACRB6, so the display remains dark. Observed with  Blackbird AST2500
> BMCs. Clearing the sync-off bits unconditionally fixes these issues.
> 
> Also do not modify VGASR1's SD bit for blanking, as it only disables GPU
> access to video memory.
> 
> v2:
> - init vgacrb6 correctly (Jocelyn)
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@...e.de>
> Fixes: ce3d99c83495 ("drm: Call drm_atomic_helper_shutdown() at shutdown time for misc drivers")
> Tested-by: Nick Bowler <nbowler@...conx.ca>
> Reported-by: Nick Bowler <nbowler@...conx.ca>
> Closes: https://lore.kernel.org/dri-devel/wpwd7rit6t4mnu6kdqbtsnk5bhftgslio6e2jgkz6kgw6cuvvr@xbfswsczfqsi/
> Cc: Douglas Anderson <dianders@...omium.org>
> Cc: Dave Airlie <airlied@...hat.com>
> Cc: Thomas Zimmermann <tzimmermann@...e.de>
> Cc: Jocelyn Falempe <jfalempe@...hat.com>
> Cc: dri-devel@...ts.freedesktop.org
> Cc: <stable@...r.kernel.org> # v6.7+
> ---
>   drivers/gpu/drm/ast/ast_mode.c | 18 ++++++++++--------
>   drivers/gpu/drm/ast/ast_reg.h  |  1 +
>   2 files changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
> index 6b9d510c509d..9b6a7c54fbb5 100644
> --- a/drivers/gpu/drm/ast/ast_mode.c
> +++ b/drivers/gpu/drm/ast/ast_mode.c
> @@ -836,22 +836,24 @@ 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 vgacr17 = 0x00;
> +	u8 vgacrb6 = 0xff;
>   
> -	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0xb6, 0xfc, 0x00);
> -	ast_set_index_reg_mask(ast, AST_IO_VGASRI, 0x01, 0xdf, 0x00);
> +	vgacr17 |= AST_IO_VGACR17_SYNC_ENABLE;
> +	vgacrb6 &= ~(AST_IO_VGACRB6_VSYNC_OFF | AST_IO_VGACRB6_HSYNC_OFF);
> +
> +	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0x17, 0x7f, vgacr17);
> +	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0xb6, 0xfc, vgacrb6);
>   }
>   
>   static void ast_crtc_helper_atomic_disable(struct drm_crtc *crtc, struct drm_atomic_state *state)
>   {
>   	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 vgacrb6;
> +	u8 vgacr17 = 0xff;
>   
> -	ast_set_index_reg_mask(ast, AST_IO_VGASRI, 0x01, 0xdf, AST_IO_VGASR1_SD);
> -
> -	vgacrb6 = AST_IO_VGACRB6_VSYNC_OFF |
> -		  AST_IO_VGACRB6_HSYNC_OFF;
> -	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0xb6, 0xfc, vgacrb6);
> +	vgacr17 &= ~AST_IO_VGACR17_SYNC_ENABLE;
> +	ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0x17, 0x7f, vgacr17);
>   
>   	/*
>   	 * HW cursors require the underlying primary plane and CRTC to
> diff --git a/drivers/gpu/drm/ast/ast_reg.h b/drivers/gpu/drm/ast/ast_reg.h
> index e15adaf3a80e..30578e3b07e4 100644
> --- a/drivers/gpu/drm/ast/ast_reg.h
> +++ b/drivers/gpu/drm/ast/ast_reg.h
> @@ -29,6 +29,7 @@
>   #define AST_IO_VGAGRI			(0x4E)
>   
>   #define AST_IO_VGACRI			(0x54)
> +#define AST_IO_VGACR17_SYNC_ENABLE	BIT(7) /* called "Hardware reset" in docs */
>   #define AST_IO_VGACR80_PASSWORD		(0xa8)
>   #define AST_IO_VGACR99_VGAMEM_RSRV_MASK	GENMASK(1, 0)
>   #define AST_IO_VGACRA1_VGAIO_DISABLED	BIT(1)

View attachment "dmesg_618rc1.txt" of type "text/plain" (119222 bytes)

View attachment "dmesg_618rc2.txt" of type "text/plain" (119268 bytes)

View attachment "git_bisect.log" of type "text/plain" (3762 bytes)