Re: [PATCH 6.17 105/849] drm/ast: Clear preserved bits from register output value

[Peter Schneider <pschneider1968@googlemail.com>]

Hi Greg,


Am 11.11.2025 um 01:34 schrieb Greg Kroah-Hartman:
> 6.17-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Thomas Zimmermann <tzimmermann@suse.de>
> 
> commit a9fb41b5def8e1e0103d5fd1453787993587281e upstream.
> 
> Preserve the I/O register bits in __ast_write8_i_masked() as specified
> by preserve_mask. Accidentally OR-ing the output value into these will
> overwrite the register's previous settings.
> 
> Fixes display output on the AST2300, where the screen can go blank at
> boot. The driver's original commit 312fec1405dd ("drm: Initial KMS
> driver for AST (ASpeed Technologies) 2000 series (v2)") already added
> the broken code. Commit 6f719373b943 ("drm/ast: Blank with VGACR17 sync
> enable, always clear VGACRB6 sync off") triggered the bug.
> 
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Reported-by: Peter Schneider <pschneider1968@googlemail.com>
> Closes: https://lore.kernel.org/dri-devel/a40caf8e-58ad-4f9c-af7f-54f6f69c29bb@googlemail.com/
> Tested-by: Peter Schneider <pschneider1968@googlemail.com>
> Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
> Fixes: 6f719373b943 ("drm/ast: Blank with VGACR17 sync enable, always clear VGACRB6 sync off")
> Fixes: 312fec1405dd ("drm: Initial KMS driver for AST (ASpeed Technologies) 2000 series (v2)")
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Nick Bowler <nbowler@draconx.ca>
> Cc: Douglas Anderson <dianders@chromium.org>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: Jocelyn Falempe <jfalempe@redhat.com>
> Cc: dri-devel@lists.freedesktop.org
> Cc: <stable@vger.kernel.org> # v3.5+
> Link: https://patch.msgid.link/20251024073626.129032-1-tzimmermann@suse.de
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>   drivers/gpu/drm/ast/ast_drv.h |    8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> --- a/drivers/gpu/drm/ast/ast_drv.h
> +++ b/drivers/gpu/drm/ast/ast_drv.h
> @@ -284,13 +284,13 @@ static inline void __ast_write8_i(void _
>   	__ast_write8(addr, reg + 1, val);
>   }
>   
> -static inline void __ast_write8_i_masked(void __iomem *addr, u32 reg, u8 index, u8 read_mask,
> +static inline void __ast_write8_i_masked(void __iomem *addr, u32 reg, u8 index, u8 preserve_mask,
>   					 u8 val)
>   {
> -	u8 tmp = __ast_read8_i_masked(addr, reg, index, read_mask);
> +	u8 tmp = __ast_read8_i_masked(addr, reg, index, preserve_mask);
>   
> -	tmp |= val;
> -	__ast_write8_i(addr, reg, index, tmp);
> +	val &= ~preserve_mask;
> +	__ast_write8_i(addr, reg, index, tmp | val);
>   }
>   
>   static inline u32 ast_read32(struct ast_device *ast, u32 reg)
> 


I think that with this patch (which fixes a bug in the original ast driver affecting AST2300), it is now safe to also 
include (in both 6.12.58 AND 6.17.8)

6f719373b943 ("drm/ast: Blank with VGACR17 sync enable, always clear VGACRB6 sync off")


which triggered that bug, and which you dropped from 6.12.55 and 6.17.5, respectively, because of my report

https://lore.kernel.org/dri-devel/a40caf8e-58ad-4f9c-af7f-54f6f69c29bb@googlemail.com/


NB: 6f719373b943 fixed (IIRC) an important issue for AST2500 users. I have tested the combination of both patches in 
mainline 6.18-rc2, and they work fine together, and Linus has both of them in his tree since 6.18-rc4.

Also, I tested both of them already on top of 6.12.5 and 6.17.5, and they were fine, too. Please see:

https://lore.kernel.org/lkml/045e6362-01db-47f3-9a4f-8a86b2c15d00@googlemail.com/



Beste Grüße,
Peter Schneider

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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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