linux-kernel - Re: PROBLEM: AST2500 BMC video output disabled by reboot (regression)

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Message-ID: <cox6kpackepnunrlhpsssvgdmjd24477cc7mide52xptmmoxyr@ijjotb3xju3v>
Date: Wed, 17 Sep 2025 22:04:45 -0400
From: Nick Bowler <nbowler@...conx.ca>
To: Thomas Zimmermann <tzimmermann@...e.de>
Cc: Doug Anderson <dianders@...omium.org>, linux-kernel@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, regressions@...ts.linux.dev
Subject: Re: PROBLEM: AST2500 BMC video output disabled by reboot (regression)

On Wed, Sep 17, 2025 at 11:14:45AM -0400, Nick Bowler wrote:
> On Fri, Aug 29, 2025 at 03:07:14PM +0200, Thomas Zimmermann wrote:
> > The ast driver doesn't do much during shutdown. Could you please out-comment
> > the lines at either [2] xor [3] and report on either effect? These calls
> > turn of the video chip's memory access and sync pulses. Not doing that might
> > resolve the problem.
> > 
> > [2] https://elixir.bootlin.com/linux/v6.16.3/source/drivers/gpu/drm/ast/ast_mode.c#L835
> > [3] https://elixir.bootlin.com/linux/v6.16.3/source/drivers/gpu/drm/ast/ast_mode.c#L839
> 
> I can try this.

OK, I tested 6.17-rc6 and reproduced the problem on this version.  Then:

  - Deleting [2] (only) appears to make no difference whatsoever. That
    is, deleting the following line in ast_mode.c:

      ast_set_index_reg_mask(ast, AST_IO_VGASRI, 0x01, 0xdf, AST_IO_VGASR1_SD);

  - Deleting [3] (only) appears sufficient to make things work again,
    that is, deleting the following line in ast_mode.c:

      ast_set_index_reg_mask(ast, AST_IO_VGACRI, 0xb6, 0xfc, vgacrb6);

Thanks,
  Nick